WHP Cruise Summary Information WOCE section designation A25 Expedition designation (EXPOCODE) 74DI230_1 Chief Scientist(s) and their affiliation Sheldon Bacon, SOC/JRD Dates 1997.08.07 - 1997.09.17 Ship DISCOVERY Ports of call Vigo, Spain to Southampton, U.K. Number of stations 143 Geographic boundaries of the stations 65°31.92''N 43°19.15''W 8°19.98''W 41°28.00'N Floats and drifters deployed see below Moorings deployed or recovered see below Contributing Authors (in order of appearance) M.A. Harding J. Smithers S. Cunningham M. Tsimplis P. Holliday H. Bryden B. Marsh M. Yelland E. Rourke S.K. Brown J. Xiong S. Holley M. Rodriguez I. Aristegui D. Smythe-Wright S. Boswell C. Harris R. Davidson C. Peckett M. Fox V. Thierry P. Mason R. Phipps S. Mitchell SOUTHAMPTON OCEANOGRAPHY CENTRE Cruise Report No. 16 RRS DISCOVERY Cruise 230 7 August - 17 September 1997 Two hydrographic sections across the boundaries of the subpolar gyre FOUREX Principal Scientist Sheldon Bacon 1998 James Rennell Division for Ocean Circulation Southampton Oceanography Centre Empress Dock Southampton SO14 3ZH, U. K. Tel: +44 1703 596441 Fax: +44 1703 596204 Email: S.Bacon@soc.soton.ac.uk DOCUMENT DATA SHEET: Author: Sheldon Bacon Title: RRS Discovery Cruise 230, 7 August - 17 September 1997. Two hydrographic sections across the boundaries of the subpolar gyre, FOUREX. Reference: Southampton Oceanography Centre Cruise Report, No. 16, 104 pp. Abstract: This report describes RRS Discovery Cruise 230, designed as a repeat of the International Geophysical Year (IGY) survey section 4, roughly from Cape Finisterre (Spain) to Cape Farewell (Greenland). IGY 4 was first surveyed in 1957, so this repeat gives a 40-year look at decadal variability in the North Atlantic from the eastern boundary regime via the junction of subtropical and subpolar gyres to the western boundary regime. Additional short sections were measured (a) midway between Cape Farewell and Denmark Strait, (b) across Denmark Strait and (c) from Iceland to Scotland in order (i) to assess the spatial variability of the western boundary regime up the east Greenland coast to Denmark Strait, (ii) to assess the exchange between the northern North Atlantic and the Nordic Seas, (iii) to create a large scale North Atlantic closed box for evaluation of the circulation, and (iv) to continue the long time series of Rockall Trough sections. Sections were measured with stations for CTD, LADCP and tracer chemistry (CFCs, oxygen, nutrients, CO2). Continuous measurements of high precision position and heading navigation data were made; also of VM-ADCP, depth and TSG. Continuous high-quality meteorological measurements were made, with a view to assessing Ekman fluxes, and comparing with fluxes inferred from Irminger Basin float data. This cruise is a U. K. contribution to the World Ocean Circulation Experiment. Keywords: ADCP; ATLN; CARBON DIOXIDE; CFC; CHARLIE-GIBBS FRACTURE ZONE; CO2; CRUISE 230 1997; CTD OBSERVATIONS; DENMARK STRAIT; DISCOVERY; IBERIAN ABYSSAL PLAIN; ICELAND BASIN; INTERNATIONAL GEOPHYSICAL YEAR; IGY; IRMINGER BASIN; LADCP; NORTH ATLANTIC; NUTRIENTS; OXYGEN; ROCKALL TROUGH; WATER EXCHANGE; WOCE. CONTENTS: SCIENTIFIC PERSONNEL SHIP'S PERSONNEL LIST OF FIGURES LIST OF TABLES ACKNOWLEDGEMENTS 1. THE CRUISE a. Scientific objectives b. Overview c. Narrative 2. CTD MEASUREMENTS a. Equipment b. Data capture and processing c. Post-cruise calibration 3. LOWERED ADCP MEASUREMENTS a. Description b. Instrument problems c. Data processing d. Bottom tracking data e. Comparison of VM - and L-ADCP 4. NAVIGATION a. Bestnav b. GPS and GLONASS c. Ship gyrocompass d. Ashtech 3D attitude determination 5. VM-ADCP MEASUREMENTS a. Description and processing b. Calibration c. Performance d. General description of observed currents 6. METEOROLOGICAL MEASUREMENTS a. Surface meteorology b. Shipborne wave recorder c. Rain buoy 7. CHEMICAL MEASUREMENTS a. Oxygen b. Nutrients c. Carbon d. Halocarbons e. Phytoplankton Speciation and Pigment Studies f. Salinity 8. OTHER MEASUREMENTS a. Thermosalinograph b. Precision Echo Sounder 9. COMPUTING 10. TECHNICAL SUPPORT 11. APPENDIX: FOUREX STATION INFORMATION SCIENTIFIC PERSONNEL Name: From: Role: BACON, Sheldon SOC-JRD Principal Scientist ARISTEGUI, Iris Soler IIM CO2 analyst BOSWELL, Steve SOC-GDD CFC analyst BRIDGER, Martin SOC-RVS Computing tech. BROWN, Sarah Portsmouth Univ.Chemistry assistant BRYDEN, Harry SOC-JRD Physics assistant CUNNINGHAM, Stuart SOC-JRD CTD & data (PI) DAVIDSON, Russell SOC-GDD Shrubbery (PI), CFC assistant DUNCAN, Paul SOC-RVS Computing tech. (senior) FOX, Maryke SOC-JRD Physics assistant & LADCP HARRIS, Craig Liverpool Univ. CFC assistant HOLLEY, Sue SOC-GDD Oxygen & nutrients (PI) HOLLIDAY, Penny SOC-GDD LADCP (PI) JOLLY, Dave SOC-RVS Mechanic MARSH, Bob SOC-JRD Navigation & VM-ADCP (PI) MASON, Peter SOC-RVS Mechanic (senior RVS tech.) MITCHELL, Simon SOC-RVS Mechanic PECKETT, Cristina SOC-GDD CFC & shrubbery assistant PHIPPS, Richie SOC-RVS Mechanic RODRIGUEZ, Marta IIM CO2 (PI) ROURKE, Lizzie SOC-JRD Oxygen & nutrients assistant SMITHERS, John SOC-OTD CTD/Electronic tech. SMYTHE-WRIGHT, Denise SOC-JRD CFC (PI) THIERRY, Virgine IFREMER Physics assistant TSIMPLIS, Mickey SOC-JRD Computing & LADCP XIONG, Jian Southampton Univ.Oxygen & nutrients assistant YELLAND, Margaret SOC-JRD Meteorology (PI) KEY SOC: Southampton Oceanography Centre GDD: George Deacon Division JRD: James Rennell Division OTD: Ocean Technology Division RVS: Research Vessel Services IIM: Instituto de Investigaciones Mariñas, Vigo IFREMER:Institut Français de recherche pour l'exploitation de la mer, Brest SHIP'S PERSONNEL Name: Rank/Rating: HARDING, Mike Master LEATHER, Ceri Mate SYKES, Syd Second Mate REYNOLDS, Pete Third Mate SUGDEN, Dave Radio Officer McDONALD, BernieChief Engineer CLARKE, John Second Engineer BELL, Steve Third Engineer JACKSON, Greg Third Engineer POOK, Tiny C.P.O. (D) LUCKHURTS, KevinP.O. (D) COOK, Stuart SG1A CRABB, Gary SG1A DICKINSON, Bob SG1A EDWARDS, Tim SG1A MACLEAN, Andy SG1A MICKMAN, Bill MM1A PERRY, Clive S.C.M. HAUGHTON, John Chef DUNCAN, Andy Mess Steward CARTER, Shiela Steward ORSBORN, Jeff Steward LIST OF FIGURES 1.1 Cruise track 1.2 Station positions 3.1 VM- and L-ADCP profile comparison 3.2 LADCP water track / bottom track comparison 6.1 Comparison of Hs corrected and uncorrected 7.1 Variations in thiosulphate normality 7.2a Duplicate difference at each station 7.2b Comparison of duplicate difference with bottles used 7.3a Silicate QC deep, QC3 and QC4 7.3b Nitrate QC deep, QC3 and QC4 7.3c Phosphate QC deep, QC3 and QC4 7.4 Salinity standard history A1 Bottle depths vs. station number LIST OF TABLES 2.1 Laboratory measurements of pressure hysteresis for DEEP01 2.2 Corrections to the conductivity offset 2.3 Laboratory measurements of pressure hysteresis for DEEP02 2.4 Salinity correction coefficients 2.5 Salinity residual statistics 2.6 Oxygen coefficients and sum square residuals 2.7 Statistics of comparison of upcast CTD minus reversing instrument value 2.8 Post-cruise laboratory measurements of pressure hysteresis for DEEP02 2.9 Post-cruise reversing instrument and DEEP02 comparisons 3.1 Upper ocean differences between VM- and L-ADCP currents 3.2 Comparison of absolute LADCP and Bottom Track velocities 4.1 Ship navigation error determined in port 4.2 Summary of Ashtech 3DF GPS performance statistics 5.1 VM-ADCP calibration exercise results 6.1 Variables and sensors logged by GrhoMet system 7.1 Summary of chemical sampling regime during cruise 7.2 Working nutrient standard concentration 7.3 Correction factors applied to the nutrient data 7.4 Standard seawater salinities 8.1 Comparison of acual depth with echo-sounder depth on station A1 WOCE format station summary ACKNOWLEDGEMENTS Without meaning to anticipate the encomia which will doubtless be forthcoming, I must thank first the Master, Captain Mike Harding. This may well have been his last research cruise as Master, in view of his impending retirement, and I wish to thank him for helping to make my first cruise as PSO such a pleasure. There was a '70's theme party during the cruise, and all were amused by Mike's wilful misinterpretation of the invitation as over-70's rather than 1970's. He appeared with walking stick and pension book, and wearing a slightly disreputable tweed jacket, in anticipation of times to come, one supposes. Mike will be sorely missed by all, and I take this opportunity to wish him well for the future. John Smithers is less an electronics technician and more a wizard. I express my sincere thanks to him, particularly in respect of the Lowered Doppler Affair. It is hard to imagine that this cruise would have been anything like the success it was without the benefit of his remarkable expertise. I am most grateful to Sue Scrowston, Andy Louch and Jackie Skelton of the RVS Operations Office for their sorting out much of the mundane logistics - hotels, flights, freight etc. I am particularly grateful to Sue and Andy for their near-instant response in the Lowered Doppler Affair, which ensured that minimal time was lost in fetching replacement gear. Thanks also to Rob Bonner (SOC-OTD) for logistical help. In the same (aforementioned) context, thanks to Nick Crisp (SOC-OTD) for arranging the replacement LADCP parts. Thanks to Chris German (SOC-CHD) for the loan of the TOBI swivel. Thanks to Robin Pascal (SOC-OTD) for flying out to Vigo to help set up the Met. gear, and likewise thanks to Gwyneth Jones (SOC Computing) for helping to set up the computers. Thanks to Aida Rios (IIM, Vigo), for her considerable help in arranging the participation of IIM scientists (Marta and Iris) to look after the CO2 measurements on this cruise. Finally, and most importantly, my sincere thanks to the responsible authorities of Greenland, Iceland, Spain and Portugal, for their gracious granting of permission to work in their respective territorial waters, without which much of this cruise would have been meaningless. The cruise was funded by the U. K. Natural Environment Research Council as part of the U. K. contribution to WOCE. Sheldon Bacon 1. THE CRUISE a. SCIENTIFIC OBJECTIVES During 1996-1997 the intense measuring effort put into the North Atlantic under the aegis of international WOCE has several aims in accordance with Goal 1 of WOCE (WOCE Implementation Plan, 1988). Firstly, meridional transports of heat, mass and freshwater will be measured, from exchanges with the South Atlantic across the Equator to exchanges with the Northern Seas between Greenland and Scotland via the Sub-Tropical and Sub-Polar Gyres. Secondly, decadal variability will be examined through exact track repeats of cruises from the International Geophysical Year (IGY) expeditions in 1957-1958, and others. IGY is the first large-scale field program which can be used as a basis for modern comparative work because its measurements are the first which approach modern standards of accuracy. Thirdly, there will be a focus on the Sub-Polar Gyre, intending to quantify the rates and to study the physics of the formation of mode waters and deep waters. The main or most immediate aims of this cruise, designated WOCE cruise A24, go under four headings: (1) Repeat survey of IGY Section 4 (Portugal to Greenland), for climate change analysis; (2) Determination of heat, mass and freshwater fluxes across IGY Section 4, and across northern 'closure' sections; thus, exchanges between the Sub-Polar Gyre and (to the north) the Nordic Seas, and (to the south) the Sub-Tropical Gyre. (3) Continuation of the Ellett (Dunstaffnage Marine Laboratory) time series section across Anton Dohrn Seamount; (4) Production of data suitable for inclusion in the WOCE North Atlantic data set. Subsidiary or longer-term aims are/were: (i) collaborating with European and North American colleagues to produce a "Summer of '97" synoptic view of the northern North Atlantic circulation, particularly WOCE sections A1 (55°N) and A2 (48°N), AR7W (Labrador Sea), with ancilliary data such as satellite altimetry (ERS2, TOPEX/POSEIDON) and floats (Arcane/ Eurofloat, PALACEs); (ii) comparisons of PALACE float profiles and inferred surface fluxes with fluxes and profiles measured on board and estimated from climatology; (iii) comparisons between model (GCM - OCCAM) output and measured / inferred circulation; (iv) testing an acoustic rainfall-measuring buoy. b. OVERVIEW Although the odd spat of nasty weather halted operations for short periods, overall the weather was splendid, enabling us to achieve all our major goals for this cruise, and several lesser ones. The weather only really deteriorated during the final week as autumn drew in, in the vicinity of Rockall. The cruise track and station positions are shown in figures 1.1 and 1.2. The cruise progressed clockwise around the northern North Atlantic, beginning in Vigo, Spain on 7 August 1997. The initial work line was 41°30'N, between 9°W and 20°W. We proceeded directly from Vigo to 9°W, 41°30N, then out to deep water (>5000 m) at 41°30'N, 12°30'W for 2 test stations, one firing all bottles at one depth, one for a bottle profile. Then we ran back to 9°W at 8 kn for an acoustic current profiling survey; the section proper began with a station on the 200 m contour at 9°13'W. Over the next few days, as we worked across the southern flank of the Galicia Bank and down the continental slope, it became apparent that the LADCP was seriously malfunctioning, and could not be persuaded, either by attention to hardware or software, to behave. We decided to have a replacement unit, complete but for pressure case, sent out to Leixões, the port of Oporto in Portugal. We broke off work immediately after station 21 (2200Z, 12 August, 12°9'W) to run in and collect the gear which had been flown out overnight, and was waiting for us on the dockside on arrival. A short wait of a few hours in port enabled an unexpected excursion for some of the scientific party to a nearby beach to bathe and skylark. We departed promptly at 1630Z. Station 22, begun at 0700Z on 14 August, was a cast to 2000 m to ensure the watertight integrity of the original LADCP pressure case with the replacement transducer head. Station 23 later the same day was an 'overlap' station, repeating 21. Thus only a day and a half were wasted; and although LADCP data are sparse for this part of the first section, we have no less than five ADCP transects, which should give a decent reference for subsequent geostrophic calculations. Station 35 was the 'corner', the last of line along 41°30'N, after which we turned north-west for the rest of the section along the rhumb (Mercator- straight) line between 41°30N, 20°W and Cape Farewell. Stations 21-34 crossed the bottom of the Iberian Abyssal Plain. Much of the remainder of the section consisted of a rather oblique cut along the Mid-Atlantic Ridge, with stations 35-40 passing seamounts of the Azores-Biscay Rise and stations 41-42 at the bottom of the north-western corner of the King's Trough, a small deep basin closed below ca. 3500-4000 m. Passing the Maxwell and Faraday Fracture Zones in the vicinity of 27°W and 29°W (along the track, respectively), we arrived at the Charlie-Gibbs Fracture Zone with stations 62-67, where 63 was at the bottom of the Southern Transform Valley and 65 at the bottom of the Northern. Stations 70-73 spanned the crest of the Reykjanes Ridge, after which we descended to the bottom of the Irminger Basin around stations 78-81. Rising up the Greenland continental slope with nominal resolution of topography at 250 m depth increments, station 92 arrived at the 200 m contour, and station 93, the final one on this repeat of IGY section 4, was our closest approach to land, in 150 m of water about 2 miles from the coast. Next we made a low-speed acoustic profiling transect of the western boundary current regime, retracing our path back to the position of station 81 in the middle of the Irminger Basin, at 5 kn. Thus a 10-minute ADCP average translates, with good navigation, to 1.5 km horizontal resolution with about <1cm/s accuracy in currents. Following this exercise, we made for the start of the next section, the East Greenland Central Section, which was selected by consultation with Alexander Sy (BSH, Hamburg), who was out in the same area at the same time as us, on the FS Meteor, but circuiting the North Atlantic in the opposite sense to us. We had hoped to meet at Cape Farewell, but our LADCP-derived delays contrived to make us miss. The Meteor however was engaged in a suite of sections down East Greenland as part of the VEINS program, and for comparative purposes, we carried out a repeat of the section most appealing to us - the most northerly one which still had a narrow continental shelf. This began in mid-basin with station 94; topographic resolution was 500 m nominal, ending in about 300 m water depth 5 miles from the coast, surrounded by grounded icebergs, in the dark. Next we headed north for the Denmark Strait Section. Running from west to east, station 103 was our furthest north at 65°31'N, finishing at station 110. Bad weather was holding us up on what should have been the final station of this section in 200 m of water, so it was abandoned and we ran before the weather to the start of the final section south of Iceland on 20°W. Also, since the Greenland shelf is so broad in the region of Denmark Strait, we began in about 500 m water depth, so there may be cause for concern over both endpoints of the Denmark Strait section. The Iceland-Scotland Section began with station 111 in 200 m water depth. The centre of the Iceland Basin was reached around stations 121-122, after which we rose up the western flank of the Rockall-Hatton Plateau. Station 127 was on the top of Hatton Bank, 129 was the deepest in the Hatton-Rockall Basin, and 132-133 were on the top of Rockall Bank. Rockall itself was passed at night in murky weather and was just visible, unlit, as a small black lump on a black background. Then the Rockall Trough was crossed by stations 134-140. Station 137 was on the top of Anton Dohrn Seamount. The last three stations, 141-143, were on the Hebridean Shelf, after which we went home. c. NARRATIVE Discovery station numbers are given, not the cruise station numbers. For stations 1-21, Discovery number = station number + 13201 (so station 1 = 13202). No Discovery number was alloted to station 22. For station 23 onwards, Discovery number = station number + 13200. The last station was 143, Discovery 13343. Friday 1st August 1997: Preparations were made for the winding on of the new CTD wire. Familiarisation procedures for new marine staff were placed in hand. Saturday 2nd August 1997: Further cleaning work around the laboratories was carried out and a slow start was made at winding on the new CTD wire. Sunday 3rd August 1997: Wire-winding was continued but great difficulties were encountered and little progress was made. Monday 4th August 1997: Wire-winding resumed and good progress was made, the cause of the difficulties having been identified. Storing operations were commenced in the forenoon and the port lifeboat lowered to the water. Storing was largely completed in the afternoon and at 1645B bunkering operations were commenced. At 2055B the winding on of the CTD wire was finally completed and at 2220B the bunkering operation was completed. Tuesday 5th August 1997: Loading of scientific equipment was commenced in the forenoon and the lorry backloaded by early in the afternoon. The inflatable dinghy was taken away under power for prolonged testing in the afternoon. Wednesday 6th August 1997: The majority of the scientific staff joined the vessel in the forenoon and spent the day setting up and commissioning their equipment. Fresh water tanks were topped up. The final members of the scientific staff joined the vessel at 2300B. Thursday 7th August 1997: Familiarisation procedures of scientific staff were commenced at 0900B. At 1000B sailing preparations were carried out and at 1054B the pilot boarded. At 1058B singling up commenced, the vessel proceeded to the master's orders and at 1103B all moorings were gone and clear. At 1112B the pilot disembarked and the vessel sailed down the Ria Vigo. From 1136B to 1144B in the wider reaches of the bay, the vessel was stopped in the water to facilitate the deployment of the starboard lifeboat to the water. At 1218B with the Isla Boreiro light bearing 025°T X 1.9', full away was ordered and the vessel proceeded into the open sea. Course was set Southwards along the meridian of 009° 00'W and at 1600B course was altered to the West along the latitude of 41°30.0'N, our first line of survey. At 1600B also, alarms were sounded and staff proceeded to emergency drill stations, followed by muster at and boarding of the boats and instruction for the deployment and boarding of the liferafts. At 2040B the Precision Echo Sounder fish (PES) was deployed. Friday 8th August 1997: At 0650B the vessel was hove to in readiness to commence the first of two test stations. At 0757B the CTD+ was hove outboard to commence station 13202, the first of two test stations, which finished 1018B. The second test station 13203 commenced at 1245B and was back inboard at 1638B. These and future stations were to consist of the lowering of a CTD rig overside with watersampling bottles attached along with an LADCP system and a fluorometer. At 1640B course was set 090°T at 8 knots of the ground to carry out an ADCP and PES survey to the point of origin of the survey line in position 41°30.0'N 009°00.0'W. The Principal Scientist and the master met together to review the progress of the cruise and make plans for the future on the first of many occasions. Frequent forked lightning displays were observed to the East as the ship headed back overnight. Saturday 9th August 1997: The day opened with but light airs giving a calm sea with a very low swell, the skies were cloudy and visibility was but moderate. As the morning progressed and the ship came into shoaler water thick fog developed. Having just cleared the fog the vessel reached the end of the survey line in position 41°30'N 008°59.91'W at 1145B. Coming about and re- entering the fog the first station of the day 13204 was occupied. In the afternoon the skies cleared and bright sunny weather gave everybody an optimistic mood. After station 13205, an LADCP fault caused a delay before the commencement of each of the next two stations. Sunday 10th August 1997: The day opened with light airs, a glassy sea and a low swell, cloudy skies prevailed at first but soon cleared to give an almost tropically warm sunny day. Stations 13208-11 were occupied. Monday 11th August 1997: The start of the day was with winds of force 4 from SSE, a slight sea was running with a low swell and skies were cloudy and overcast, a slight haze reduced the visibility. Stations 13212-7 were occupied. Tuesday 12th August 1997: The day opened cloudy and overcast with light rain, the winds were from the South South West at about 8 knots, the sea was rippled with a low swell. Stations 13218-13221 were occupied. Wednesday 13th August 1997: At 0012B with the rig from station 13222 inboard and secured course was set to 097°T for the port of Leixões in Portugal to where spares for the LADCP were being airfreighted from the U.K., the LADCP not having been functioning properly. The day opened with light variable airs and a rippled sea with a low swell, skies were fine and clear with clear horizons. Between 1047B and 1104B the PES fish was recovered. At 1443B the pilot boarded in the approaches to Leixões (the seaport for Oporto), the vessel secured on berth at 1540B. The port authorities boarded in order to complete the necessary formalities and the spares were loaded. The vessel was cleared to sail by the port authorities at 1830B, letting go the berth at 1831B the vessel sailed down the harbour, the pilot disembarking at 1843B. Full away was rung at 1854B with the breakwater bearing 060°T by 0.9' and course was set to 332°T. The vessel proceeded at full speed to the parallel of 41°30'N and then altered course to 270°T. Thursday 14th August 1997: The day opened with the wind from North by West at 9 knots with a slight sea and low to moderate swell, skies were generally fine and horizons clear. At 0909B a deployment of the casing for the new LADCP casing to 2000 m was commenced in order to give it a pressure test. This was not given a Discovery station number, but it was given a scientific one (22). The PES fish was re-deployed at 0930B. At 1030B the CTD rig was brought inboard and course set to the West. Station 13223 was occupied in the evening. Friday 15th August 1997: The day opened generally cloudy with a fine haze, winds were from the North West at about 10 knots giving a rippled sea with a low swell. At 1600B all hands were exercised and trained at emergency drills, completing with a muster at boat stations. Stations 13224-13227 were occupied. Saturday 16th August 1997: The day opened with cloudy skies, the wind being from the South West by West at 11 knots giving a slight sea accompanied by a low swell. On station 13230 29 minutes were lost when the rig had to be recovered after initial deployment, a system malfunction having occurred with some of the equipment mounted on the frame. Stations 13228-30 were occupied. Sunday 17th August 1997: Light rain fell most of the night, clearing away with the dawn when skies were cloudy and clear, with the wind from West by North at 8 knots giving a rippled sea with a low swell. Stations 13231-3 were occupied. Monday 18th August 1997: A series of small depressions passing just north of the vessel caused freshening winds overnight and slowed the vessel's progress somewhat between stations. The last station on the direct East to West line, 13235, was then occupied in the morning. Stations 13234-7 were occupied. Tuesday 19th August 1997: In the early morning freshening winds blew from the South West at about force 4, with the dawn they backed into the North West bringing rain showers, seas were slight to moderate with a low swell. Stations 13238-41 were occupied. Wednesday 20th August 1997: After further overnight rain the day opened with the wind from South West by West at 18 knots giving a moderate sea with a low swell, skies were clouded with low overcast and visibility through the day was generally moderate. In the forenoon the crew was mustered and both lifeboats swung out to the embarkation deck. A safety committee meeting was held in the ship's library at 1030B. In the afternoon a slight diversion from track was made in response to a distress call relayed from Falmouth Marine Rescue Co- ordination Centre, course was altered at 1350B and resumed at 1425B after receipt of message informing us that the alarm had been accidentally triggered. Stations 13242-5 were occupied. Thursday 21st August 1997: Fog, mist and rain overnight gradually cleared away in the morning and at dawn the wind was from South West by West at 20 knots with moderate seas and a low swell, skies were cloudy and overcast with moderate visibility. Stations 13246-9 were occupied. Friday 22nd August 1997: A cool gray, cloudy, overcast dawn with light variable airs, a rippled sea and low swell presaged. Emergency fire and boat drill took place between 1615B and 1630B. Stations 13250-3 were occupied. Saturday 23rd August 1997: The weather at the start of the day gave promise of fine weather, skies being fine with some cumulus cloud, winds were variable and light at about 4 knots with a rippled sea and a low swell. Stations 13254 to 13257 were occupied. In the evening, in deteriorating weather, slow progress was made from station 13257 to 13258. Sunday 24th August 1997: At 0300B having arrived upon the next station position the decision was made to suspend operations due to winds of 40 to 45 knots which were blowing accompanied by a heavy swell. At dawn the skies were heavily clouded and overcast, winds were from North West by West at about 30 knots with moderate to rough seas and moderate to heavy swells. At 0856B with conditions moderating the vessel came about and at 1013B resumed station. Stations 13258-9 were occupied during the remainder of the day. Monday 25th August 1997: At 0200B the ship's clocks were retarded one hour to Alpha time. Light rain overnight persisted into the forenoon, the day opening with winds from the North North East at 6 knots giving a rippled sea accompanied by a low swell with cloudy overcast skies. Stations 13260-4 were occupied. Tuesday 26th August 1997: The day opened with freshening winds from North West by West at 30 knots, a moderate sea and swell were accompanied by skies that were cloudy and fine with good visibility. Stations 13265-8 were occupied. Wednesday 27th August 1997: The day opened with winds from North West by West at 20 knots, giving a moderate sea and swell, with heavy cloud and overcast with frequent passing showers. Stations 13269-73 were occupied. Thursday 28th August 1997: The day opened cloudy overcast and clear, winds were from the North West by West at 13 knots with a slight sea and a low swell. Stations 13274-7 were occupied. Friday 29th August 1997: The day opened cloudy and overcast with the wind from SSW at 13 knots, seas were slight with a low swell. At 1122A speed was reduced to about 8 knots to allow for engine room maintenance work. At 1600A emergency drill was held with various instruction classes followed by man overboard drill when at 1626A a 'dummy' (representing the Master) was dropped overside. The dummy was brought on board with the vessel in the hands of the Mate. The vessel was underway again by 1641A. Station 13281 was notable in that during the period of the work our first iceberg of the cruise was sighted seven miles distant, closer to a growler proved impressive to those unused to working in high latitudes, these sightings were about 135 miles from land and just within the maximum indicated limit for icebergs in the month of August. Stations 13278-81 were occupied. Saturday 30th August 1997: Speed in darkness was now reduced to five knots. The day opened cloudy and clear with a wind from West North West at 18 knots giving a slight to moderate sea with a low swell. At 1600A, the weather having deteriorated with winds reaching force 8 to 9 the vessel hove to and scientific work was suspended pending an amelioration of conditions. At 2330A it having become apparent that then worst was over the vessel came about and steamed towards the next station position. Stations 13282-4 had been occupied. Sunday 31st August 1997: The vessel resumed station at 0115A and work commenced at 0134A. The day opened fine and clear with winds force 3 from West by North giving a slight sea accompanied by a low to moderate swell. During the afternoon, our second significant piece of ice was passed at about two miles distant, a towering pinnacled (Arctic) giant rising to about 297 feet. Being now within the maximum iceberg limit speed in the hours of darkness was restricted to a maximum of 5 knots. Stations 13285-92 were occupied. Monday 1st September 1997: The final station of our line from Portugal to Greenland was to be 13293 just over two miles from the Greenland coast East of Cape Farewell, close to Cape Hoppe. Series 1 was reported as a failure having gone to 150 m depth at 0031A, series 2 reached a depth of 150 m at 0055A. At 0107A, the equipment having been brought inboard course was then set to carry out an under way profiling run. The day dawned fine and clear with light variable airs giving a rippled sea with a low swell. The profiling run continued throughout. Tuesday 2nd September 1997: Profiling continued until 0350A when arrived at position 58°05.5'N 040°37.44'W course was altered to 036°T. At daybreak skies were fine and clear the wind however had gathered strength and was blowing from the North at about 25 knots giving a moderate sea accompanied by a low swell, speed was increased at this time to maximum. Despite being on maximum speed however progress was relatively slow, the growth of weed and barnacles on the hull and the swell combining to reduce speed at times to seven knots. During the hours of darkness, being within the maximum iceberg limit, speed was reduced to five knots. Wednesday 3rd September 1997: Weather conditions moderated overnight and the dawn was one of those beautiful occasions with winds from the North at 12 knots, a roseate sunrise illuminating a deep blue-gray sea under clear skies and a few circling seabirds ever hopeful skimming the slight sea and low swells in our wake. Passage towards the next line of survey continued until arrival on station 13294 at 1308B. At 1823A whilst approaching the next station (13295) an experimental rain buoy was deployed astern and remained there for most of the station. Thursday 4th September 1997: The day dawned with very light airs, a glassy sea and a low swell under fine and cloudy skies. Stations 13296-13301 were occupied. Friday 5th September 1997: At 0219A on station 13302 the system went down to a depth of 293 m. It was not advisable to work closer in towards the coast due to the large number of icebergs and at 0234A course was set 118°T, then at 0300A course was set 061°T towards the start of the line of positions running across the Southern approaches to the Denmark Strait. Saturday 6th September 1997: In the forenoon, visibility was reduced and became quite thick just after noon, in consequence the afternoon was spent at reduced speed as the vessel approached the first station (13303) of the line across the southern approaches to the strait. When approaching station 13305 speed was reduced at 2044A and the rain buoy deployed at 2053A. The rain buoy deployment was concluded at 2155A. At 2350A the vessel hove to on station 13306, but due to deteriorating weather work was suspended at 2400A. Sunday 7th September 1997: At 0720A the vessel proceeded to resume station. Winch problems caused a small delay on station 13307. Stations 13306-9 were occupied. Monday 8th September 1997: After station 13310, the vessel reached the next station in deteriorating weather conditions, winds coming out of the North West at 40 to 45 knots and science was suspended. At 0712A the decision was made to abandon the station and in consequence course was set for the line of stations commencing from the Southern coast of Iceland and proceeding South along the meridian of 20°W. Tuesday 9th September 1997: At 0800A the vessel arrived just a few miles to the East of the new (1963) volcanic island of Surtsey and resumed scientific work on station 13311 at 0802A. The day had opened with much ameliorated conditions, winds were from the West at 5 knots, with a rippled sea and low swell, skies were clouded and there was a slight haze but visibility was good. Stations 13311-6 were occupied. Wednesday 10th September 1997: the vessel arrived on the next station at 0412A having made slow progress due to deteriorating weather conditions, the vessel remained hove to for a while and at 0704A station was resumed. The day opened cloudy/overcast and clear with a moderating wind blowing at about 23 knots from WNW, seas were moderate to rough and the swell was moderate. Stations 13317-20 were occupied. Thursday 11th September 1997: The day opened with fresh winds from the North West at 25 knots giving a moderate sea and swell, cloudy and fine with occasional showers. From 1414A to 1506A for station 13322, a trial deployment was made with a CTD sensor mounted inside a water bottle in parallel with the sensor mounted on the frame in attempt to check that time necessary for water within an open water bottle to match that of its surroundings, unfortunately the the trial failed. At 1600A emergency drill was carried out with various classes of instruction in fire fighting and life saving followed by a muster at boat stations. Stations 13321-5 were occupied. Friday 12th September 1997: The day opened with cold Northerly winds at 15 knots and frequent violent squalls of wind with rain showers, moderate seas and swells were running and skies were clouded and overcast. In the evening station 13331 took place at 2024A, the vessel now coming up onto the Rockall Bank. The last station of the day took place in increasingly unsettled weather with heavy swells coming down from the North. Stations 13326-32 were occupied. Saturday 13th September 1997: Having rounded Rockall, station 13333 to the South of St. Helen's reef was occupied. When the next station was attempted, problems were experienced with the wire going slack as the ship rose and fell in the heavy swells. The decision was made to press on to the next station in the hope of some amelioration in conditions occurring. The day dawned with the winds from the North West at about 35 knots, with rough seas and heavy swells, skies being cloudy and fine with good visibility. Station 13335 took place after lunch, however this station was not without incident as during recovery at 1430A there was a failure of the CTD termination. Coming up onto the Anton Dohrn Bank for station 13337. Stations 13333-7 were occupied. Sunday 14th September 1997: The day opened with the wind from West South West at 20 knots, giving a moderate sea and swell, skies were cloudy and overcast with continuous light rain. With station 13341, the vessel was coming onto the shelf just West of the Hebrides. After station 13342, the vessel remained on site whilst vertical profiling was carried out. On the final station of the cruise (13343), the system was finally landed on deck at 1752A, the PES fish was brought inboard at 1800A and sampling was completed at 1820A at which time a course of 142°T was set home in rapidly deteriorating conditions. Monday 15th September 1997: At midnight conditions had deteriorated so much that course was adjusted to 180°T in order to ease the extreme movments of the ship in the heavy South Westerly swells, at 0200A course was again adjusted to 210°T. At 0500A conditions reached their most extreme as a front approached and the vessel manoeuvred variously, once the front had passed an easing of the wind strength plus a marked veer in direction enable the vessel to come about and assume a course of 110°T, this course was maintained until 0800A when it became necessary to put a further dog's leg in our progress when course was altered to 180°T. The day opened with the wind from South West at 30 knots, seas were still rough with a moderate swell, skies were generally cloudy with good visibility. At 0912A in much eased conditions course was finally altered to 108°T directly towards the North Channel. Progress through the Irish Sea however proved slow the run of the tides proving contrary to the vessel's progress. Some slight progress towards cleaning up the scientific laboratories was made by a now thoroughly worn out scientific complement. Tuesday 16th September 1997: The day opened with the wind from right ahead being South by West at 18 knots, seas were moderate, skies were cloudy and overcast with good visibility. Tuskar rock was passed, distant 13.7 miles at 0800A. Progress was slowed again by contrary tides and the freshening wind which although it veered to the South West did not decrease until the vessel was approaching the region of Land's End late in the evening. The vessel turned Eastwards to head up channel off Wolf rock at 2148A. Wednesday 17th September 1997: The weather in the English Channel was extremely fine and favourable and good progress was made, some lost ground being recovered. The day opened fine calm and clear and remained that way with glassy sea and sunny skies. Start Point was passed at 0640A distant 4.7 miles. End of passage was rung at 1300A and the Needles fairway buoy was passed at 1308A, at 1357 we were abeam of Hurst Point. At 1429A the pilot boarded just before Hamstead Ledge, the vessel entered Empress Dock at 1623A and was alongside the berth at 1635A, finished with engines was rung at 1640A and the vessel was all secure at 1650A. Scientific staff disembarked soon after. Captain M. A. Harding For me, the highlight of the cruise was the East Greenland work. On the way in to Cape Farewell, we passed an enormous castellated iceberg with a huge hole right through the middle so you could see through to the other side. The lower parts of the berg were polished by wave action, and the interior of the hole had that shade of ice blue which you don't see anywhere else. The weather was glorious - calm, clear, often sunny by day, and starry by night. There were regular and improving auroral displays, with greens, whites and occasional reds, and lots of swirling arches and wavy curtains. The section in to Cape Farewell was completed with station 93, less than 2 miles from the extraordinary landscape of rocky pinnacles with ice and snow which is the coast of Greenland. The sun had set over the Cape as we finished, and the water was so very calm that Jupiter was reflected as a shining path. Seals were swimming near the ship. On the second approach to Greenland (not quite as close as the first) we had another remarkable sunset, with sunbeams shining through the peaks of the coast mountains to illuminate a thin layer of mist in a peachy-orange colour. Sheldon Bacon Figure 1.1: Discovery Cruise 230 track Vigo to Southampton. Bathymetry is 200m (solid), 1500m (dots), 3000m (solid). Figure 1.2: Discovery Cruise 230 station positions. Bathymetry is 200m (solid). 1500m (dots), 3000m (solid). 2. CTD MEASUREMENTS a. EQUIPMENT The equipment used during the cruise was as follows: - Neil Brown MKIIIb/c CTDs DEEP01 and DEEP02 - Chelsea Instruments Fluorometer S/N. 88/2050/95 - Chelsea Instruments Transmissometer S/N. 161/2642/003 - FSI OCM-D-112 S/N 1325-011592 - FSI OTM-D-112 S/N 1333-011592 - Simrad Altimeter 200 m range. - LADCP and battery pack - FSI Rosette Pylon No.1 - GO and FSI 10 Litre Niskin Bottles - SIS Thermometers S/N T741 and T989 - SIS Pressure Meters S/N 3192H and 3694H Both CTDs are MKIIIb instruments converted to a MKIIIc format. Deep02 was specially modified for this cruise to accept data from two FSI modules: one FSI OTM (Platinum Resistance Thermometer Module) and one FSI OCM (Conductivity Module). These mount on a specially modified 10 litre GO water bottle which has external rubbers linking the endcaps as opposed to an internal Epoxy coated spring. During this cruise 143 stations were occupied with a depth range of 130-5478 m. As the 10 mm CTD wire does not have the load capacity to reach depths in excess of 4500 m (approx), the 17 mm deep tow cable was used and linked to the CTD package with a TOBI swivel. Although the combination of swivel and shackle is quite large (1 m approx), there was sufficient clearance to allow the package to be deployed without the need to remove the ship's rail. This arrangement was used until the deeper stations had been completed after the Vigo to Greenland section and performed well. However, in heavier sea conditions above Force 6, handling became more difficult due to the closer proximity of the package to the ship's side during deployment and recovery. The sheave over which the cable runs is much further inboard than the 10 mm CTD cable. From station 103 onwards, the 10 mm CTD cable was used with a swivel fitted between the cable and package. This increased the working clearance which proved fortuitous as the majority of bad weather occurred after the cable change. CTD Stations 1-135 were occupied using instrument DEEP01. There was an initial problem with loss of the Fluorometer signal at approximately 600 m on each upcast. This proved to be a faulty lead connecting the Fluorometer to the CTD. After replacement, this gave no further problems. The CTD and other associated sensors worked without fault for the duration of the cruise. The FSI pylon performed reliably, but after some time failed to fire bottle number 23. As sufficient bottles were available, this was not changed for the spare unit. The LADCP was fitted to the package for all stations but was not without problems (see LADCP report for details). CTD DEEP02 was mounted on the frame along with the modified 10 Litre bottle carrying the FSI OTM and OCM modules. Although the FSI sensors worked both Conductivity and Oxygen sensors were unusable along with a loss of Altimeter signal. The cast was abandoned and DEEP01 reinstalled and the modified bottle removed. During the upcast of station 136 electrical contact with the CTD was lost. On recovery this was traced to a fault in the swivel which had gone electrically short circuit. The swivel was removed and DEEP02 plus bottle mounted. The Conductivity and Oxygen sensors had been replaced and the loss of Altimeter signal traced to a broken connection within the CTD. The remaining stations 136-142 were completed with this arrangement. The bottle was removed for station 143 for fear of damage in the heavy seas as the OTM and OCM protrude beyond the safety of the CTD frame. During the cruise a new software package to acquire and display CTD data was under development. Although much remains to be done to bring this to a finished product, it proved essential for the stations where CTD DEEP02 was used. Due to the non standard format of this instrument, the GO software normally used was able to log raw data but not display the multiplexed analogue channels. The most important of these for operational use is the Altimeter, necessary to avoid sea bed contact. The new software was able to handle the data format from DEEP02 and display all data channels. The level A system failed to log data on three stations but the data were recovered with the use of appropriate software designed for the purpose. John Smithers b. DATA CAPTURE CTD data were passed from the CTD Deck unit to the Level A. The level A averaged the raw 16 Hz data to data at 1 Hz. Before averaging, the data are checked for pressure jumps and median de-spiked. The gradient of temperature over the 1 second sample of data is calculated. From the Level A, data are passed to the Level B (logging) and then to Level C (archiving). Bottle firing times were logged using a separate Level A. As with previous cruises, the CTD Level A caused 'serial overruns' when accepting and processing data from the CTD deck unit. This caused a loss of data of as much as 20 seconds per cast. The problem was alleviated by removing the clock input to the Level A. The Level A did not consume processor time synchronising with the clock but was able to handle CTD data. Serial overruns were still observed but they did not lead to data loss. The internal clock on the CTD Level A is sufficiently accurate over a cast if the Level A is allowed to communicate with the clock between stations. The CTD unit DEEP01 was calibrated in the laboratory on the 11th of June 1997. A final decision on the calibration will be made after a post-cruise calibration. Attached to the CTD were a Chelsea 0.5 m transmissometer and a fluorometer. These instruments passed their data via the CTD multiplexed channels. TEMPERATURE Temperature raw counts were first scaled by (2.1) and then calibrated using (2.2): (2.1) Traw = 0.0005 x Traw (2.2) T = -1.94178E-2 + 0.998608 x Traw To correct the mismatch in the temperature and conductivity measurements temperature is 'speeded up' by (2.3) (2.3) T = T + tau(dT/dt) where the time rate of change of temperature is determined over a one second interval. After inspection of 'stairs' beneath Mediterranean water where step function changes were observed, the time constant chosen to minimise salinity spikes was tau = 0.175s. Temperatures are reported using the ITS-90 scale. ITS- 68 is used for computing derived quantities. Temperatures are converted to ITS-68 by (2.4), as suggested by Saunders (1990). (2.4) T68 = 1.00024 x T90 PRESSURE Raw pressure counts were scaled by (2.5) and then calibrated using (2.6): (2.5) Praw = 0.1 x Praw (2.6) P = -10.94 + 1.0027284 x Praw + 1.36753E-6 x P2raw -1.0313E-10 x P3raw The pressure sensor is temperature dependent: the CTD gave a larger pressure when it was colder. The correction (2.7) gave deck pressures which average to -0.0191 dbar with a standard deviation of 0.1220 dbar whilst the CTD was on the deck for temperatures varying between 3°C and 23°C, (2.7) P = P + 0.14(ptlag - 25.4) where ptlag is a lagged version of the CTD temperature, and is constructed by (2.8) and (2.9): (2.8) W = exp(-tdel/tconst) (2.9) ptlag(t0 + tdel) = W x ptlag(t0) + (1 - W) x T(t0 + tdel) where T is the CTD temperature, tdel is the time interval in seconds over which ptlag is updated with tconst = 400 s. Pressure is adjusted to compensate for hysteresis between down and up casts: the pressure hysteresis is a function of the maximum pressure of the cast: (2.10) Pout = Pin - { dp6000(Pin) - [(Pin/Pmax) x dp6000(Pmax)]} where dp6000(P) is the hysteresis and is given in Table 2.1, Pmax is the maximum pressure of the cast and Pin is the upcast CTD pressure. CONDUCTIVITY Raw conductivity was first scaled by (2.11) and then calibrated with (2.12): (2.11) Craw = 0.001 x Craw (2.12) C = 0.046595 + 0.9877211 x Craw The offset and slope were determined using bottles deeper than 2000 dbar over stations 001 to 047. Over groups of stations small offsets were added to this correction compensating for fluctuations in the CTD or in the bottle sampling. The corrections applied to the offset are listed in Table 2.2. The conductivity sensor was calibrated for the cell material deformation correction (2.13): (2.13) C = C x (1 + alpha x (T - T0) + beta x (P - P0)) where alpha = -6.5E-6 °C-1, beta = 1.5E-8 dbar-1, T0 = 15 °C and P0 = 0 dbar. CTD INSTRUMENT DEEP02 After station 135 and to the end of the cruise, station 143, CTD DEEP02 was used. DEEP02 had been modified, pre-cruise, to accept inputs from two FSI Ocean Temperature and Conductivity modules. These modules were fitted inside a Niskin bottle to investigate the effect of flow through the bottle. The intention is to investigate the relationship of water surrounding the Niskin sample bottle to that inside the bottle. DEEP02 was calibrated in a similar mannor to DEEP01. The following equations were applied (Calibrations from Oct. 1994), (2.14) T = -2.8434E-3 + 1.0067956 x Traw + 7.287E-6 x T2raw (2.15) tau = 0.2 (2.16) P = 3.42 + 1.002348 x Praw - 3.9467E-6 x P2raw (2.17) P = P + 0.28(ptlag - 41.86) with R2 = 0.97 for n = 7/8 points for 10.6°C < ptlag < 13.5°C (2.18) C = 9.08698E-3 + 1.02002066 x Craw All offsets, lagged temperatures and conductivity cell model were applied as outlined above for DEEP01. The pressure hysteresis data used are given in Table 2.3. SALINITY After the conductivity calibration, salinity residuals (bottle salinity - CTD salinity) showed a depth dependence. This dependence looks like a temperature effect in the upper 500 m of the water column and a pressure effect below. The shape of the residuals over the station groupings was modelled using pressure and temperature, (2.19) dsalin = a + bP + cT where dsalin is the correction to salinity. This correction was then added to the CTD salinity. Table 2.4 lists the coefficients determined. Salinity residual statistics are given in Table 2.5. Post-cruise an intercomparison of standard sea water used during the cruise revealed that the standard sea water used for stations 001 to 044 lead to salinity samples being 0.0015 fresh. Therefore, 0.0015 has been added to CTD salinities for these stations. Full details may be found in section 7.f. ANALYSIS OF BOTTLE SALINITIES IN THE EASTERN NORTH ATLANTIC Stations 020 to 034 were taken at a latitude of 41.5°N between 12°W and 20°W, within the Eastern North Atlantic (ENA). These 15 stations had 73 bottle salinity samples taken at potential temperatures colder than 2.5°C. Saunders (1985) first proposed that the deep basin of the ENA may be used as an oceanic calibration facility given that systematic measurement errors between instruments (and standard sea water) are bigger than the in situ variations of temperature and salinity. Saunders proposed that between 15-30°W and 20-46°N the relationship between potential temperature and salinity could be accurately described by the linear fit (2.20): (2.20) S = 34.698 + 0.098 x theta Our bottle salinity samples are 0.0044 fresher with a standard deviation of 0.0008 than this. Later Mantyla (1994) used two cruises which covered the ENA to propose refinements to this line accounting for latitudinal variations. At 41.5°N the relationship given by Mantyla is (2.21): (2.21) S = 34.9163 + 0.1000075 x (theta - 2.25) For this cruise we have (2.22): (2.22) S = 34.9143 + 0.100304 x (theta - 2.25) with R2 = 0.9954. Therefore at 2.25°C our data are 0.002 fresher than the Mantyla data. For 73 samples spanning 2.0 to 2.5°C the mean difference is 0.0021 fresher with a standard deviation of 0.0008. At 2.25°C the salinity predicted by the Saunders relation is 34.9185. The S/theta gradient varies by about 0.015 psu/°C between 20°N and 50°N (the S/theta gradient is about 0.1 psu/°C at 41.5°N). The difference in S/theta gradient in the two equations above is much smaller than any latitudinal variation. We therefore conclude that the variation between our data and that of Mantyla is due to variations in standard sea water and does not suggest any environmental difference. Due to a standard sea water problem, post-cruise it was found that stations 001 to 044 were 0.0015 fresh. This value has subsequently been added to our data making our data 0.0005 fresher than Mantyla, 1994. OXYGEN The oxygen model of Owens and Millard (1985) was used to calibrate the oxygens (2.23): (2.23) O2 = rho x oxysat(S,T) x (Oc - chi) x exp {alpha x [f x TCTD + (1 - f) x Tlag] + beta x P} where rho is the slope, oxysat(S,T) is the oxygen saturation value after Weiss (1970), Oc is oxygen current, chi is the oxygen current bias, alpha is the temperature correction, f is the weighting of TCTD the CTD temperature and a lagged temperature Tlag computed exactly as the pressure temperature lag earlier, and beta is the pressure correction. Five parameters, rho, alpha, beta, f, chi were fitted for each station. This approach minimises the residual bottle oxygen minus CTD oxygen differences but places complete reliance on the bottle oxygens being correct. Oxygens were calculated in µmol/l. DEEP02, stations 136 to 143 have no CTD oxygen data. Table 2.6 gives the parameters and the sum square residual for each station. TRANSMITTENCE, FLUORESCENCE, AND ALTIMETRY On DEEP01, Fluorescence was converted to voltages (2.24); this is a calibration of the voltage digitiser in the CTD. Transmittance was similarly calibrated to voltages (2.25). The altimeter had the calibration (2.26) applied. (2.24) fvolts = -1.7196E-3 + 1.21971E-3 x fraw + 3.48596E-10 x f2raw (2.25) trvolts = -1.7196E-3 + 1.21971E-3 x trraw + 3.48596E-10 x tr2raw (2.26) alt = 0.2 + 5.148E-2 x altraw - 5.8E-8 x alt2raw On DEEP02, fluorescence (2.27), transmittance (2.28) and altimeter (2.29) calibrations were: (2.27) fvolts = -3.44E-4 + 1.21971E-3 x fraw - 2.813E-11 x f2raw (2.28) trvolts = -3.44E-4 + 1.21971E-3 x trraw - 2.813E-11 x tr2raw (2.29) alt = 4.73E-2 + 5.41E-2 x altraw - 1.9E-8 x alt2raw DIGITAL REVERSING TEMPERATURE AND PRESSURE METERS Two digital reversing temperature meters (RTM) were used, T746 and T989 and two reversing pressure meters (RPM) P6132H and P6394H. T746 and P6394H were at position one on the CTD rosette, T989 and P6132H were at position four. T746 failed on station 054 due to low battery power. No spare batteries were available so the thermometer could not be used throughout the remainder of the cruise. After station 054 T989 was placed in position one on the rosette. P6132 was calibrated by (2.30): (2.30) Pcal = -6.7 + 1.02 x Praw - 3.3E-6 x P2raw This calibration was obtained from the first 92 stations where it was observed that residuals from this instrument had a quadratic shape with depth. The other instruments have had no calibrations applied. Table 2.7 summarises data from the reversing instruments. Throughout the cruise no trends or offsets were identified in pressure or temperature. There were insufficient data to determine if there are any biases between CTD DEEP01 and DEEP02. Post-cruise calibration of pressure and temperature sensors will be our method for identifying calibration shifts. Stuart Cunningham and Mickey Tsimplis c. POST CRUISE CALIBRATION DEEP01 DEEP01 was used on the first 135 out of 143 stations. The post cruise calibration showed that the pressure sensor was stable and was continuing to give the same pressure response. Therefore, nothing was done to pressure. The post cruise temperature calibration revealed that the temperature sensor was under-reading by 0.0040°C. Too few reversing temperature measurements were made to reveal if temperature offsets occurred during the cruise. Therefore, a linear difference in time between the pre and post cruise calibrations suggests that 0.0028°C should be added to the temperatures recorded during the cruise. A calibration of +0.0028°C has been added to stations 001 - 135. All variables dependant on temperature have been recalculated. DEEP02 DEEP02 was used on stations 136 - 143 in the Rockall Trough. The post cruise calibrations were sufficiently different from the pre cruise calibrations that the post cruise calibrations were applied to the raw data, ignoring the pre cruise calibrations. The following equations were applied (Calibrations from Nov. 1997). TEMPERATURE Temperature raw counts were first scaled by (2.1) and then calibrated using (2.31): (2.31) T = 6.0194E-4 + 1.00702 x Traw Equations and values (2.3) and (2.15) still apply. Temperatures are reported using the ITS-90 scale. PRESSURE Raw pressure counts were scaled by (2.5) and then calibrated using (2.32). Equation (2.17) is replaced by (2.33). The pressure hysteresis data of table 2.3 are replaced by those of table 2.8. (2.32) P = -2.8 + 0.9928896 x Praw - 1.33E-6 x P2raw + 2.015E-10 x P3raw (2.33) P = P + 0.28 (ptlag - 19.36) CONDUCTIVITY Raw conductivity was first scaled by (2.11) and then calibrated with (2.34). The cell deformation correction (2.13) was unchanged. (2.34) C = 0.0176 + 0.97959165 x Craw SALINITY Following the conductivity calibration salinity residuals were examined for pressure and station dependance. There are few stations and no measurements deeper than 2500 dbar, so no residal shape was identified. The residual statistics are appended to table 2.5. DIGITAL REVERSING TEMPERATURE AND PRESSURE METERS Comparisons between CTD DEEP02 and reversing instruments are given in table 2.9. All relevant information is as reported above for these instruments. Neither the means or variances were different from those obtained when using DEEP01. Stuart Cunningham REFERENCES Mantyla, A. W., 1994: The treatment of inconsistencies in Atlantic deep water salinity data. Deep-Sea Res., 41 1387-1405. Owens, W. B., and R. C. Millard, 1985: A new algorithm for CTD oxygen calibration. J. Phys. Oceanogr., 15 621-631. Saunders, P. M., 1986: The accuracy of measurement of salinity, oxygen and temperature in the deep ocean. J. Phys. Oceanogr., 16, 189-195. Saunders, P. M. (1990) The International Temperature Scale 1990, ITS-90. International WOCE Newsletter No. 10, p 10. Weiss, R. F., 1970: The solubility of nitrogen, oxygen and argon in water and seawater. Deep-Sea Res. 17 721-735. TABLE 2.1: Laboratory measurements of pressure hysteresis for DEEP01 made on 31/10/94 at 9.44°C. Intermediate values of pressure hysteresis are found by linear interpolation. P dP6000(P) dbar dbar 0 0.0 400 3.9 1000 6.0 1500 5.9 2000 4.8 3000 2.0 3500 1.0 5000 0.0 6000 0.0 TABLE 2.2: Corrections to the conductivity offset. Station Correction Notes numbers mmho/cm 001 - 035 -0.0024 westward leg to turning stn. 036 -0.0047 037 - 039 -0.0026 040 - 049 -0.0010 050 - 069 0.0000 070 - 089 0.0028 090 - 093 0.0118 fresh and shallow stations 094 - 097 0.0028 098 - 102 0.0118 fresh and shallow stations 103 -0.001 104 - 110 bts-us=-5.45e-8xstatno+6.722e-2 104 - 110 -0.007 111 - 129 0.0052 130 0.0012 131 - 135 0.0073 135 last station with DEEP01 136 - 140 0.0000 136 first station with DEEP02 141 - 143 -0.0031 143 end of cruise TABLE 2.3: Laboratory measurements of pressure hysteresis for DEEP02. Intermediate values of pressure hysteresis are found by linear interpolation. P dP5500(P) dbar dbar 0 0.0 100 0.9 200 1.6 300 2.1 400 2.3 500 1.9 1000 4.3 1500 4.6 2000 4.0 2500 3.7 3000 2.7 3500 2.1 4000 1.5 4500 0.9 5500 0.0 TABLE 2.4: Salinity correction coefficients Stations a b c Notes 001 - 035 0.838 -0.000017 -0.460610 westward leg 036 - 049 0.800 -0.000191 -0.419474 050 - 069 1.782 -0.000113 -0.508926 070 - 089 1.875 -0.000234 -0.616385 090 - 093 1.947 -0.000594 -0.404307 094 - 097 1.875 -0.000234 -0.616385 098 - 102 1.947 -0.000594 -0.404307 103 - 135 2.360 -0.000254 -0.453748 135 last DEEP01 136 15.079 -0.003484 -0.871264 first DEEP02 137 -1.927 -0.004599 0.480528 138 -3.178 0.003279 0.511432 139 7.312 -0.001061 -0.200623 140 11.539 -0.002944 -0.578537 141 - 143 0 0 0 end of section TABLE 2.5: Salinity residual statistics Stations all p all p all p p>2000 p>2000 p>2000 mean stdev n mean stdev n 001 - 035 0.0000 0.0012 650/688 0.0000 0.0009 152/163 036 - 049 -0.0001 0.0018 317/321 0.0000 0.0007 51/52 050 - 069 -0.0001 0.0011 409/451 0.0000 0.0008 74/77 070 - 089 0.0000 0.0015 507/534 -0.0002 0.0010 60/63 094 - 097 0.0000 0.0015 507/534 -0.0002 0.0010 60/63 090 - 093 -0.0002 0.0016 82/111 098 - 102 -0.0002 0.0016 82/111 070 - 102 0.0000 0.0012 557/645 104 - 110 0.0000 0.0009 111 - 129 p>1500 0.0000 0.0007 51/55 130 - 135 0.0000 0.0011 001 - 135 0.0000 0.0018 2554/2659 0.0000 0.0009 366/385 136 - 143 0.0000 0.0016 110/112 <-- post cruise 001 - 143 0.0000 0.0011 2483/2781 0.0000 0.0009 370/389 TABLE 2.6: Oxygen coefficients and sum square residuals num rho alpha beta f chi n sumsq µmol/l 001 2.2825 -0.0270 0.0001684 0.2603 -0.033 22 0.71 002 2.2628 -0.0263 0.0001957 0.2653 -0.055 19 3.04 003 2.4315 -0.0241 0.0007435 0.0000 -0.111 6 1.11 004 2.2825 -0.0270 0.0001684 0.2603 -0.033 22 0.71 005 2.2844 -0.0279 0.0000638 0.2358 -0.005 13 1.40 006 2.6819 -0.0270 0.0003048 0.7497 -0.126 15 9.18 007 2.4749 -0.0336 0.0001282 0.5787 -0.033 20 1.59 008 2.4324 -0.0300 0.0001735 0.5399 -0.060 17 1.86 009 2.2763 -0.0221 0.0002494 0.2377 -0.083 15 1.79 010 2.3307 -0.0265 0.0001970 0.3299 -0.058 20 2.40 011 2.3818 -0.0270 0.0002109 0.2517 -0.067 19 2.03 012 2.3456 -0.0236 0.0002328 0.0078 -0.075 19 1.65 013 2.2872 -0.0250 0.0002059 0.2344 -0.056 20 2.22 014 2.3238 -0.0273 0.0001851 0.2970 -0.044 21 2.75 015 2.2988 -0.0257 0.0001956 0.2651 -0.047 21 3.38 016 2.3099 -0.0271 0.0001919 0.3056 -0.045 20 1.66 017 2.3219 -0.0252 0.0002421 0.2369 -0.073 18 2.84 018 2.3162 -0.0249 0.0002150 0.1653 -0.058 15 2.01 019 2.3629 -0.0285 0.0001726 0.2765 -0.033 21 1.39 020 2.4225 -0.0287 0.0001792 0.3771 -0.041 21 1.49 021 2.4291 -0.0296 0.0001732 0.3683 -0.037 20 1.21 022 2.3886 -0.0286 0.0001741 0.2549 -0.033 20 1.61 023 2.3886 -0.0286 0.0001741 0.2549 -0.033 20 1.61 024 2.3886 -0.0286 0.0001741 0.2549 -0.033 20 1.61 025 2.4124 -0.0249 0.0002266 0.2634 -0.067 22 1.43 026 2.4270 -0.0283 0.0001877 0.2922 -0.045 22 1.35 027 2.3836 -0.0288 0.0001754 0.2180 -0.033 21 1.59 028 2.4391 -0.0283 0.0001899 0.1251 -0.046 18 1.25 029 2.3690 -0.0307 0.0001493 0.3272 -0.012 19 1.63 030 2.3380 -0.0272 0.0001888 0.2055 -0.038 18 1.29 031 2.3542 -0.0293 0.0001407 0.1468 -0.005 19 1.25 032 2.4067 -0.0290 0.0001747 0.2487 -0.035 18 1.72 033 2.4206 -0.0301 0.0001696 0.3481 -0.030 19 0.98 034 2.4523 -0.0281 0.0001866 0.1682 -0.043 20 1.02 035 2.2878 -0.0269 0.0001771 0.2669 -0.019 19 1.49 036 2.3696 -0.0286 0.0001691 0.2794 -0.025 19 1.15 037 2.6065 -0.0326 0.0001459 0.1831 -0.026 18 0.99 038 2.2542 -0.0272 0.0001688 0.2040 -0.008 18 1.70 039 2.2542 -0.0272 0.0001688 0.2040 -0.008 18 1.70 040 2.3552 -0.0302 0.0001615 0.3183 -0.013 18 1.73 041 2.3939 -0.0310 0.0001483 0.2621 -0.011 19 1.88 042 2.3557 -0.0307 0.0001372 0.2276 -0.001 19 1.63 043 2.3362 -0.0295 0.0001350 0.2239 0.004 15 1.83 044 2.2448 -0.0295 0.0001289 0.2208 0.018 18 2.34 045 2.3659 -0.0306 0.0001301 0.1685 0.006 22 1.31 046 2.3208 -0.0294 0.0001391 0.3293 0.006 20 1.53 047 2.2890 -0.0305 0.0001183 0.2582 0.026 20 1.59 048 2.4680 -0.0360 0.0000775 0.0842 0.042 18 1.19 049 2.3151 -0.0315 0.0001177 0.1381 0.024 20 1.64 050 2.3796 -0.0335 0.0001086 0.1631 0.027 18 1.83 051 2.3648 -0.0320 0.0001186 0.1692 0.018 19 2.03 052 2.3658 -0.0317 0.0001164 0.1449 0.023 19 2.03 053 2.4367 -0.0347 0.0000997 0.0315 0.030 21 1.50 054 2.3321 -0.0353 0.0000864 0.1329 0.051 17 1.48 055 2.3887 -0.0336 0.0001113 0.0737 0.019 22 1.75 056 2.2867 -0.0324 0.0001171 0.2662 0.029 18 1.74 057 2.4469 -0.0330 0.0001462 0.3965 -0.013 17 1.19 058 2.2070 -0.0323 0.0001179 0.3879 0.037 20 1.11 059 2.3013 -0.0325 0.0001095 0.1302 0.032 21 1.61 060 2.3664 -0.0316 0.0001234 0.2593 0.010 19 1.59 061 2.3066 -0.0303 0.0001216 0.1403 0.019 20 0.80 062 2.2269 -0.0295 0.0001201 0.2851 0.029 18 1.12 063 2.2864 -0.0326 0.0001183 0.2685 0.027 17 1.04 064 2.2957 -0.0321 0.0001178 0.1529 0.027 17 1.20 065 1.9973 -0.0291 0.0000906 0.1577 0.092 18 1.59 066 2.2988 -0.0317 0.0001160 0.1189 0.025 15 1.22 067 2.2861 -0.0331 0.0001183 0.2724 0.024 18 1.26 068 2.2786 -0.0305 0.0001307 0.3364 0.014 20 1.03 069 2.2814 -0.0305 0.0001131 0.1592 0.027 20 1.19 070 2.3036 -0.0295 0.0001210 0.1306 0.018 18 0.48 071 2.2927 -0.0321 0.0001226 0.2229 0.021 18 0.72 072 2.2868 -0.0327 0.0001166 0.2158 0.027 18 1.03 073 2.2770 -0.0314 0.0001248 0.2770 0.022 17 1.57 074 2.2802 -0.0309 0.0001297 0.2710 0.019 21 0.94 075 2.2844 -0.0327 0.0001218 0.2533 0.025 22 1.17 076 2.1222 -0.0308 0.0001008 0.0920 0.062 21 1.58 077 2.1222 -0.0308 0.0001008 0.0920 0.062 21 1.58 078 2.1828 -0.0308 0.0001058 0.0211 0.048 19 1.12 079 2.2759 -0.0296 0.0001229 0.2951 0.019 19 1.08 080 2.2847 -0.0340 0.0001149 0.2511 0.029 19 1.36 081 2.3079 -0.0297 0.0001288 0.1159 0.009 17 1.19 082 2.0535 -0.0313 0.0000974 0.1836 0.075 20 1.19 083 2.3156 -0.0325 0.0001301 0.1584 0.012 19 1.50 084 2.2733 -0.0313 0.0001157 0.1963 0.026 15 1.54 085 2.3008 -0.0328 0.0001172 0.2376 0.023 21 1.12 086 2.3008 -0.0328 0.0001172 0.2376 0.023 21 1.12 087 2.2812 -0.0401 0.0000892 0.0706 0.052 22 1.40 088 2.2657 -0.0414 0.0000997 0.2733 0.051 17 1.03 089 2.2158 -0.0439 0.0000639 0.3214 0.082 15 1.25 090 2.2440 -0.0395 0.0000822 0.3276 0.047 11 0.88 091 2.2968 -0.0262 0.0001929 0.2442 -0.009 9 0.86 092 2.4012 0.0506 0.0012555 0.5984 -0.208 4 0.97 093 2.1295 -0.0395 0.0001195 0.0000 0.070 5 2.15 094 2.2812 -0.0342 0.0001146 0.2662 0.029 20 1.16 095 2.2994 -0.0332 0.0001078 0.1666 0.033 096 2.2994 -0.0332 0.0001078 0.1666 0.033 17 1.13 097 2.2815 -0.0289 0.0001296 0.0321 0.018 18 1.83 098 2.2734 -0.0365 0.0001079 0.2735 0.039 16 1.24 099 2.2884 -0.0256 0.0001662 0.2640 -0.008 17 1.42 100 2.2862 -0.0265 0.0001815 0.2577 -0.006 13 1.24 101 2.3244 -0.0221 0.0002181 0.1490 -0.028 10 1.51 102 2.2439 -0.0359 0.0001770 0.2856 0.024 6 0.58 103 2.3530 -0.0595 0.0000412 0.0000 0.069 9 1.38 104 2.2769 -0.0403 0.0000218 0.0000 0.075 8 1.67 105 2.3269 -0.0125 0.0002820 0.1692 -0.076 14 1.34 106 2.2973 -0.0268 0.0001603 0.2879 -0.006 19 1.81 107 2.2587 -0.0344 0.0001165 0.3550 0.033 22 1.18 108 2.2881 -0.0287 0.0001474 0.2767 0.005 14 1.75 109 2.2819 -0.0281 0.0001598 0.2716 0.001 14 1.32 110 2.2290 -0.0352 0.0001361 0.1328 0.040 11 1.50 111 2.2798 -0.0731 -0.0005178 0.1834 0.351 6 1.22 112 2.2412 -0.0433 0.0000336 0.0232 0.112 9 1.12 113 2.2748 -0.0262 0.0001944 0.1882 -0.012 12 1.27 114 2.3349 -0.0431 0.0000342 0.0000 0.096 13 1.43 115 2.3349 -0.0431 0.0000342 0.0000 0.096 13 1.43 116 2.2806 -0.0303 0.0001356 0.2869 0.014 18 1.38 117 2.2795 -0.0297 0.0001416 0.2619 0.011 18 1.46 118 2.3396 -0.0333 0.0001178 0.1466 0.022 17 1.36 119 2.2927 -0.0277 0.0001506 0.2554 -0.001 18 1.67 120 2.4523 -0.0335 0.0001445 0.5229 -0.014 20 1.21 121 2.3220 -0.0323 0.0001284 0.2501 0.016 20 1.22 122 2.3252 -0.0343 0.0001170 0.3029 0.023 20 1.42 123 2.2708 -0.0292 0.0001507 0.2537 -0.004 14 2.37 124 2.2764 -0.0316 0.0001295 0.2799 0.020 15 1.55 125 2.2765 -0.0311 0.0001509 0.3369 0.008 15 0.98 126 2.3183 -0.0267 0.0001909 0.3770 -0.020 11 1.42 127 2.2652 -0.0256 0.0001868 0.1504 -0.013 9 1.20 128 2.5701 -0.0293 0.0002347 0.4126 -0.064 10 0.93 129 2.3546 -0.0366 0.0000932 0.4402 0.032 11 1.36 130 2.3133 -0.0593 -0.0002698 0.0000 0.310 8 3.40 131 2.2930 -0.0329 0.0001654 0.3108 0.019 5 0.86 132 2.2733 -0.0498 0.0001655 0.1263 0.104 5 0.12 133 2.2682 -0.0532 0.0000126 0.1704 0.139 4 0.24 134 2.2600 -0.0357 0.0001008 0.4163 0.044 9 1.21 135 2.2826 -0.0277 0.0001842 0.2604 -0.019 10 0.79 TABLE 2.7: Upcast CTD value minus reversing instrument. Temperature differences have been calculated within ±0.5°C then ±2 sd, pressure differences have been taken within ±25 dbar then ±2 sd. n points is the number of points in the mean over the total number of data points. all P mean sd n points T989 °C 0.0139 0.0088 104/122 T746 °C 0.0280 0.0098 47/52 P6394 dbar 1.2 1.8 116/130 P6132 dbar 0.1 1.0 101/119 P>2000 dbar mean sd n points T989 °C 0.0149 0.0041 46/58 T746 °C 0.0277 0.010 45/50 P6394 dbar 0.86 1.6 77/97 P6132 dbar -0.24 1.1 27/40 TABLE 2.8: Post-cruise laboratory measurements of pressure hysteresis for DEEP02. Intermediate values of pressure hysteresis are found by linear interpolation. P dP5500(P) dbar dbar 0 0.0 100 0.9 200 1.6 300 2.1 400 2.3 500 1.9 1000 4.3 1500 4.6 2000 4.0 2500 3.7 3000 2.7 3500 2.1 4000 1.5 4500 0.9 5500 0.0 TABLE 2.9: Upcast CTD value minus reversing instrument. Temperature differences have been calculated within ± 0.5°C then ± 2sd, pressure differences have been taken within ± 25 dbar then ± 2 sd. n points is the number of points in the mean over the total number of data points. For post- cruise DEEP02 calibration. all P mean sd n points T989 °C 0.0033 0.0005 6/7 T746 °C 0 P6394 dbar 2.5 3 6/7 P6132 dbar -0.5 1.7 6/7 3. LOWERED ADCP MEASUREMENTS a. DESCRIPTION The LADCP package consisted of a RDI 150kHz BroadBand ADCP (phase III) with a pressure case rated to 6000 meters and 4 downward facing transducers with 20 degree beam angles. The LADCP was fitted centrally in the CTD rosette frame and power during casts was supplied from a second pressure case containing a 48 volt alkaline battery pack mounted horizontally near the bottom of the frame. The battery pack pressure case was aligned so that batteries could be replaced by simply removing the end cap of the case. A short lead was left permanently attached to the unit and tied to the frame to enable the external power/communications lead to be attached pre- and post-deployment. A 15 m communications/power lead was routed through the Bottle Annex and Deck Lab and connected to a dedicated PC and power supply situated on the port side of the Deck Lab. While attached to the instrument the cable was taped to the CTD frame and draped over a deck light bracket to keep it clear of the boots and heads of samplers. During casts the free end of the cable was fed back through the opening into the Bottle Annex to keep it dry. For most of the cruise the CTD package was kept permanently at the aft end of its deck tracks, so it was necessary to extend the 15m cable with a spare short lead to enable connection. During set-up prior to a cast and for downloading data, external power was supplied to the unit via the communications lead. There is sufficient diode protection inside the ADCP for the battery supply to be overriden by applying external power at a slightly higher voltage (i.e. 50 volts, with the battery packs used on this installation). Prior to each cast the instrument was subjected to a series of tests and sent a configuration (command) file which determines the mode of operation. The test results and deployment files were recorded for each cast. With some exceptions, the instrument was set to Water and Bottom Tracking Mode with 16 m bins and 10-bin ensembles for the whole cruise. The LADCP clock was checked and corrected if it was found to be more than 1 second from GMT. The power and communication cable was disconnected and a blank plug placed on the lead to the instrument and taped to the frame for security. A total of 16 battery packs were available for the cruise, and 8 eventually used. The 48 volt alkaline battery packs (32 x 1.5 volts) were replaced when the power level dropped to around 34-35 volts to avoid potential data loss. The life of the battery packs ranged from 37,000m to 110,000m of casts, with an average of 91,000m per pack. The packs which had a shorter life were those used in regions with many shallower casts, ie a relatively high time spent within Bottom Tracking range. One pack was used solely in water tracking mode and was found to use approximately the same amount of battery power as the casts set to Bottom Tracking Mode over deeper casts (>1500m). b. INSTRUMENT PROBLEMS During the early part of the cruise performance of the LACDP was not wholly satisfactory. Data appeared to be logged correctly during the downcast but not for the whole of the upcast. Also, the power/communications cable was inadvertently disconnected whilst data was being downloaded. Further communications with the instrument failed after this. The pressure case was opened and the fault traced to 2 communication line fuses which had blown. These were of a wired in type of which no spares we carried. Fortunately, 2 were available from unused channels which were removed and resoldered in the appropriate lines. Communications were then re-established. The original problem with data loss continued however, and lack of any spare boards or parts resulted in a port call to Leixões (Oporto). Spare boards, transducer assembly and a complete electronics chassis were flown to Oporto. Rather than attempt to identify the exact cause of the problem it was decided to replace the transducer assembly and electronics chassis in total. The power conditioning circuit board from the original instrument was retained as this included the appropriate connections to the endcap. Before fitting the electronics chassis a cast to 2000 metres was performed with just the pressure case and transducer assembly to test the water tight integrity. This proved successful and the instrument was finally assembled. The complete set of tests available from the software suite was run and the instrument passed all satisfactorily. Data logging for both up and down casts was now satisfactory and it was also noted that the Ping from the transducers could now be heard or at least sensed which was not the case with the original instrument. c. DATA PROCESSING This section briefly outlines the method of LADCP current calculation and the data processing path taken on D230. In essence the LADCP measures instantaneous relative velocities of scatterers in the water column and these can be converted into profiles of absolute currents by an elaborate processing path. The scatterer velocities are measured by utilising the Doppler frequency shift, phase changes and correlation between coded pulses transmitted and received by the four tranducers. The raw data must be scaled to velocity units by taking into account the depth and temperature dependent sound velocity (from CTD data). The directions can be inferred from trigonometric calculations based on the geometry of the transducer set, the orientation of the package (measured with a flux gate compass) and the local magnetic variation from true north. The depth of the instrument was first calculated by integrating the measured vertical velocity and later fine-tuned by matching to the CTD time and pressure data. Velocity data are collected in 16m bins and each ping produces 10 bins which make up an ensemble. In order to remove relative velocites introduced by the motion of the package during the cast, shear profiles were calculated by differentiating the velocities within each ensemble. The data are then integrated up over the cast to produce a shear profile with a zero net velocity. This process also removes the barotropic component of the velocities which must be reinstated either from the ships displacement (from differential GPS data) or from the relative motion of the package over the sea floor (Bottom Tracking). The final velocity profile is therefore a sum of the baroclinic and barotropic components. The processing of LADCP data is achieved using software developed by Eric Firing at the University of Hawaii. His software uses a combination of 'perl' scripts and MATLAB 'm' files to process the data, which are stored in a CODAS database. The processing stages were those outlined in the SOC LADCP Data Processing Manual written by N.Crisp, L. Beal and R. Tokmakian and are summarised below. 1) Extract binary ADCP files from instrument, and scan it using 'scanbb' to give useful information about the cast - e.g. time in water, out of water, number of good ensembles, and the cast depth as an estimate of integrated vertical velocity. 2) Load the ADCP data into a CODAS database, including magnetic variation (information provided by the Bridge Officer at each station), nominal cast position, and only including good ensembles specified by the previous 'scanbb' step. Raw data files (*.000), database files (*.blk), configuration and control files (*.cmd, *.cnt, *.def), scan files (*.scn), and deployment log and test files (*.txt, *.log) were copied to a location on the SUN using PC_NFS. All these files and their original file structure were periodically compressed and archived to an EO before some were removed from the SUN to conserve space. The necessary database and control files remained in individual cast directories on the SUN and were backed- up daily along with all cruise data. 3) The database files from the PC were converted to SUN format CODAS files using the program 'mkblkdir'. 4) The perl script 'domerge' calculated mean shear profiles (the baroclinic component of the current) and applied corrections and editing options which were kept constant throughout the cruise. Data were averaged into 5m bins. The MATLAB script 'do_abs' calculated absolute velocities, and produced a standard set of profiles. In the first instance the uncorrected data (down, up and mean profiles) were viewed and plotted as unreferenced shear profiles with the depth-averaged component set to zero. 5) Next the calibrated CTD data were interactively matched to the ADCP vertical velocities within MATLAB (set_***.m), and the true depth information merged into the CODAS database for the cast, together with sound speed data corrected for temperature and salinity (add_ctd then re- run domerge and do_abs). 6) One method of obtaining absolute currents is to use GPS data during the cast to restore the depth-averaged (barotropic) velocity component (equivalent to the ship's displacement) which was removed when first calculating the shear. For all but 7 days of the cruise the GPS GLONASS 1- second data stream was used for calculating the ships displacement. For days ??? to ??? the GPS 4000 1-second data stream was used. The GPS data were subjected to a 5-second filter (pfiltr; 30 seconds for GPS 4000) and subsampled every 5 (30) data points to create a global GPS file (ascii file sm.asc, saved as Matlab file sm.mat). The GPS files were created every few days rather than at the end of the cruise, so in total 10 sequential sm.mat files were generated. Some GPS files caused 'non- monotonic' errors during do_abs so a short Matlab script was written to identify and remove any rogue non-monotonic rows in the sm.mat files. The absolute profiles calculated by do_abs were plotted and saved to file. 7) Each cast was rotated through the heading of the cruise track to calculate current components normal to the cruise track for comparison with geostrophic shear profiles from the CTD data (rotate.m). This stage also creates an ascii file containing the mean vertical, east, north, along-track and across-track components. The ascii file was imported into pstar (pascin) with position data extracted from the headers of the corrected CTD files (pinq) rather than the uncorrected positions found in the file latlon.asc created by domerge (ladcp.exec). d. BOTTOM TRACKING DATA On Stations 42 to 143 the LADCP was set to Bottom Tracking (BT) mode in order to obtain good measurements of the bottom currents particularly in the deep western boundary currents. In this mode the LADCP produces alternating pings for water tracking and Bottom Tracking when in range of the sea floor. The Firing software does not deal with Bottom Tracking data, so new pstar scripts were written to process the data. Following is a brief outline of the steps taken during the processing: 1) Ascii data are extracted from the binary BBADCP files (using conversion file D230BT.FMT and program 'bblist'). Variables extracted include ensemble number, time (Julian Day), BT velocities (east/u, north/v, vertical/w, error), water track velocities (east/u, north/v, vertical/w, error) and the BT range from the bottom from Beams 1/2 and 3/4. 2) Ascii data read are into pstar (pascin), absent data changed from 9999 to -999 (pedita), and time converted into days of the year (JDay plus 1) then seconds (ptime). User inputs the LADCP clock lag in seconds as necessary. 3) Currents are rotated by the local magnetic variation (pcmca2, pcalib) and the water velocity over ground calculated for each bin (water track velocities minus bottom track velocities) (parith). This step removes the package motion from the velocities. 4) The data are merged with CTD data on time (pmerg2) in order to match pressure (depth) data with the velocity profiles. No correction for velocity of sound variation has been made. Each 1 second ensemble then has an associated depth, and next the incremental depth of each bin is calculated according to the binsize specified in the LADCP configuration file. 5) Finally the data are averaged into depth bins (pbins) specified by the user (eg 16m) and can be compared to the absolute velocity profiles derived from the GPS data. e. COMPARISON OF SHIPBOARD AND LOWERED ADCP MEASUREMENTS INTRODUCTION AND SUMMARY In order to estimate the accuracy of full-depth profiles of horizontal velocity obtained from the Lowered Acoustic Doppler Current Profiler (LADCP), we undertook a number of comparisons of the LADCP measurements with shipboard ADCP measurements in the upper water column and with bottom-tracking estimates of the flow in the bottom 200 m of water column during Discovery cruise 230. For comparisons in the upper water column at 31 stations, ADCP and LADCP velocity profiles exhibit generally similar structure so the difference at each station is estimated as an offset plus a standard deviation about the offset between ADCP and LADCP east or north velocities. The average absolute offset in east or north velocities is approximately 2.4 cm s-1, while the average standard deviation for the vertical variability in structures is approximately 1.7 cm s- 1. For the bottom layer comparisons, the vertical structures of the LADCP and Bottom Track velocity profiles are very similar, as might be expected since the baroclinic structure is measured by the same instrument. More importantly, the Bottom Track currents show reasonable agreement with the absolute currents near the bottom as estimated by combining baroclinic LADCP velocity profiles with ship movement during station. For four stations in the deep western boundary current off Greenland where the bottom currents exceed 10 cm s-1, the absolute velocity components for the two methods agreed within about 2 cm s-1. Such agreement should help put estimates of the transport of the deep boundary current on a firmer foundation. As discussed above, the LADCP measurements are initially processed as shear profiles and then vertically integrated to provide baroclinic profiles of east and north velocities. Baroclinic profiles are here defined to have zero depth- averaged values. The depth-averaged velocity components are then obtained by combining the movement of the ship during station as determined from GPS navigation with the time series of absolute velocities measured by the LADCP. A natural question arises as to the accuracy of the resulting absolute LADCP velocity profiles. We have tried to answer this question of accuracy in two ways. First, we compare the LADCP velocities in the upper water column with the station- averaged ADCP velocities measured with the shipboard system. Secondly, we compare the estimates of absolute velocities from the LADCP with bottom- tracking estimates of the near-bottom flow. We also have plans to pursue a third approach to compare the overall LADCP velocity structure with geostrophic estimates of the baroclinic shear from simultaneous CTD casts, but this approach was not undertaken during Discovery 230. UPPER OCEAN COMPARISONS The ADCP measurements, as discussed in Section 5, were initially acquired as time averages over two minutes. For a typical CTD station lasting 2.5 hours, there are approximately 75 vertical profiles of velocity through the upper water column down to about 400 m depth by the shipboard ADCP system. In the on-station ADCP velocity profiles, there is inherent variability with a typical magnitude of 10 cm s-1 due to noise in ship's position which was estimated to be between 5m and 20m from time series of positions logged while Discovery was at the pier in Vigo and then Porto . An error of 10 m in ship's position translates into an error in velocity over two-minutes of 12 cm s-1. Because the individual two-minute ADCP profiles penetrate to different depths, time-averaged velocities at each depth were considered to be of reasonable quality when at least 10 two-minute averaged velocities were available during the period of the station. For such averages, the navigation-induced error in the ADCP velocities should be less than 2 cm s-1. An overall average velocity profile was estimated for the ADCP east and north velocities measured during each of 31 stations. During a station the LADCP transits through the upper 400 m of the water column on the downcast over approximately 20 minutes and again on the upcast over about 20 minutes. We display the downcast and upcast profiles in addition to the average profile to indicate the variability in the LADCP currents. It is the average of the up and down LADCP profiles that is compared quantitatively to the shipboard ADCP profile for each station (Figure 3.1). For comparisons at 31 stations, the shipboard ADCP and LADCP velocity profiles exhibit broadly similar vertical structure but with a varying mean offset. In order to quantify the agreement then, we estimate the mean difference between the shipboard ADCP and LADCP profiles and the standard deviation about the mean difference for each profile of east or north velocity on each station (Table 3.1). The average difference over all 31 stations is less than 1 cm s-1 indicating that there is no substantial bias between the ADCP and LADCP velocities. The similarity in vertical structure is apparent from the standard deviations about the mean difference for each station which are only 1.58 cm s- 1 for east velocity and 1.81 cm s-1 for north velocity and there are only 3 instances where the standard deviation exceeds 3 cm s-1. The overall measure of agreement which we prefer is the average absolute difference between the shipboard ADCP and LADCP velocities. For east and north velocities, the average absolute differences are 2.33 cm s-1 and 2.45 cm s-1 respectively. In summary, the absolute LADCP velocities exhibit similar vertical structure to the shipboard ADCP velocities and the mean offset between the LADCP and ADCP velocities measured on station is 2.4 cm s-1. BOTTOM BOUNDARY COMPARISONS For most of the LADCP profiles during Discovery 230, the instrument was set up to make alternate bottom-tracking pings as the instrument approaches the ocean bottom. Such bottom-tracking provides direct estimates of the movement of the instrument which can then be added to the velocity profiles relative to the instrument to create absolute velocity profiles for the water column within about 200 m of the bottom. Such absolute bottom velocities are effectively independent of the bottom velocities estimated in the overall absolute profiles because any problems in measuring velocity through the water column will not influence the Bottom Track velocities but will cause errors in bottom currents in the absolute profiles. We compare absolute LADCP bottom velocities with Bottom Track velocities at 4 stations in the deep western boundary current off Greenland where the deep velocities are larger than 10 cm s-1. For each of these stations, the scatter in Bottom Track velocity and the mean Bottom Track velocity profile are compared to the absolute LADCP over the bottom 200 m (Figure 3.2). Again we estimate a mean offset and a standard deviation about the offset between the absolute LADCP velocity and Bottom Track velocity (Table 3.2). The scatter in Bottom Track velocities surprised us. In general, the vertical structure of the average profiles is very similar, which should not be surprising because the baroclinic profile effectively represents the same measurements for each. The offset, however, is an independent quantity as discussed above. For these 4 stations in strong bottom currents, the offset varies from 0.04 to 4.14 cm s-1 with an average absolute offset of 2.5 cm s-1. The perceptive reader might have noticed that in 7 out of 8 cases, the offset suggested by these bottom comparisons has the same sign as that suggested by the surface comparisons. CONCLUSION Based on comparison of absolute LADCP velocities with on-station shipboard ADCP velocities in the upper water column and with Bottom Track velocities in the bottom 200 m of the water column, we conclude that the present accuracy of absolute LADCP velocities is approximately 2.5 cm s-1. Additional Bottom Track comparisons are needed to determine if the absolute LADCP velocities can be improved by consideration of the combined offsets for the independent surface and bottom comparisons. Penny Holliday, John Smithers, Harry Bryden and Bob Marsh TABLE 3.1: Upper ocean differences between ADCP and LADCP currents (cm s-1). uADCP- vADCP- uLADCP vLADCP Station mean std dev mean std dev 30 -1.36 2.14 -4.22 1.90 31 0.96 1.36 -0.35 2.62 32 -3.05 1.69 0.44 0.90 33 -4.28 0.98 -4.10 2.07 34 3.32 2.77 0.63 2.91 35 -2.98 1.94 0.03 2.09 36 1.64 1.25 1.51 0.76 37 -1.00 1.19 1.14 1.91 38 3.36 1.57 7.51 1.51 39 -4.42 0.62 -3.04 1.60 40 -0.85 2.14 2.46 2.52 41 -0.36 1.87 -1.00 1.60 56 -4.52 1.42 -0.65 1.91 57 -1.21 2.32 4.67 3.76 60 -1.18 1.43 0.61 1.16 80 3.64 0.96 -2.38 0.90 81 1.71 1.15 0.76 0.64 82 2.77 1.22 -4.25 1.25 83 4.12 1.23 -0.41 1.50 84 2.65 0.86 -1.66 1.69 85 5.47 1.16 -5.92 0.84 86 -2.15 1.34 0.23 1.08 87 0.97 1.10 1.10 1.15 88 1.29 1.25 -1.48 1.84 89 -0.82 2.52 -2.12 1.35 94 0.42 0.97 -1.54 0.71 95 1.62 0.68 3.51 0.60 96 5.12 0.71 -1.45 1.98 97 3.47 1.38 -5.10 1.59 98 0.70 5.28 1.04 2.91 99 4.63 2.39 6.97 6.86 Average 0.63 1.58 -0.23 1.81 Average Absolute 2.33 2.45 Difference TABLE 3.2: Comparison of Absolute LADCP and BottomTrack Velocities in Strong Currents off Greenland uBT- vBT- uLADCP vLADCP Station uBT vBT Mean Std Dev Mean Std Dev 84 13.62 -15.57 1.10 0.90 -1.47 1.89 94 -12.09 4.09 3.68 1.48 4.14 0.98 95 -14.91 10.40 0.65 0.85 2.80 0.92 96 -2.57 12.57 0.04 1.19 2.27 1.22 Average Absolute Difference 2.0 cm s-1 Figure 3.1: VM- and L-ADCP profile comparisons for station 85; VM-ADCP: short dash; L-ADCP: down (chain), up (long dash), mean (solid). Figure 3.2: LADCP water track/bottom track comparison: station 85 bottom track velocity scatter plot with 10m bin average (dashed line) and water track velocity (solid line). 4. NAVIGATION a. BESTNAV A standard PSTAR navigation file was maintained throughout the cruise. This was appended daily with the RVS "bestnav" position data at 30 second intervals. Daily processing: Level 0 acquisition of RVS 'bestnav' navigation data from level A. b. GPS AND GLONASS An Ashtech GG24 GPS-GLONASS receiver configuration (incorporating a Trimble- 4000 receiver) enabled the acquisition of navigation data (ship position, heading, speed over ground, satellite fix parameters) from more than one source. Data were acquired every second from the GPS satellite constellation, and also from the more accurate "mix" of GPS (US, dithered) and GLONASS (Russian, undithered) constellations, dubbed "Glos". Daily processing: Step 1: Level 0 acquisition of GPS navigation data (both GPS-4000 and Glos mix); Step 2: Level 1 quality control of Glos data: data is deleted wherever poor positioning accuracy is indicated by satellite fix parameters: 0 > PDOP > 10 0 > TDOP > 4 0 > VDOP > 4 0 > HDOP > 4 Step 3: Determination of ship velocity and 2-minute averaging. The east and north components of ship velocity over ground were plotted daily to reveal any gaps in the GPS and Glos, and to indicate the timing and nature of ship manoeuvring (a subsequent aid in the manual editting of ADCP absolute velocity datasets, to eliminate spurious currents arising from ship turns, and to separate on station and underway profiles) UNCERTAINTIES IN SHIP'S POSITION AND VELOCITY DUE TO NAVIGATION While Discovery was at the pier in Vigo, positions were monitored using both GPS and Glos positioning systems. The standard deviation in 2068 10-second GPS positions was about 20 m in east and north components while the standard deviation in Glos positions was about 10 m (see table 4.1). Because the navigation is primarily used in the ADCP processing for determining ship's velocity over two-minute averages, 2 minute differences in ship's position at the pier were also estimated. For the GPS positioning, the two-minute differences had a standard deviation of 25.2 m in north and 17.6 m in east components. Such standard deviations would lead to uncertainties in ship's velocity over two-minutes of 21.0 cm s-1 in north velocity and 14.6 cm s-1 in east velocity. The standard deviations of two-minute differences in Glos position were only 5.3 m in north and 5.0 m in east components. Such standard deviations would lead to uncertainties in ship's velocity over two-minutes of only 4.4 cm s-1 in north and 4.2 cm s-1 in east velocities. An opportunity also arose to monitor positions in Porto for 2.5 hours while Discovery was at the pier. The standard deviations in position were similar to those in Vigo, 20 m for the GPS positions but only 6 m for Glos. Because the Vigo record is longer, we use the Vigo position uncertainties as a measure of the uncertainties in ADCP velocities due to navigational uncertainties during Discovery Cruise 230. UNDERWAY CHANGES IN GPS ACQUISITION We had to alternate between relatively noisy ship positioning, determined from the dithered GPS satellite constellation, and the more accurate Glos positioning [having accidentally chosen to switch from GPS/GLONASS mix to pure GLONASS from 1200 on day 226 to 1800 on day 232]. c. SHIP GYROCOMPASS Two S.G.Brown gyrocompass units are installed on the bridge. Ship heading was logged every second via a level A microprocessor. Daily processing: Step 1: Level 0 acquisition of gyro heading data (logged every second) from level A. d. ASHTECH 3DF GPS ATTITUDE DETERMINATION The Ashtech 3DF GPS is a system of 4 satellite-receiving antennae mounted on the foredeck and bridge roof of the ship, and a receiver unit in the bridge house. Every second the Ashtech measures ship attitude (heading, pitch and roll) accurately, and this data is used in a post-processing mode to correct ADCP current measurements for 'heading error' (as the ADCP uses the less accurate but definitely continuous ship gyro headings to resolve east and north components of current). Accompanying each attitude are measures of maximum measurement rms error (mrms), and maximum baseline rms error (brms). To set up the Ashtech, the following 'best' parameter values were set using menu 4 on the receiver unit: in the ATTD SETTINGS sub-menu: max mrms 0.007m filter N max brms 0.060m max angle 10 deg. one sec sampling enabled in the ATTD CONTROL sub-menu: max cycle 0.20 cyc Kalman filter reset N Note however that setting the latter Y enabled acquisition of the first successful attitude data on the evening of day 223. Daily Processing: Step 1: Level 0 acquisition of Ashtech data (heading, pitch, roll, mrms, brms, logged every second) from level A; Step 2: Level 1 merging of gyro and Ashtech data; Step 3: Level 2 basic quality control of Ashtech data, averaging over 2- minute periods, and determination of heading error, 'a-ghdg' (correction applied to gyro data as determined from Ashtech-to-gyro comparison): Step 4: Plotting the daily time series of gyro heading, a-ghdg, plus pitch, roll and mrms statistics, to enable inspection for remaining outliers and further editing, and linear interpolation of the tidied-up a-ghdg time series The performance of the instrument throughout the cruise was not without problems. Unfortunately the Ashtech can be rather temperamental. It must maintain good satellite fixes to continue logging. Once fixes are lost for too many minutes, logging is interrupted, and it is necessary to switch off and on the receiving unit, and to reset the parameters (which re-assume undesired default values). Several specific problems arose in the course of the cruise. Problem 1: On sailing the instrument proved to be badly parameterized. On day 224, after five days, having determined appropriate parameters, we finally started to acquire accurate ship attitude data. Problem 2: Logging stopped at 0300 on day 226, and this was not noticed until 0330 on day 227. It was necessary to switch the receiver off, clearing the internal memory, and resetting the appropriate parameters. Thereafter the instrument was carefully monitored, and performed at an acceptable level over the following 10 days. Problem 3: There was a sudden failure to determine attitude after 1141 on day 237 (although position continued to be accurately fixed thereafter), discovered upon daily processing of Ashtech data on day 238 (watch checks only confirmed that Ashtech data was being logged). Initial efforts to solve the problem focussed on rebooting the receiver and resetting parameters, but to no avail. On day 239 it was noted that only three of the four antennae were locking to four satellites (the minimum number of satellites required to compute attitude, but only if locked-onto by all four antennae), raising suspicions that the fault lay with the hardware, specifically an antenna or cable connection. Sequential connection and disconnection, from the receiver, of the four antennae cables confirmed that no information was available from antenna 4. On day 240 the problem was finally traced to a faulty amplifier (which serves to improve signal- to-noise ratio) on the cable to antenna 4 (situated starboard on the boat deck). Salt deposits inside and outside the amplifier casing suggested seawater ingress, and tests revealed that the 9V signal from the antenna was being drawn off by the amplifier, reducing it to 5V. With no replacement amplifier available on board, the amplifier was removed and the cable re-terminated. After this renovation the Ashtech successfully continued to compute attitude from 1803 on day 240, having failed, on this occassion, for a total duration 3 days 6 hours 22 minutes. Problem 4: During days 244 and 245, the Ashtech software hung on three separate occasions, and it was necessary to switch on and off, and, on the second occassion, to clear both the receiver internal memory and the data memory. These problems were accompanied by overheating of the receiver unit, which is in direct sunlight, persuading us to construct and fit a makeshift heat shield. However, the problems also coincided with strong variations in the Earth's electromagnetic fields (evidenced at night by the Aurora Borealis), which possibly interfered sporadically with satellite signals. Overall the instrument performed acceptably, although considerable effort was necessary to maintain attitude determination. Performance (on days when the Ashtech logged continuously) are quantified in Table 4.2. Where we failed to obtain good Ashtech heading data for more than an hour or so (as throughout, or for part of, days 220-223, 226, 236-240, 245 and 246), the heading error, a-ghdg, was estimated from gyro heading. In order to make this estimation, we derived a quadratic relationship between a-ghdg and gyro heading, asymmetric about true north, using a scatterplot of a-ghdg against heading data (in the manner of King and Cooper, 1993). Working fits were determined twice during the cruise, the first fit being based on a-ghdg data collected along 41.5°N (to estimate a-ghdg over days 220-223 and 226), the second fit being based on all data collected upto day 236 (to estimate a-ghdg over days 236-240, 245 and 246). The entire cruise dataset will be subsequently used to derive a comprehensive relationship between a-ghdg and heading, possibly accounting also for latitudinal variations of gyro error. Bob Marsh TABLE 4.1: Ship navigation error determined in port. Vigo Porto GPS Glos GPS Glos sd lat (m) 24.7 9.7 19.4 22.9 sd lon (m) 17.0 11.8 8.3 4.2 sd 2minydif (m) 25.2 5.3 sd 2minxdif (m) 17.6 5.0 yvelerror (cm s-1) 21.0 4.4 xvelerror (cm s-1) 14.6 4.2 TABLE 4.2: Summary of Ashtech performance statistics. Julian Day Number of bad 2-min. Daily Number averaged headings %GOOD 224 256 61.0 225 281 64.4 228 278 61.4 229 174 75.8 230 237 67.1 231 311 56.8 232 128 82.2 233 211 70.7 234 228 68.3 235 216 70.0 241 163 77.4 242 220 69.4 243 200 72.2 244 319 55.7 247 131 81.8 248 160 77.8 249 69 90.4 250 259 64.0 251 444 38.3 252 188 73.9 253 214 70.3 254 173 76.0 255 134 81.4 256 166 76.9 257 245 66.0 5. VM-ADCP MEASUREMENTS a. DESCRIPTION AND PROCESSING The instrument used was an RDI 150 kHz unit, hull-mounted approximately 2 m to port of the keel of the ship and approximately 33m aft of the bow at the waterline. On this cruise the firmware version was 17.10 and the data acquisition software was 2.48. With the exception of a few interruptions (see Problems below), the instrument was operated continuously from day 219 (on leaving Vigo) to day 258 (after setting course for Southampton). For most of this time the ADCP was used in the water tracking mode, recording 2 minute averaged data in 64 x 8m bins from 8 m to 512 m water depth. On the continental shelves off Iberia, Greenland and Iceland (in water shallower than 500m), the instrument was switched to a combined water and bottom tracking mode, maintaining 64 x 8m bins. While in bottom-tracking mode, a FH-command (setting the number of water-track pings between bottom-track pings) of FH00001 was entered in the "Direct Control" menu - ensuring one water-track ping per bottom-track ping. The ADCP was otherwise operated with a default configuration. Daily processing: Step 1: Level 0 acquisition of ADCP water tracking and bottom tracking velocities from level A, and conversion to level C PSTAR format. Step 2: Level 1 correction of the times of ADCP velocity profiles, taking account of an approximate -1 second per hour PC clock drift. Step 3: Level 2 correction of the east and north components of ADCP velocities, accounting for the gyro heading error (determined as the difference between Ashtech and gyro headings). Step 4: Level 3 calibration of the shear profiles, taking account of errors in signal amplitude and transducer alignment, using a working calibration based on bottom tracking into/out of Porto (later confirmed as sufficiently accurate by more extensive bottom-tracking off Greenland and Iceland). Step 5: Level 4 merging of profiled velocities of water (relative to ship) with ship velocity determined from GPS [2 versions, depending on whether we had GPS-4000 or a GPS-Glonass mix] Step 6: Manual editting to remove spurious currents implied by sudden changes in ship velocity (e.g. coming on/off station), or short gaps in navigation data. Occassional processing: Editing of daily absolute velocity files to create on station and underway files, to compare with LADCP mesurements and CTD-derived geostrophic calculations respectively. Daily plotting: 1 plotted contoured "percent good" (PCG) over 0-500m, to determine quality of profiling (PCG > 25% is necessary to accept data for processing). 2 plotted 2-minute averaged currents, filtered with filter of width 10 minutes, in top 200m, to make first inspection of the raw absolute current data, especially useful for identifying problems with underway (steaming) data. 3 averaged data in bins 13-24, to determine 104-200m average current, averaged this over 10 minutes, and applied top-hat filter of width 50 minutes, and plot against latitude and longitude (as did Saunders and King, 1995), to identify features of the circulation. 4 where appropriate (e.g. across acoustic sections, wherever strong features were observed), resolved the east and north components of raw absolute current to along- and cross-track components, plotted contoured cross-track profiles. b. CALIBRATING THE ADCP The ADCP is routinely calibrated to take account of the orientation of the transducer on the hull (a misalignment angle - on RRSDiscovery the transducer orientation is intended to be fore-aft, pointing in the direction of steaming). Calibration exercises are undertaken to determine an amplitude factor A and the alignment angle error phi On this cruise we used heading- corrected bottom tracking data to determine 2-3 hour averages of ship velocity (and hence speed over ground and heading), compared with GPS-derived ship velocity. The choice of 2-3 hour averaging periods is mindful of the noise in GPS-derived ship velocity. Details of when and where we switched from water tracking to bottom tracking are as follows: 1 steaming in/out of Porto (to change the LADCP) on day 225; 2 on the Greenland shelf between station 102 (at the coastward termination of the second East Greenland Current section) and station 103 (commencing the Denmark Straits section) on day 248; 3 on the Icelandic shelf, approaching station 111 (commencing the 20°W section) on day 252. The Greenland shelf bottom tracking was quality controlled and split into six approx. 2.5 h duration segments. The bottom tracking south of Iceland was likewise quality controlled and split into two approx. 3 h segments. Details of the quality control are as follows. Bottom tracking data were not used wherever: (a) depth exceeded 500 m; (b) GPS-derived ship heading changed by more than 10° between 2-minute ensembles (c) GPS-derived ship speed changed by more than 10 cm s-1 between ensembles. The results of the calibration exercises are shown in Table 5.1. Differences between estimates of A and phi and sizeable standard deviations are possibly due to noise in the heading correction. However, note that A and phi have changed very little since Discovery cruise 223 (for which A = 1.0054 and phi = 3.57; see Leach and Pollard, 1998). We also used bottom tracking data to confirm that the Ashtech minus gyro heading correction was correctly determined. c. ADCP PERFORMANCE The typical %good on/off station indicated depth penetration, along the 41.5°N line, of 300 m and 200 m respectively. The ADCP suffered complete deterioration of depth penetration on day 236, steaming into heavy seas, and occasionally thereafter, notably in transit between the Cape Farewell acoustic section and the East Greenland Central Section. When sea conditions were favourable, depth penetration improved markedly with latitude, to over 500 m during the Cape Farewell acoustic section, implying better back-scatter at higher latitude. Problems encountered: 1. On three occasions the level A logging failed, and was restarted by RVS: (i) at 0800 on day 239 for 3 hours; (ii) at 0125 on day 245 for 1 hour 45 minutes (at the end of Cape Farewell acoustic section); (iii) at 1150 on day 245 for 3 hours 45 minutes. On these occassions ADCP data was later retrieved from the appropriate PINGDATA files, which are saved (one every 9 hours 16 minutes) on the PC. 2. A hardware failure at around 1800 on day 254, indicated on the PC by errors in all 4 acoustic beams and VERY WEAK TRANSMITTED SIGNAL error message. This was fixed by Dave Jolly of RVS, by reseating the boards in the VM chassis, and profiling resumed at around 1500 on day 255. On this occassion ADCP data was irretrievably lost. 3. On three occasions the PC was found to have hung and needed a reset: (i) at 2335 on day 239 for 25 minutes; (ii) at 1252 on day 255, for 4 hours; (iii) at 1048 on day 257, for 22 minutes. On these occasions ADCP data was irretrievably lost. d. GENERAL DESCRIPTION OF OBSERVED CURRENTS After completion of test CTD station 1 on day 220, the ship steamed eastward at 5 kn over the ground along 41.5°N, from 12.5°W to 9°W, in an acoustic survey of the eastern boundary. A northward coastal current of up to 30 cm s-1 was observed between 9°W and 10°W. Apart from the northward coastal current, we measured generally weak, although highly variable, currents along the 41.5°N line. On turning to follow a northwest heading at 20°W, currents strengthened, and underway profiling indicated a cyclonic feature, about 100 km across, centred on 26°W, 47°N, with northward currents, followed by southward currents, of up to 40 cm s-1 (associated with temperature and salinity anomalies of -0.5°C and -1.3 psu). We then encountered a southeastward flow of up to 60 cm s-1 between 28.5°W, 49°N and 29.5°W, 50°N, presumed to be a southern branch of the North Atlantic Current (NAC). The main branch of the NAC was observed as a northeastward flow of up to 70 cm s-1, between 30.9°W, 51°N and 31.5°W, 51.5°N. A northeastward flow of up to 25 cm s-1 was encountered between 32°W, 52°N and 33°W, 53°N, in the vicinity of the Charlie Gibbs Fracture Zone. We thereafter observed strong mesoscale variability in currents across the Irminger Sea, before reaching the East Greenland Current (EGC). The EGC exhibited southwestward flow of up to 60 cm s-1 between 41°W, 58.25°N and 43.5°W, 59.75°N, in the approach to Cape Farewell, and was very successfully resampled in a subsequent acoustic section back to 40.5°W 58°N. Weaker currents of up to 35 cm s-1 were observed on the following EGC transect, between 39°W, 62.5°N and 40.5°W, 63°N. Further strong southwestward flow was measured in transit between that transect and occupation of the Denmark Strait section. On commencement of the Denmark Strait section (30-31°W, 65-65.5°N), strong (> 80 cm s-1) northwestward currents were observed. Across the majority of the Strait we observed moderately strong (20 - 30 cm s-1) northward surface flow, with the exception of strong southward flow centred on 29°W, 65°N (also measured by LADCP). Unfortunately the ADCP hardware failure on days 254-255 coincided with the second transit across the NAC (observed on station 121 with the LADCP at 60°N, 20°W). Near-surface currents of up to 30 cm s-1 were observed across the Rockall Trough, with northward and southward flows respectively on the west and east sides of the Anton Dohrn Seamount. Strong eastward currents (up to 70 cm s-1) were observed at, and between, the final two CTD stations on the Hebridean shelf, possibly evidence for the coastal current. ADCP current measurements along 4X were generally excellent, with very few gaps due to heavy seas, and no soft- or hardware related loss of data. Apart from problems with instrument reliability, good quality ADCP current measurements along the 20°W/Ellett section were frequently interrputed by heavy seas, and the profiling along this section was generally less successful than along the 4X section. Bob Marsh REFERENCE Leach, H. and R. T. Pollard, 1998: Discovery Cruise 230. Cruise Report, in preparation. TABLE 5.1: ADCP calibration exercise results. A is amplitude scaling factor, phi is heading correction in degrees to starboard. Calibration exercise A sd phi sd Porto port call incoming 1.0032 3.503 outgoing 0.9998 3.418 all data 1.0012 3.454 Off Greenland segment 1 0.9959 0.0122 3.414 0.591 segment 2 1.0004 0.0078 3.393 0.568 segment 3 1.0002 0.0091 3.424 0.624 segment 4 1.0003 0.0065 3.400 0.275 segment 5 1.0008 0.0080 3.487 0.455 segment 6 1.0008 0.0080 3.372 0.548 all data (15h) 0.9997 0.0086 3.415 0.522 South of Iceland segment 1 1.0025 0.0095 3.333 0.548 segment 2 1.0018 0.0098 3.408 0.466 all data (6h) 1.0021 0.0097 3.370 0.509 6. METEOROLOGICAL MEASUREMENTS a. SURFACE METEOROLOGY AIMS The aims of the surface meteorological measurements during cruise D230 included: i Continuous measurement of mean meteorological variables such as air and sea temperatures, wind speed and wind direction, downwards radiation (long wave, short wave and photosyntheticaly active) and atmospheric pressure. ii Determination of the momentum transfer (wind stress) and heat fluxes between the ocean and atmosphere. All the instruments and logging systems functioned well throughout the cruise, and a high quality data set was obtained. SENSORS DEPLOYED (a) Mean Meteorology The GrhoMet meteorological instrumentation system uses the RVS Rhopoint network for connection to foremast, hull and laboratory sensors. In addition to the normal RVS instrument suite, further JRD/OTD sensors were mounted on the foremast and connected into the logging system. A total of 18 variables were logged (Table 6.1). These measured air temperature, air pressure, wind speed, wind direction, downward long wave, short wave, and photosynthetically active radiation (PAR). The system acquired data at 5 second sampling rate and generated data files in raw and calibrated format which were written to the hard disk of a PC in the main lab. The GrhoMet system also output raw (uncalibrated) data via an RS232 link to the level B in SMP format, where the data was logged by the RVS computer system. The scientific clock was read through a serial port and used to update the PC clock once every 6 hours when a new data file was opened. (b) Wind stress A Gill Instruments Solent Sonic Anemometer (R2 Asymmetric Model, serial number 38) was mounted on the starboard side of the foremast platform. The anemometer was operated in Mode 1 and the 21 Hz sampled data were logged using a PC system situated in the main lab. This recorded the raw data stream on optical disk and also calculated and recorded wind speed spectra and spectral levels. These were based on about 12 minutes sampling period (n=1024*15) starting each quarter hour. SENSOR PERFORMANCE (a) Air temperature and Humidity Four sensors provided dry bulb air temperature data: two psychrometers, the RVS air temperature sensor, and the temperature signal from the RVS humidity sensor. The air temperature from the humidity sensor was low by almost 1 degree compared to the data from the other three sensors. It was thought from previous cruise comparisons that the two psychrometers may over-estimate the dry bulb temperature by up to 0.15°C when the downward solar radiation is in excess of a few hundred W/m2. However, examination of night-time data showed that the RVS air temperature sensor may read low by a similar amount for air temperatures of 15°C or more. Accurate calibration of the RVS sensor is required before the true cause of any trends can be determined. When extremes of temperature or large downwards solar radiation were absent, the two psychrometers and the RVS sensor agreed well: for downwards longwave radiation of less than 100W/m2 and temperatures between 6 and 13°C, the mean difference was 0.03 ± 0.12°C or better. The starboard psychrometer dry bulb readings also exhibited an intermittent cold bias, probably due to dripping from the wet bulb wick. This problem occurred mainly during the first two weeks of the cruise. The mean difference between the wet bulb temperature values from the two psychrometers was negligible ( -0.03 ± 0.05°C). The humidity estimates from the two psychrometers were in very good agreement, with a mean differences of 0.0 ± 1.0%. The RVS humidity sensor compared well with the psychrometers: in this case the mean difference was 2.0 ± 1.5%. (b) Radiative fluxes Examination of night-time data showed that the starboard solarimeter read high by about 5 W/m2. The port solarimeter showed a negligible bias. As observed on previous cruises, these instruments were sometimes shaded by the foremast extension and other instruments mounted nearby. Selecting the highest reading from the two instruments is recommended, but is not a complete solution since the two sensors were sometimes shaded simultaneously. The port PAR sensor underestimated by 2.5 W/m2, and the starboard overestimated by 1.2 W/m2 (again, night time data only). Use of data from the starboard sensor is recommended since it exhibited less scatter. The comparison between the two longwave sensors showed an underestimate from LW2 of 5 W/m2 for the higher values of downward longwave radiation, and 10 W/m2 for the lower, clear sky values. From past instrument comparisons, LW1 is believed to be the more accurate sensor. (c) Wind velocity and wind stress Twelve minute averages of the mean relative wind speeds from the R.M. Young propeller-vane and the Solent Sonic anemometer were compared. For the entire data set, the mean difference was -0.1 ± 0.7 m/s. However, after selecting data for periods when the wind was blowing within 30 degrees of the ship's bow (i.e. the anemometers were well exposed), the mean wind speed difference was - 0.3 ± 0.5 m/s. The mean 10 minute average wind speed during the cruise was about 8m/s; the maximum 10 minute wind speed observed was about 21 m/s. The wind stress estimates obtained from the sonic anemometer corresponded to drag coefficient values similar to those found on previous cruises. (d) Sea Surface temperature The GrhoMet system logged sea surface temperature (sst) data from the hull contact sensor which is located in the forward hold at a depth of about 3.5 m. The data from the hull sensor were compared to those from the thermosalinograph (TSG) which sampled water from an intake located at a depth of about 5 m. The TSG sensor produced bad data when the ship was in Oporto (day 225.5 to 225.7) and again during day 221. The cause of the latter period of bad data is not known. Previous comparisons between the hull sensor and the TSG suggested that the hull sensor underestimated sst by 0.5°C. Although this offset was incorporated in the calibration of the data from the hull sensor, the end of cruise comparison between the TSG and hull sensor data showed a residual offset which varied with temperature. For temperatures above 11°C the hull sensor underestimated by 0.12 ± 0.06°C, and below 11°C the hull sensor underestimated by 0.26 ± 0.13°C. Margaret Yelland b. SBWR MEASUREMENT DETAILS The MK IV version of the Ship borne Wave Recorder (SBWR), developed through a collaborative programme between Ocean Technology Division of SOC and W. S. Ocean Systems Ltd., has been installed on Discovery since cruise 224. The electronic control and processing unit of the MK III system has been replaced by a PC running an application developed using LabWindows CVI. This converts pressure and accelerometer signals into a wave height value which is periodically processed to produce a wave energy spectrum. The logging system outputs summary data, such as significant wave height (Hs), to the RVS level B. However, this data has not been corrected for instrument response. The spectral data were periodically downloaded from the PC hard disk, and corrected for instrument response before recalculating Hs. Figure 6.1 shows that the uncorrected data underestimates Hs by around 40% on average. Margaret Yelland c. ACOUSTIC RAIN BUOY An acoustic rain gauge buoy was made available (by G. Quartly of JRD and K. Birch of OTD) for deployment trials during the cruise. Two attempts were made to deploy the buoy while the ship was hove-to, on station, during days 235 and 239. These attempts were unsuccessful since the buoy only drifted a few tens of meters from the ship, rather than the required 500 m. In order for the buoy to stream away from the ship it was necessary for the deployment to take place while the ship was steaming at half or one knot. This method was tried successfully on days 246 and 249. Some practical problems were encountered during these deployments; the rope used was very thin (about 3 mm) which made recovery by hand a slow process; the hydrophone cable tended to kink and tangle (which could be prevented by use of a "fishing reel" arrangement); it was necessary for someone to be on deck to observe the behaviour of the buoy throughout the deployment. Communication with the buoy is relatively straightforward but the supplied terminal software has to be used. Several attempts were made to adjust the data rates. It should be possible to set up the buoy to output data every 90 seconds with 7 subsamples in that period. This was achieved on the bench but data rates reverted to 1 sample every 90 seconds after a 20 minute period. Either the buoy software is not the same as was thought or the manual did not provide clear enough instructions to do this. An external communications port is necessary to make any changes without having to open the buoy up every time. Further time on the bench is required to fully understand all of the available options, otherwise the instrument was relatively easy to use and communicate with. Margaret Yelland and John Smithers TABLE 6.1: Variables and sensors logged by the GrhoMet system. The variable names in the data files are shown [thus]. For each instrument (RVS) indicates that the sensor is part of the standard ship's system; (JRD/OTD) that the instrument was added for the cruise. ------------------------------------------------------------------------------ Variable Position Instrument Note ------------------------------------------------------------------------------ Wet and Dry Bulb St'b'd side of Psychrometer IO2003 (1) [psyptd psyptw] foremast platform to day 229.7, IO1030 (forward sensor) thereafter. (JRD/OTD) ------------------------------------------------------------------------------ Wet and Dry Bulb St'b'd side of foremast Psychrometer IO2002 [psystd psystw] platform (aft sensor) (JRD/OTD) ------------------------------------------------------------------------------ Humidity & air Port side of foremast Vaisala HMP 35D temp. [hum humt] platform (RVS) ------------------------------------------------------------------------------ Air temp [atemp] St'b'd side of foremast Vector Inst. 209 platform (RVS) ------------------------------------------------------------------------------ Longwave [lw1] Top of foremast Eppley PIR 31170 (port sensor) (JRD/OTD) ------------------------------------------------------------------------------ Longwave [lw2] Top of foremast Eppley PIR 31171 (starboard sensor) (JRD/OTD) ------------------------------------------------------------------------------ ShortWave [ptir] Gimbal mounted on Kipp & Zonen CM6B port side of 962301 (RVS) foremast platform ------------------------------------------------------------------------------ ShortWave [stir] Gimbal mounted stbd Kipp & Zonen CM6B side of foremast 962276 (RVS) platform ------------------------------------------------------------------------------ Photosynthetically Gimbal mounted on Didcot DRP-1 0151 active radiation port side of (RVS) [ppar] foremast platform ------------------------------------------------------------------------------ Photosynthetically Stbd side of Didcot DRP-1 5143 active radiation foremast platform (RVS) [spar] (not gimballed) ------------------------------------------------------------------------------ Wind Speed & Port side of RM Young AQ 11276 Direction [ws1 wd1] foremast platform (RVS) ------------------------------------------------------------------------------ SST [sst1] Hull mounted approx. PRT (RVS) 5 meters depth. ------------------------------------------------------------------------------ Pressure [baro] Lab Vaisala DPA21 (RVS) ------------------------------------------------------------------------------ Time Lab Ship's clock (RVS) ------------------------------------------------------------------------------ Notes: (1) The fan on psychrometer began to fail on day 226, but the instrument could not be replaced until calm weather permitted on day 229. Figure 6.1: Comparison of Hs corrected (Hs corr) and Hs uncorrected (Hs raw) 7. CHEMICAL MEASUREMENTS Samples for salinity, oxygen and nutrients were drawn from all bottles on all stations. A summary of the sampling regime for all other quantities described below is given in table 7.1, and again in the WOCE format station summary table, reproduced here as Appendix 1. a. OXYGEN Dissolved oxygen samples were drawn from each niskin bottle following the collection of samples for CFC analysis. Between one and four duplicate samples were taken on each cast, from the deepest bottles. The samples were drawn through short pieces of silicon tubing into clear, pre-calibrated, wide necked glass bottles and were fixed immediately on deck with manganese chloride and alkaline iodide dispensed using precise repeat Anachem bottle top dispensers. Thanks to Pete Mason for the construction of a reagent stand for use on deck. Samples were shaken on deck for approximately half a minute, and if any bubbles were detected in the samples at this point, a new sample was drawn. The samples were transferred to the constant temperature (CT) laboratory, and then shaken again thirty minutes after sampling and stored under water until analysis. The temperature of the water in the Niskin bottles was measured using a hand held electronic thermometer probe. The temperature was used to calculate any temperature dependant changes in the sample bottle volumes. Samples were analysed in the CT laboratory starting two hours after the collection of samples. The samples were acidified immediately prior to titration and stirred using a magnetic stir bar set at a constant spin. The Winkler whole bottle titration method with amperometric endpoint detection (Culberson, 1987) was used with equipment supplied by Metrohm. The spin on the stir bar was occasionally disturbed by the movement of the ship and also by the uneven bases on some of the glass bottles, leading to less effective stirring of the sample and thus longer titration times, although this probably did not effect the accuracy of the endpoint detection. The Anachem dispensers were washed out with deionised water, each time the reagents were topped up, to avoid any problems caused by the corrosive nature of the reagents. The normality of the thiosulphate titrant was checked against an in house potassium iodate standard of 0.01 N at 20°C at the beginning of each analytical run and incorporated into the calculations. A total of seven standards were used throughout the duration of the cruise. Blank measurements were also determined at the start of each run to account for the introduction of oxygen with the reagents and impurities in the manganese chloride, as described in the WOCE Manual of Operations and Methods (Culberson, 1991). Thiosulphate standardisation was carried out by adding the iodate after the other reagents and following on directly from the blank measurements in the same flask, as on the cruises D223 and D227. Changes in the thiosulphate normality are shown in figure 7.1. The thiosulphate normality precision was poor initially for the first reagent batch in use. The precision improved for the second batch, between stations 027 and 047, although the thiosulphate normality was low. From station 048 the thiosulphate normality results remained constant despite further changes in reagent batch. Tests were also carried out on each batch of alkaline iodide used during analysis, since some variability has occured on previous cruises when the iodide batch was changed. Absolute duplicate differences for each station are shown in figure 7