RV Pelagia Cruise Report: Cruise 64PE155, Project MARE-1, Mixing of Agulhas Rings Experiment C. Veth Chief Scientist NIOZ, Texel, 2000
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Table of contents nr. Chapter page 1 Cruise Narrative 5 1.1 Highlights 5 1.2 Cruise Summary Information 5 1.3 List of Principal Investigators 11 1.4 Scientific Programme and Methods 11 1.5 Major Problems Encountered during the Cruise 18 1.6 List of Cruise Participants 19 2 Underway Measurements 20 2.1 Navigation 20 2.2 Echo Sounding 20 2.3 Thermo-Salinograph Measurements 20 2.4 Vessel mounted ADCP measurements 20 2.5 Meteorological measurements 20 3 Hydrographic Measurements -Descriptions, Techniques, and Calibrations 21 3.1 Rosette Sampler and Sampler Bottles 21 3.2 Temperature Measurements 22 3.3 Pressure Measurements 22 3.4 Salinity Measurements 23 3.5 Oxygen Measurements 23 3.6 Nutrient measurements 24 3.7 CTD Data Collection and Processing 25 3.8 LADCP Data Collection and Processing 25 3.9 Data Management 25 4 Acknowledgements 26 Appendix A (Weekly messages) 27 Appendix B (cruise summary file) 42 3
The research reported here is part of the contribution to the Dutch Clivar programme CLIVARNET that is funded by ALW, subsidiary of the Netherlands Organization for Scientific Research (NWO), contract no. 750.710.01 4
1 Cruise Narrative 1.1 Highlights a: MARE-project, RV Pelagia cruise 64PE155 in the Agulhas retroflection area (leg 6 in "Pelagia around Africa) b: Expedition Designation (EXPOCODE): 64PE155 c: Chief Scientist: Drs. C. Veth Netherlands Institute for Sea Research (NIOZ) P.O.Box 59 1790AB Den Burg/Texel The Netherlands Telephone: +31(0)222-369414 Telefax: +31(0)222-319674 e-mail: veth@nioz.nl d: Ship: RV Pelagia, Call Sign: PGRQ length 66 m. beam 12.8 m draft 4 m maximum speed 12.5 knots e: Ports of Call: Cape Town (South Africa) Cape Town (South Africa) f: Cruise dates: February 27, 2000 to March 19, 2000 1.2 Cruise Summary Information Summary In the evening of February 27th RV Pelagia left port in Cape Town and headed south for the expected northern edge of Agulhas ring ASTRID. The position of ASTRID was determined by satellite altimetry (TOPEX/ERS-2) analysis in combination with infra-red images of the Agulhas retroflection area. We arrived at the northern edge on February 28th around noon and planned to start with two perpendicular sections with the undulating platform SCANFISH in order to determine the approximate size, shape and position of the ring. It was decided to make two SCANFISH transects (north-south and west-east) through the satellite-estimated centre at about 37.5 S, 18.5 E. Based upon the SCANFISH results two CTD-sections were planned through the calculated centre at an angle 5
of 45 (northeast-southwest and southeast-northwest). Technical problems with the tow-link of the SCANFISH resulted in a delay of some hours, but at about 17:00 UTC the north-south SCANFISH transect along the 18.5 East meridian through ASTRID started. The SCANFISH was set to undulate in the depth interval 15-190m. In order to monitor the crossing of the ring edge without interrupting the data collection, the salinity and temperature were recorded by hand each time the undulator reached the 100 dbar pressure level(later at 50 dbar). With a cruising speed of about 6 knots the SCANFISH system made undulations of a period duration of 15 minutes, corresponding to one upand-down motion in 1.5 nautical mile. The north-south transect was prematurely interrupted by mechanical problems with one of the steering flaps before the southern edge of the ring was crossed. The estimated repair time was so long that we decided to steam to the starting point of the second SCANFISH transect at 37 45' S, 16.5 E, the west-east transect. This transect was completed at 37 45' S, 20 E without interruption. The SCANFISH sections ended on March 2nd at about 17:00 UTC. At the end of the undulator section a first test with the microstructure profiler FLY was performed. Technical problems with the special winch and line-pulling system caused this test to end prematurely. The first CTD-section started at March 3rd in the morning. At intervals of 10 nm CTD/Rosette casts were performed. Most casts to a depth of 1000 m and occasionally to the bottom (between 4000 and 5000 m). The hydrographical CTD/Rosette series was interrupted occasionally by a geological station with an extra CTD-cast and two multinet hauls. The NE-SW transect was not entirely completed due to bad weather. The transect ended on March 5th at 21:00 UTC. It was decided not to wait for the weather to improve and continue the transect, but to steam to the beginning of the second (SE-NW) CTD/Rosette transect that started on March 6th at about 10:00 UTC. Halfway this transect, near the centre of the ring ASTRID, two ARGOS buoys were deployed separated by a distance of about 10 nm. The CTD transect was completed on March 9th in the afternoon. During the crossing of the northwest edge of the ring a series of microstructure casts have been made. In collaboration with the "home base" at the University of Utrecht it was decided to complete the nonfinished north-south SCANFISH transect and the northeast-southwest CTD-transect with a number of extra CTD-casts. After completing this part of the programme, R.V. Pelagia steamed to the assumed location of the second Agulhas ring "LAURA", centerd around position 35 S, 14.5 E. Between ASTRID and LAURA a series of CTD-cast were done to get information on the region between the two rings. From infra-red imagery it seemed that small satellite rings appeared next to ASTRID. The first series of CTD/Rosette casts, with additional microstructure measurements in LAURA looked very different from the profiles measured in ASTRID and after consultation the "home base" at the IMAU it was clear that the ring LAURA was probably at a different location. The infra-red images of this region are hard to interprete due to an abundance of clouds and all kinds of surface water streamers. Near the newly proposed position of the ring LAURA, around 37 S, 14.5 E a series of CTD/Rosette casts was performed from northwest to southeast and after consultation Utrecht it was decided to finish the programme with a last long CTD/Rosette section through LAURA in the direction of ASTRID as far as time permitted. In the assumed centre of LAURA an ARGOS buoy was 6
deployed and at the far end of the measurement series an extra ARGOS buoy was deployed in ASTRID, because it appeared that one of the first two buoys didn't give a signal. All the CTD-series were interrupted occasionally by geo-stations with an extra CTD-cast and two multinet hauls. Cruise Track Figure 1. Cruise track of RV Pelagia cruise 64PE155. The dots idicate hydrographic stations and the dashed circle the position of ring Astrid at the beginning of the cruise as determined by satellite altimetry. Hydrographic Stations Details on the hydrographic stations can be found in the Cruise Summary (Appendix A) A total of 89 hydrographic stations was performed. On 3 of these stations a SCANFISH undulator haul was done. In these cases a station was defined by the period between deployment and recovery of the SCANFISH. On 84 stations standard CTD casts were recorded (see figure 2). In 7 cases an extra shallow (400 m) CTD cast was recorded to take samples for the geological research and two multinet hauls were performed. Water samples were taken at most stations inside ring Astrid and ring Laura 7
(see figure 4.) for the determinations of nutrients, salinity and dissolved oxygen and on geological stations for geological and biological research. Three water samplers in the rosette system were fitted with reversing electronic pressure sensors. A Lowered Acoustic Doppler Current Profiler (LADCP) was attached to the CTD/Rosette frame to measure the vertical profiles of the current speed and direction. At 20 stations a microstructure probe FLY was deployed. ARGOS drifters were deployed at 4 stations. The positions of the hydrographic stations are indicated in figure 2. At the hydrographic stations the SBE9/11+ CTD was lowered with a speed of about 1 m/s. Due to the use of a bottom indicator switch we were able to sample to within quite a short distance from the bottom (5 m). Figure 2. The CTD-stations Hydrographic Sampling SCANFISH Two perpendicular transects were made with a SCANFISH MKII-1250 (GMI (Denmark)), undulating wing-type platform supplied with a Seabird SBE9/11+CTD and a Chelsea Fluorometer. The SCANFISH undulated in a vertical depth range between 15 and 190 m at a sailing speed of 6 to 7 knots. 8
Scanfish sections -34 Latitude (South) -36-38 st3 st1 st1 st2 st3 st2-40 12 14 16 18 20 22 Longitude (East) Figure 3. Scanfish transects CTD/Rosette During the up-cast of each CTD/rosette station water up to 24 samples were taken at regular depth intervals. The vertical distribution of the sampling locations and depths is indicated in figure 4. Water samples taken 0 Station number 0 10 20 30 40 50 60 70 80 90 1000 Pressure (dbar) 2000 3000 4000 5000 Figure 4. Vertical distribution of the water samples versus station number. 9
Lowered ADCP At most stations current velocity and direction from the entire CTD-cast depth were measured with two synchronized, self-contained 300 khz ADCP's mounted on the CTD/Rosette frame. One of the two is downward looking (the master), and the other one upward looking (the slave). ARGOS drifters During the cruise four ARGOS drifters were deployed. The drifters used were standard spherical WOCE/TOGA mixed layer drifters (diameter 30 cm), fitted with a holey sock drogue at 15 m. The drogues had a length of 7 m, and a diameter of 1 m. The ARGOS ptt numbers of the drifters are 19289, 19286, 19288 and 19287 (See figure 5, A, B, C, and D respectively, for the deployment positions). Number 19288 failed to work. ARGOS drifters -34 Latitude (South) -36-38 C D B A -40 12 14 16 18 20 22 Longitude (East) Figure 5. Positions of drifter deployments. Microstructure Profiler FLY II At a number of stations (see figure 6), mainly near the edges of Agulhas rings, the micro-structure probe FLY II has been deployed in order to determine the rate of dissipation of turbulent kinetic energy. The FLY II deployment requires a special winch and linepuller, in such a way that the instrument can make a free-falling motion through the water. Initial technical problems with the winch and linepuller caused two deployment failures with this instrument. The turbulence level in the ring edges turned out to be so low, that the micro-structure probe had difficulty measuring the dissipation level. The threshold level of the instrument is about 2e-9 W/kg for. Near the ring edges the turbulent dissipation levels were of this order of magnitude, but 10
in the ring centre probably lower. That means that only an upper level of turbulent dissipation can be estimated. Microstructure Profiler FLY -34 Latitude (South) -36-38 -40 12 14 16 18 20 22 Longitude (East) Figure 6. Position of micro-structure probe deployments *.SUM file (the stations summary) A hard copy of the preliminary *.SUM file describing all stations is added in the appendix B. 1.3 List of Principal Investigators Name Responsibility Affiliation Drs. C. Veth LADCP, FLY-measurements. NIOZ/Texel Dr. H.M. van Aken Ocean hydrography, ARGOS drifters. NIOZ/Texel Drs. A.K. v. Veldhoven Ocean hydrography NIOZ/Texel J.H.F. Jansen Geology, Multinet, aquaflow pump NIOZ/Texel Dr. G.M. Ganssen Geology, Multinet, aquaflow pump VU/Amsterdam Ing. S. Ober CTD/rosette & (L)ADCP-technology NIOZ/Texel Dr. M. Rouault meteorology and air-sea interaction UCT/Cape Town 1.4 Scientific Programme and Methods The physical research within the MARE (MARE =Mixing of Agulhas Rings Experiment) programme. The Physical Department of NIOZ participated in the MARE-project that is one of the main components of the Dutch contribution to CLIVAR (Climate Variability and Prediction). Climate variability at interannual, decadal to millennial time scales is coupled to variations in the ocean s 11
thermohaline circulation (THC). Interocean exchange of water (between the Indian Ocean and the South Atlantic) around South Africa is thought to be a key link in the maintenance of the THC. As a result, variability in interocean exchange induces variability in the global THC and can affect the production of North Atlantic Deep Water (NADW) and associated climate variability over Europe. Interocean exchange between the Indian Ocean and Atlantic Ocean, or Agulhas leakage, occurs on an intermittent basis. It is determined largely by the shedding of Agulhas rings which is extremely variable on an interannual time scale. As a result Agulhas rings seem to be the most likely source of South Atlantic circulation anomalies that, in the long run, influence NADW production. However, at present it is unclear what fraction of Agulhas Ring Water is transferred to the THC by mixing with the Benguela Current in the Cape Basin. The main objective of the MARE programme is to determine the proportion of the Agulhas leakage that contributes to the northward branch of the THC, to estimate its variability at interannual to millennial time scales and to determine the impact of anomalous Agulhas leakage on the strength of the Atlantic overturning circulation and associated (actuo- and palaeo-) climate fluctuations over the northeast sector of the Atlantic Ocean. Sub-questions ( operational questions ) to answer are: 1. What is the decay rate of an individual ring: how fast does a ring loose its properties to the surrounding water? 2. What is the relative role and magnitude of air-sea interaction in the initial stage of ring formation and in later stages? 3. What is the relative role of interleaving with subsequent, vertical-shear induced turbulence and double-diffusive processes on the exchange of Agulhas ring water with its surroundings? 4. What is the influence of bottom topography on the decay of Agulhas rings? To achieve these goals, three cruises were planned of which the first two have already been carried out. During the first cruise (MARE I, March 2000), that was carried out within the framework of Pelagia around Africa, a newly formed Agulhas ring (called "Astrid") was investigated, south of Cape Town. At this stage the ring was visible with satellite altimetry measurements (sea surface height, SSH) as well as with satellite sea surface temperature measurements (SST). The same Agulhas ring "Astrid" will be investigated again during the second cruise (MARE II, July 2000) on board S.A. Agulhas. At that time the ring will only be visible in the SSH. In between the cruises the ring will be tracked by satellite altimetry and ARGOS-buoys. During the cruise the Agulhas ring was crossed a couple of times with an undulating CTD-system, CTD (Conductivity, Temperature, Depth) and (Lowered) ADCP (Acoustic Doppler Current Profiler) equipment, which yielded profiles and sections of temperature, salinity, potential density and the velocity components and chemical parameters from the water samples taken. 12
The Physical Department of NIOZ is mainly involved in the sea-going part of the project. The other participating institutes (IMAU- Utrecht, KNMI-De Bilt) emphasize the modelling and theoretical study of this subject. The objective of the geological research within the MARE programme is threefold. In the first place, we want to develop a record of the history of the Agulhas Rings during the late Quaternary, with its glacial to interglacial variations of the global climate. Special emphasis lies on a highresolution record of the last 20 000 years, the last deglaciation. The second objective is to describe the South Atlantic oceanographic system during periods when it looked different from today, possibly even without incoming Agulhas Rings. For these two objectives, sediment cores will be taken during the forthcoming MARE 2 and MARE 3 expeditions. Thirdly, we will develop proxies for the Agulhas Rings; proxies being characteristic properties of the sediment that describe the marine environment above. For this purpose, we sampled plankton with the multinet. This device is towed through the water with low speed (2 to 2.5 knots) while it is slowly raised. By successively opening and closing the different nets, the plankton is collected out of nine intervals between 800 m, 500, 300, 150, 100, 75, 50, 25, and 10 m water depth, and the sea surface. In these samples, the plankton composition will be studied, particularly of the foraminifera, a group of calcareous zooplankton. At each multinet station, two CTD casts were taken to determine the nutrient contents and stable isotope composition of the seawater at different depth in the water column. From all CTD depths, water samples were filtered on board for the measurement of stable isotope composition and chlorophyll a of the Particulate Organic Matter, and to study the coccolithophores, a calcareous phytoplankton group. Additionally, the same type of plankton and water samples were collected twice a day from 5 m below the water surface with the aquaflow pump, to get informed about the spatial distribution of the proxies.thanks to the close co-operation with the physical oceanography staff, we have been able to successfully sample a nice collection of samples for the study of foraminifera and other components and proxies in the two visited Agulhas Rings Astrid and Laura, and their margins. 13
GEO-stations -34 58 Latitude (South) -36-38 75 39 36 20/21 12 28 8-40 12 14 16 18 20 22 Longitude (East) Figure 7. The positions of the geo-stations (with station numbers) The objective of the biological research within the MARE programme is twofold 1. Diversity and dynamics of microbial communities in an Agulhas Ring Cytological techniques, such as flow cytometry, and molecular biological methods, such as PCR, DGGE and DNA sequence analysis are used.. These parameters have been studied in relation to general parameters determined during the cruise, such as nutrient concentrations. Also the phytoplankton has been studied with flow cytometry. Water samples of different sites, inside and outside the ring, and at different were filtered in order to collect the micro-organisms. DNA of these organisms will be isolated. The DNA will be used in several molecular biological techniques in order to compare the structure of the different communities in space and time, and to identify the dominant populations. The study must give a better understanding about the structural and functional stability of the microbial communities in the different water masses inside and outside Agulhas rings. (Sub project participants: Marcel Veldhuis, Gijsbert Kraay, Harry Witte, en Gerard Muyzer) 2. Phytoplankton composition and viability The second biological aim of the MARE-1 cruise was to determine the phytoplankton composition on a transect across the agulhas rings named Astrid and Laura as well as the viability of this plankton component. For this purpose discrete water samples (6 to 12) were taken of the upper 300m of the water column. Samples were analyzed using a flow cytometer, which determines the size, chlorophyll, and phycoerythrin fluorescence of each individual particle. Based on these optical characteristics phytoplankton can be classified on the species level or clustered in-groups with similar size and fluorescence content. The most abundance species which could be assigned was Synechococcus. This species can easily be distinguished from the other phytoplankton components by its orange fluorescence caused by the pigment phycoerythrin. Next in abundance is a variety of eukaryotic phytoplankton species, largely varying in size with depth and along the transect but these were far 14
below in abundance in comparison with the Synechococus. Further classification will be done on base of the detailed pigment composition. For this purpose samples were taken at most CTDfys1000 stations at 4 different depths. In order to have an indication of the health of the phytoplankton, subsamples were examined for their viability. This assay was performed directly after sampling. But before we have any idea of usefulness this new technique we must carefully analyze the data at home. (Gijsbert Kraay & Marcel Veldhuis) Preliminary Results At this stage of the project we will only show some preliminary results of the physical oceanographical research, mainly based upon the CTD-data. The CTD-data were already processed and analyzed on board, in order to adapt the cruise plan to the actual situation encountered in the field. At the start of the cruise the Agulhas ring Astrid coincided rather well with the position obtained from satellite-altimetry data. The two perpendicular SCANFISH sections were used to estimated the central position of the ring. An example of a SCANFISH measurement is given in figure 8, the westeast temperature and salinity section. 15
Figure 8. West-East SCANFISH sections of temperature and salinity The central part of the ring shows a homogeneous watermass of warm and salty water of Indian Ocean origin. Near the edges strong horizontal and vertical gradients are visible. The CTD-sections, that run from Northeast to Southwest (stations 5 to 20) and Southeast to Northwest (stations 21 to 39) confirm this picture. Examples of CTD-sections (combined stations 21 to 39 and 50 to 59) to a depth of 1000 m, are given in figure 9. Theta-S plots of the stations indicate that the water warmer than about 12ºC in the ring is different from the water outside of the ring. The 12ºC isotherm is a kind of border between the warm and salty ring water and the "background" water. This isotherm has a depth of about 600 m in the centre of the ring and 200 m in the ring edges. The isopycnals bulge downward under the weight of the warm and salty dome of Indian Ocean water in such a way that strong geostrophic currents run more or less in a circular way around the ring centre. These currents, that Figure 9. CTD-sections of potential temperature, salinity and density through ring Astrid (station 21 39) and outer-ring area. 16
have a speed of about 3 to 4 knots at the surface near the ring edge, clearly influenced the navigation of R.V.Pelagia. With the Lowered ADCP measurements, the picture of the geostrophic currents that run in circles was confirmed. The layers of water rotated roughly as a rigid body, but with a different speed at different depths. That means that inside the ring the main velocity gradient was directed vertically. At the ring edge the main velocity gradient also had an important horizontal component. A picture of the measured Lowered ADCP velocities, averaged over layers of a thickness of 200 m, is shown in figure 10. As well as the geostrophic current derived from the density sections as the directly measured currents indicate that the rotational motion is detectable as deep as the bottom (4500 5000 m). This means that an Agulhas ring is a feature that is present over the whole water column. Figure 10. The velocity field in the ring Astrid as measured with the lowered ADCP during the MARE-I cruise. The velocities are averaged over depth intervals of 200 m, indicated by the rings in the figure, except for the surface layer that is averaged over 100 m. The maximum velocities at the surface are of the order of 1.5 m/s (3 knts). The individual discs seem to rotate almost as a rigid body with a strong velocity shear near the ring edge. Rotation of the ring is detectable as deep as the bottom 17
at 5000 m. The velocity field measured by LADCP corresponds well with the geostrophic velocity field as determined by density sections. In the gradient zones near the ring edges the isopleths of temperature and in particular salinity are very irregular although the isopycnals are smooth. This indicates interleaving of water masses at the ring edge, probably due to the strong horizontal velocity gradients at the edge. The stations 50 to 59, 60 to 69 and 70 to 89 constituted sections that have been worked in a search for another Agulhas ring: Laura. An indication for a weak ring has been found centered around station 75. The final section of stations 75 to 89 to the east entered ring Astrid again and showed that the ring had moved to the north-northwest over a distance of about 100 km in 2 weeks. This was later confirmed by ARGOS drifter positions. 1.5 Major Problems encountered during the Cruise A number of problems was encountered during the cruise, most of them were solved without great loss of time. a. The gyro-compass was broken down. This was detected in the harbour of Cape Town during the implementation of the software for the vessel-mounted ADCP. It was possible to find the right spare part in Cape Town in time, thanks to the shipping agent. b. The tow-link of the SCANFISH undulator showed considerable wear and had to be repaired. A different way of mounting this tow-link solved the problem. c. An axis of one of the undulator flaps was not secured well enough and caused almost the loss of the steering flap. Thanks to the ships technician this problem was solved. d. The line-puller of the FLY II micro-structure probe showed very much friction. Reparation of a ball bearing by the ships technician solved the problem. e. The FLY II winch cable easily gives problems when the FLY is recovered after a deployment. The winding process has to be guided manually, otherwise complicated knots are made. f. Some of the temporary sailors didn't have much experience with the handling of equipment. For this reason the captain advised to end a CTD section prematurely because of increasing wind at a moment where the regular crew should have operated still. Also due to this lack of experience a tow link of the multinet was damaged during the handling of the A-frame. g. On the way back to Cape Town, the ABC-system, that, among other things, saves the data of the underway measurement system, broke down due to a hard-disc crash. The problem was solved temporarily and partially by giving more tasks to another disc. h. The memory of the Lowered ADCP has to be read out regularly because of the limited size of the memory. This reading is going very slowly and requires very much time. Only thanks to the deployment time of the multinet, it was possible to empty the LADCP memories without too much time loss. Occasionally it was necessary to drop a LADCP measurement at a less 18
important station. It would be very nice to find a way to read the LADCP memories faster. This problem must be addressed to R&D Instruments Inc. i. The LADCP files tend to break up into parts. This problem seriously disturbs the processing of the LADCP data. A relation between file break-up and the battery power has been suggested, but in that case the battery voltage specifications have not been met. This problem will be addressed to R&D Instruments Inc. 1.6 Lists of Cruise Participants Scientific crew person responsibility Institute C. Veth Chief Scientist, micro-structure, CTD, LADCP NIOZ H.M. van Aken Hydro-Watch, CTD, ARGOS drifters, Data Management NIOZ J.D.J. Derksen Computer Network, Electronics NIOZ G.M. Ganssen Geology, Multinet, aquaflow pump, VU B.P. Groot Oxygen determination NIOZ M.A. Hiehle Hydro-Watch, Salinity Determination, Data Management NIOZ J.H.F. Jansen Geology, Multinet, aquaflow pump NIOZ G.W. Kraay Flowcytometer, pigments, NIOZ S. Ober CTD system, Thermometry, ADCP, LADCP NIOZ M. Rouault Meteorology UCT M. van der Vegt Hydro-Watch, CTD IMAU A.K. v. Veldhoven Hydro-Watch, CTD NIOZ N.C. Vervaet Hydro-Watch, CTD IMAU E.M. v. Weerlee nutrients determination NIOZ C.P. Whittle Hydro-Watch, CTD UCT NIOZ: Netherlands Institute for Sea Research, Texel IMAU: Institute for Marine and Atmospheric Research, Utrecht University UCT: University of Cape Town (South Africa) Ships crew John Ellen Marco van Duijn Bert Puyman Jan Pieterse captain first mate second mate first engineer 19
Jan Kalf second engineer A. de Wolf cook Roel v.d. Heide ships technician M. Verschoor sailor AB G. Struik sailor AB M. v.d. Ruit sailor AB 2 Underway Measurements 2.1 Navigation RV Pelagia has several different navigational systems. We used the Differential GPS receiver for the determination of the position. For a large part of the cruise only GPS was applied, because coast stations were not available within the reach of the ship. At certain periods of the day, mainly during the night shift of 00:00 to 04:00 GMT, strong jumps in the GPS-position occurred. The data from the GPS receiver were recorded every ten seconds in the ABC data logging system. After the removal of a few spikes these data were sub-sampled every 5 minutes. 2.2 Echo Sounding The 3.5 khz echo sounder as well as the navigational Furuno echo sounder were used on board to determine the water depth. The uncorrected depths from these echo sounders were recorded in the ABC data logging system. Over steeper parts of the bottom topography the depth digitizer of the 3.5 KC echo sounder was occasionally not able to find a reliable depth. The maximum range of the Furuno echo sounder to obtain reliable results was about 800 m. 2.3 Thermo-Salinograph Measurements The Sea Surface Temperature, Salinity, and Fluorescence measured continuously with an AQUAFLOW thermo-salinograph system with the water intake at a depth of about 3 m. For the calibration of the salinity sensor, water samples were taken regularly. 2.4 Vessel mounted ADCP measurements A 75 khz vessel mounted ADCP (RDI) recorded the water current field continuously down to a depth of 600 m below the ship. The quality of these data depend very much on reliable GPS and gyrocompass data. A first look to the VM-ADCP data shows that they are good, but it will take some time in the post-processing phase to get the final data. 20
2.5 Meteorological measurements A number of meteorological data are recorded by the ABC underway logging system. The following instruments are the standard meteo-set of the R.V. Pelagia as "selected ship", partially provided by the Royal Dutch Meteorological Institute (KNMI): Instrument range accuracy Wind anemometer 0-75 m/s 0.10 m/s Wind direction 0-360 degrees 1 degree Dry bulb air temp -30 50 ºC 0.05 ºC Rel. humidity 0 100 % 0.5% Air pressure 940 1060 hpa 0.06 hpa Global radiation 0 2000 W/m2 1% of measured value (PAR-sensor) In addition to the standard instrument set, specific sensors were installed by the participants of the University of Cape Town: 1.Radiation measurements: To calculate the net heat budget at the surface, measurement of the incoming short wave and long wave radiation was required. The outgoing long wave radiation is calculated as a function of the sea surface temperature and the outgoing short wave radiation is taken as a fraction of the incoming short wave radiation (7%). The incoming short wave radiation was measured with an Eppley precision spectral pyranometer. It has a thermopile detector with a long term stability. The pyranometer has two hemispheres, with the inner hemisphere blocking the infrared radiation from the outer one. The incoming long wave radiation was measured with an Eppley precision infrared radiometer. It measures the exchange of radiation between a horizontal blackened surface and the sky. Both devices were sampled with a Campbell CR10 datalogger system. 2.Backup relative humidity and temperature measurements Relative humidity and temperature measurements were supplied by a Vaisala HMP35D relative humidity sensor, housed in a radiation shield. This instrument utilises a HUMICAP, a thin film capacitive sensor that has become a reseach standard. The humidity and temperature were sampled every 10 s with a datalogger system. 3.CR10 Datalogger: The CR10 is a measurement and control system protected in a sealed and rugged canister. It provides sensor measurement, timekeeping, communication, data reduction and data and program storage. The datalogger interfaces with a wiring panel where the slow response sensors (pyranometer and radiometer) can be powered and sampled. Communication was also maintained with a laptop computer so that the data stored on the datalogger could be backed up to magnetic tape daily. 21
3 Hydrographic measurements - Descriptions, Techniques, and Calibrations 3.1 Rosette Sampler and Sampler Bottles A 24 position rosette sampler was used, fitted with 5 and 10 litre NOEX sampler bottles. A multivalve system, developed at NIOZ, allowed closing the sampler bottles by computer command from the CTD operator. The general behaviour of the samplers was good. No errors in the functioning of the rosette sampler itself could be detected. A problem with the NOEX sampler bottles that occurred during the MARE-0 cruise was solved. For a number of bottles an O-ring had to be replaced by one with another diameter and softer type of rubber. 3.2 Temperature Measurements No reversal thermometers were used for the calibration of the CTD-thermometer S/N 2118, but in principle the pre- and post-cruise factory calibration was adapted as standard. In laboratory circumstances the stability was < 0.001 ºC/year and the accuracy 0.002 ºC. For monitoring the behaviour of the temperature sensor of the CTD-system during the cruise a high precision SBE35 reference temperature sensor was mounted on the CTD-rack, which recorded the temperature every time a sampler was closed. Preliminary results of comparison of the primary CTD temperature sensor with the SBE35 sensor indicate that the temperatures showed a strong agreement. Comparison of CTD-thermo sensor and SBE35 are summarized in the table: diff. TEMP all >500 >1000 >2000 cleaned mean 0.0125 0.0070 0.0040 0.0013 0.0013 stdev 0.0473 0.0213 0.0120 0.0016 0.0012 median 0.0019 0.0019 0.0011 0.0011 0.0011 max 0.4511 0.0814 0.0719 0.0054 0.0036 min -0.0903-0.0903-0.0085-0.0011-0.0003 N 133 78 48 30 25 The final calibration of the temperature sensor S?N 2118 was completed before the end of the cruise. 3.3 Pressure Measurements Pre- and post-cruise factory calibrations of the CTD pressure sensor S/N 48349 showed a stability of 3 dbar over a period of 5 year. In the thermometer racks, mounted on sampler bottles, also SIS reversing electronic pressure sensors were placed. Before the cruise these sensors were calibrated by the manufacturer. Also readings of the deck pressure was performed with the SIS sensors to determine the zero offset. Comparison between the CTD-pressure sensor and the SIS sensors is summarized in the table: diff. PRES dprs dprs dprs dprs 22
(p6532) (P6500) (P6543) all mean -1.0-1.6-0.8-1.2 stdev 1.2 1.3 1.2 1.3 max 0.9 0.6 1.1 1.1 min -3.1-5.0-3.3-5.0 median -1.1-1.5-0.6-1.2 The final calibration of the pressure sensor S/N 48349 was completed before the end of the cruise. 3.4 Salinity Measurements Water was drawn from the samplers into a 0.25 litre glass sample bottle for the salinity determination after 3 times rinsing. The sample bottles had a stopper as well as a screw lid. The salinity of water samples was determined by means of an Guildline Autosal 8400B salinometer. The salinometer was installed in a laboratory container, fitted with an air conditioning system. This kept the surrounding air temperature constant within 1 C. The readings of the instrument were performed by computer, giving the average and statistics of 10 consecutive readings. For each sample 3 salinity determinations were carried out. The standard water used was from batch P134 with a K 15 ratio of 0.99989 (S=34.996). From each deep CTD/rosette cast an extra duplicate sample was drawn. Salinity determinations from the duplicate samples obtained from independent runs were used to determine the reproducibility of the salinity determination. The results from the water sample salinities will be used to determine the calibration of the CTD conductivity sensor. A preliminary analysis of the difference between the water sample salinity and the CTD salinity sampled is presented in the table: diff. SAL all > 1000 dbar > 2000 dbar mean 0.0003 0.0002-0.0009 stdev 0.0051 0.0053 0.0010 RMS 0.0051 0.0053 0.0013 median -0.0005-0.0006-0.0009 max 0.0391 0.0391 0.0015 min -0.0125-0.0125-0.0032 N 113 82 36 Post- and pre-cruise calibration of the conductivity sensor S/N 995 showed a stability of <0.0001 (S/m)/year and an accuracy of 0.0002 S/m. 3.5 Oxygen Measurements 23
For the oxygen determination water samples were drawn in volume calibrated 120 ml pyrex glass bottles. Before drawing the sample each bottle was flushed with at least 3 times its volume. The determination of the volumetric dissolved oxygen concentration of water samples was carried out by means of a spectrophotometer Winkler technique, recently developed at NIOZ [see Su-Chen Pai et al., Marine Chemistry 41 (1993), 343-351]. Before and after the cruise the spectro-photometer was intercalibrated with an automatic end point determination Winkler method. The stock solution of KJO 3 used in the analysis was prepared and calibrated in the laboratory by using gravimetric methods. The stock solutions were stored at low temperature (~4 C). At each cast where samples for the oxygen determination were drawn, duplicate samples and were drawn from the deepest water sampler in order to determine the precision of the analysis. 3.6 Nutrient Measurements From all sampler bottles samples were drawn for the determination of the nutrients silicate, nitrite, nitrate and phosphate. The samples were collected in polyethylene sample bottles after three times rinsing. The samples were stored dark and cool at 4 ºC. All samples were analysed for the nutrients silicate, nitrite, nitrate and phosphate within 10 hours with an autoanalyzer based on colorimetry. The laboratory container was equipped with a Technicon TRAACS 800 autoanalyzer. The samples, taken from the refrigerator, were directly pored in open polyethylene vials (6 ml) and put in the auto-sampler trays. A maximum of 60 samples in each run was analysed. The different nutrients were measured colorimetrically as described by Grashoff (1983): Silicate reacts with ammoniumolybdate to a yellow complex, after reduction with ascorbic acid the obtained blue silicamolybdenum complex was measured at 800 nm (oxalic acid was used to prevent formation of the blue phosphatemolybdenum). Phosphate reacts with ammoniummolybdate at ph 1.0, and potassiumantimonyltartrate was used as an inhibitor. The yellow phosphatemolybdenum complex was reduced by ascorbic acid to blue and measured at 880 nm. Nitrate was mixed with a buffer imidazole at ph 7.5 and reduced by a copperized-cadmium coil (efficiency > 98%) to nitrite, and measured as nitrite (see nitrite). The reduction-efficiency of the cadmium-column was measured in each run. Nitrite was diazotated with sulphanilamide and naftylethylenediamide to a pink coloured complex and measured at 550 nm. The difference of the last two measurements gave the nitrate content. Calibration standards were prepared by diluting stock solutions of the different nutrients in the same nutrient depleted surface ocean water as used for the baseline water. The standards were kept dark and cool in the same refrigerator as the samples. Standards were prepared fresh every day. The samples were measured from the surface to the bottom to get the smallest possible carry-over effects. In every run a mixed control nutrient standard containing silicate, phosphate and nitrate in 24
a constant and well known ration, a so-called nutrient cocktail, was measured, as well as control standards, sterilized in an autoclave or by gamma radiation. These standards were used as a guide to check the performance of the analysis. The autoanalyzer determined the volumetric concentration (μmol/dm3) at a temperature of approx. 20ºC. In order to obtain the densimetric concentration in μmol/kg the volumetric concentrations were divided by the density of sea water at 20ºC, sample salinity, and zero sea pressure. 3.7 CTD Data Collection and Processing The SBE 9/11+ CTD-system was fitted with temperature sensor S/N 2118 and conductivity sensor S/N 995. For the data collection the Seasave software, version 4.224, supplied by SBE, was used. The CTD data were recorded with a frequency of 24 data cycles per second. After each CTD cast the data were copied to a hard disk of the ship's computer network. Back-up copies were made on CD-ROM. On board the up-cast data files were sub-sampled to produce files with CTD data corresponding to each water sample, taken with the rosette sampler. On board the down-cast CTD data were processed with the preliminary calibration data, and reduced to 1 dbar average ASCII files, which were used for the preliminary analysis of the data. Full re-calibration and data processing will be carried out at NIOZ, Texel. 3.8 LADCP Data Collection and Processing Current velocity and direction data from the entire water column were measured with two synchronized self-contained 300 khz ADCP's mounted on the CTD frame. One of the two is downward looking (the master), the other one upward looking (the slave). Data collection takes place during the down- and up-cast of the CTD/Rosette. The data are subsequently stored in solid state memory inside the ADCP. The LADCP data collection was started a few minutes before the deployment of the CTD and was stopped immediately after the CTD was back on deck. Then the data were transferred from the internal solid state memory to the dedicated service computers, and subsequently copied in the appropriate directory on the ships computer network. A MATLAB master script file developed by Martin Visbeck, LDEO, version 4.0, Jan. 2000, has been used for data processing, data reduction and calculations of the currrent velocity and direction profiles. The master script file refers to several other sub-script files. Each of these sub-script file has a specific task controlled by the master script. Essential in the calculations are the correct input of start and stop times and start and stop positions of the CTD (time and positions are derived from the GPSsystem). The MATLAB programme plots the results of the measurements and calculations as well as 25
several quality parameters. The content of the master script and the results of the water profile and bottom track calculations are also stored in three separate ASCII files. See Chapter 1.5.h and 1.5.i for problems with the LADCP. 3.9 Data Management All raw data were copied to a cruise directory on the network computer of R.V. Pelagia in different groups of sub-directories. Subsequent processed data, final products, documents and figures were copied to separate sub-directories within the cruise directory. Daily back ups were made. At the end of the cruise copies of the whole cruise directory have been made on CD-ROM. By help of a range of measurement forms all data were tracked. A final overview of the hydrographic stations, water samples, and the available raw data was made in a cruise summary file and a water sample file. 4 Acknowledgements The research reported here is a part of the NIOZ/IMAU/KNMI contribution to the Dutch CLIVAR programme (CLIVARNET). The project was funded by ALW), subsidiary of the Netherlands Organisation for Scientific Research (NWO), contract no. 750.710.01. The see-going party of the MARE-community thanks the MARE home front for their continuous support in helping to locate the rings. In particular I like to mention Will de Ruyter, Sybren Drijfhout, Peter-Jan van Leeuwen, Taco de Bruin and Johann Lutjeharms. We thank the ships crew and the personnel of the supporting technical departments of NIOZ for their professional support and active participation in the preparation and execution of the MARE-1 cruise. Also Theo Buisman, Marieke Rietveld and the Cape Town shipping agency Meihuizen Int. for their logistic support. 26
Appendix A De weekberichten, The weekly messages (in Dutch) Proloog op de weekberichten van MARE-1 In deze proloog zal ik uitleggen waarom we met de Pelagia op deze afgelegen plek van de wereldzee zijn, ten zuiden van Kaap de Goede Hoop, en wat we hier proberen te meten. MARE staat voor Mixing of Agulhas Rings Experiment. Het MARE-project is een samenwerkingsproject van een aantal Nederlandse instituten (IMAU Univ. Utrecht, KNMI DeBilt en het NIOZ Texel) en buitenlanse instituten, vooral de Universiteit van Kaapstad. Het onderzoek vindt plaats in het kader van het internationale project CLIVAR betreffende variabiliteit van het klimaat en de voorspelling er van. De gedachte achter MARE is als volgt: In de wereldzeeën lopen een aantal stromen. Sommige hiervan worden aangedreven door de wind, andere door dichtheidsverschillen in het water. De dichtheid van water hangt af van bijvoorbeeld de temperatuur (warm water is lichter omdat het uitgezet is), of van het zoutgehalte (zout water is zwaarder dan zoet water). Deze eigenschappen van water houden een soort lopende band van bewegend water in stand aangedreven door verwarming en verdamping van water rond de evenaar en afkoeling in noordelijker streken. In het dagelijks gebruik van oceanografen wordt deze lopende band The Great Conveyor Belt genoemd. In de poolstreken koelt het water af en zal dan onder het toegenomen gewicht gaan zinken. Dit vindt vooral plaats in de noordelijke randzeeën van de Atlantische Oceaan. Dit koude water verspreid zich over de bodem van de oceanen en komt, na langzaam weer veranderd te zijn door menging met het andere water, naar boven in vooral de Stille Oceaan en de Indische Oceaan. Aan het oppervlak stroomt het water dan weer terug naar de Atlantische Oceaan en via het Caribisch gebied, waar het water flink warmer wordt en zouter (door verdamping), gaat het water even later als Warme Golfstroom weer naar het noorden. In west-europa danken wij daar ons zachte klimaat aan. Een heel rondje duurt bij elkaar gemiddeld 500 tot 1000 jaar. Wat heeft dit nu met Agulhas Ringen te maken? Het water van de Indische Oceaan dat terug wil stromen naar de Atlantische Oceaan doet dat niet gewoon als een nette stroom. Langs de Afrikaanse zuid-oostkust stroomt de Agulhasstroom weliswaar naar het zuidwesten, maar even voorbij Kaap de Goede Hoop komt deze stroom de grote stroom tegen die om Antarctica heendraait van west naar oost. Zo buigt de Agulhasstroom af naar het oosten en keert terug naar de Indische Oceaan. Dit hadden de oude zeevaarders al ontdekt en er hun voordeel mee gedaan. Vlak bij de kust had men een sterke tegenstroom, maar een stukje naar het zuiden had men de stroom mee op weg naar naar Indië. Bij het omdraaien van richting van de Agulhasstroom gaat wel eens wat mis en snoert de stroom eerder af. Het achtergebleven stuk stroom maakt een draaiende beweging en bestaat uit Indische-Oceaanwater dat nu zelfstandig in een omgeving van 27
Atlantisch water zit dat nagenoeg stilstaat. Deze draaiende bel Indische-Oceaanwater heet Agulhas Ring. Deze ring beweegt zich met een boog in de richting van zuid-amerika en keert tenslotte ook weer naar het zuiden om te verdwijnen in de grote stroom om Antarctica. De ring is echter aan slijtage onderhevig en zal water verliezen. Het is nu dit verloren water dat eigenlijk terugkeert in de lopende band. Het MARE-project. In het MARE-project sporen we zo n ring op. Dat kan met behulp van satellietgegevens bijvoorbeeld door het meten van het zeeniveau (Topex/Poseidon, ERS) of de oppervlaktetemperatuur. Omdat Indische-Oceaanwater wat warmer is, is het wat uitgezet en staat er als het ware een warm bobbeltje op het zeeniveau dat in het midden enkele decimeters hoger staat dan het omringende Atlantische water. Zo n bobbeltje heeft een diameter van 300 tot 400 kilometer. Deze ring zullen we doormeten en zien hoeveel extra warmte en zout er in zit vergeleken met Atlantisch water. Door na een half jaar en ook weer na een jaar de ring weer door te meten, kunnen we bepalen hoeveel water van de ring is afgestaan aan de Atlantische omgeving van de ring. We kijken dus eigenlijk naar de afgestane schilletjes van de ring. Agulhasringen en klimaat. Het afgestane ringwater brengt extra warmte en extra zout in de Atlantische Oceaan. Met computermodellen is uitgerekend dat er een verband is tussen het aantal ringen dat bij Kaap de Goede Hoop de hoek om komt en het klimaat in de noordelijke Atlantische Oceaan. Grofweg gezegd en ook zeker nog niet bewezen: minder ringen geeft een zwakkere Golfstroom en dus minder warmte in onze richting. Om meer over deze hypothese te weten te komen moeten we in het verleden kijken. De ringen brengen ook plankton mee uit de Indische Oceaan. Dit plankton gaat dood en zakt naar de bodem van de oceaan. Restanten van sommige soorten zijn terug te vinden in laagjes in de zeebodem. Geologen kunnen van die laagjes de ouderdom bepalen. Als er veel ringen zijn in een bepaalde geologische periode in het verleden, kunnen we eventueel veel restanten van Indische-Oceaanplankton aantreffen, en zo ook het omgekeerde. Interessant is het te weten of er bijvoorbeeld in de ijstijd, toen de Golfstroom heel anders liep minder of geen Agulhasringen waren. Misschien is een verandering in het aantal ringen dat per jaar ontstaat een indicatie voor een klimaatverandering. Wat doen wij nu aan boord? Aan boord van de Pelagia wordt een eerder uit satellietbeelden opgespoorde ring bezocht. Deze wordt vanuit verschillende richtingen doorgevaren en zoutgehalte- en temperatuurverdelingen ervan bepaald. Er worden metingen van de turbulentie gedaan in de randen om de wrijving met het omringende water te bepalen en dus de slijtage van de ring. Er wordt naar het soort plankton gekeken en hoe dit uitzinkt naar de bodem. Het laatste gebeurt met een verankering die al eerder is uitgezet. Telkens na een half jaar bezoeken we de ring weer en kijken hoe alles is veranderd. Om de ring weer terug te vinden gooien we voor de zekerheid ook nog wat boeien in zee, midden in de ring, die via satellieten hun positie aan ons doorgeven. Bij de tweede en derde tocht worden ook kernen uit de zeebodem genomen om de geologische laagjes met of zonder Indische-Oceaanalgen op te sporen. 28