Preliminary results
In total, 164 CTD casts were done during EddyPump. The first 45 of these were performed at hydrographic stations aligned along the 10°E meridian. The majority of the other casts were spread over two CTD station grids, one termed Eddy1 station grid (see Table 3.1.1.1) centred around 51°12' S, 12°40' W within a large-scale open-ocean phytoplankton bloom and the other located to the north off South Georgia (see Table 3.1.1.2). The remaining CTD casts were done at Station PS79/85 (52°S, 8°W) located in a chlorophyll-poor open-ocean region and at St. PS79/86 (52°S, 12°W) to provide water column data in support of later interpretation of benthic samples taken there in the framework of the SYSTCO-II project.
Tab. 3.1.1: ANT-XXVIII/3 EddyPump: Eddy1 Station Grid during PS79
A B C D E
An additional Lagrangian Station, no. PS79/137, was located rows 4-5 / columns D-E, following the track of the buoy deployed earlier at the core.
Fig. 3.1.2: Horizontal distribution of density st at 300 m depth in the Eddy1 grid, based on CTD stations 98, 101 – 112 and 115 - 126 for synopticity reasons. The density contours also imply the geostrophic flow field, with streamlines running parallel to the density contours and the flow direction oriented in a way that the light water is located to the left (southern hemisphere). The flow speeds measured by the VM-ADCP (Section o.3)
in the mapped area however were rather low and hardly exceeded 30 cm s-1.
Tab. 3.1.2: ANT-XXVIII/3 EddyPump: South Georgia Basin Station Grid
G H I J K
6 APF meander Sts: 167
APF meander Sts: 168
APF meander Sts: 171 He
High Chl Filament Sts: 150
High Chl Filament Sts: 149
48°48' S
5 Low Chl Filament Sts: 166
Low Chl Filament Sts: 169
Low Chl Filament Sts: 172
Low Chl Filament Sts: 151
Moderate Chl Sts: 148
49°12' S
3. Physical oceanography
Final Core Station PS79/174 located rows 3-4 / columns H-I.
3.2 Physical Oceanography: helium sampling for assessment of upwelling
Matthew Donnelly1, Volker Strass2 not on board:
Oliver Huhn2, Monika Rhein2
1SoES
2IUP Univ. Bremen
3AWI Work at sea
During ANT-XXVIII/3 a total of 305 helium samples were obtained out of a potential 306 sample tubes: one tube was found to be corroded. Of the 305 samples taken, 8 tubes are potentially compromised owing to the shearing of bolts or the detachment of the plastic tube whilst closing the bottom bolts; however all of these samples are sealed and therefore available for analysis. A further 4 samples may have been compromised by initially unknown interference with other samplers of the Niskin bottles. Due to the demand for water from the Niskin bottles, helium sampling at some stations was undertaken across two casts.
Preliminary results
The helium sampling was primarily focused on two eddy regions. Two stations were surveyed on the approach to the first eddy in high and low chlorophyll areas.
At the first eddy a 5-station star survey was conducted, followed by an intensive 6 station north-western extension of the survey grid. The centre of the star was surveyed twice. In addition, the final station at the first eddy region was also surveyed. At the second eddy region north of South Georgia, there was another 5 station star survey. The stations at which helium samples were taken are indicated in Tables 3.1.1 and 3.1.2.
The samples taken will be used to calculate helium isotope disequilibria. Variations in the ratio of 3He, which is introduced into the deep ocean by hydrothermal activities,
and 4He allow estimation of deepwater upwelling rates and of the entrainment in the mixed layer. It is expected that the so-determined upwelling and entrainment rates, which will be analyzed in combination with the microstructure turbulence measurements described below in Section 3.5, will significantly contribute to constrain estimates of exchange rates between deep water and mixed-layer water mass properties in the Atlantic sector of the ACC.
3.3 Physical Oceanography: En-route measurements of currents and backscatter strength with the vessel-mounted acoustic doppler current profiler (VM-ADCP)
Volker Strass1, Hendrik Sander2
1AWI
2OPTIMARE Work at sea
Vertical profiles of ocean currents down to 335 m depth were measured with a Vessel Mounted Acoustic Doppler Current Profiler (type ‘Ocean Surveyor’;
manufacture of RDI, 150 kHz nominal frequency) nearly continuously when outside territorial waters. The transducer was installed 11 m below the water line in the ship’s keel behind an acoustically transparent plastic window for ice protection.
Echoes reflected by particles moving relative to the ADCP return with a change in frequency. The ADCP measures this change, the so-called Doppler shift, as a function of depth to obtain water velocity at a maximum of 128 depth levels. The instrument settings for this cruise were chosen to give a vertical resolution of current measurements of 4 m in 80 depth bins and a temporal resolution of 2 min for short time averages.
Determination of the velocity components in geographical coordinates, however, requires that the attitude of the ADCP transducer head, its tilt, heading and motion is also known. Heading, roll and pitch data are read by the ADCP deck-unit from the ship’s gyro platforms. The ship’s velocity was calculated from position fixes obtained from the Global Positioning System (GPS) or Differential GPS if available, and was taken over from the ship’s navigation system. A timeout error of the navigation data interface resulted in a data gap of several hours during 03. Feb. 2012.
Accuracy of the ADCP velocities mainly depends on the quality of the position fixes and the ship’s heading data. Further errors stem from a misalignment of the transducer with the ship’s centre-line and a constant angular offset between the transducer and the GPS antenna array, and a velocity scale factor. The ADCP data calibration and processing was done by using the CODAS3 software package (developed by E. Firing and colleagues, SOEST, Hawaii).
The ADCP also recorded the echo intensity, or backscatter strength, which can be analyzed in order to provide an estimate of zooplankton abundance. This estimate will be compared with the zooplankton abundance data derived from net catches.
3. Physical oceanography