The 24th of August has been the second day in a row with rather benign weather on the continental shelf off the Western Antarctic Peninsula. Winds have been moderate out of the southwest and the skies partly cloudy to clear in the late afternoon, but the cold air remains. Occasional icebergs have been sighted out along the edge of the shelf, but they are lone sentinels compared to the numbers we saw closer to shore and in Marguerite Bay. We have completed the steam along the shelf break and we are now working our way from the edge of the continental shelf toward the Adelaide Island coast along transect line 3. Our current position at 1628 is -66° 30.229S; -71° 03.379W. Winds are out of the southwest (231) at about 20 kts and the air temperature is -17.5°C. The barometer is increasing slowly at 984.8 mlb.
The 23rd of August was spent steaming along the outer margin of the continental shelf to map the sea floor along that region and finishing the abbreviated observations at stations on the southern portion of the grid. About mid-day, we learned that the Gould was able to move northward without the Palmer's assistance and after the evening communication, it was clear that they were no longer waiting for us to come and lead them up the coast toward Palmer Station through the pack ice. This development required the modification of our plan to meet up with the Gould. An alternative science plan was prepared to enable ideas that have been expressed over the last few days to be incorporated, since the pressure to leave the survey area early and assist the Gould was lessening. To fill in the gaps in bathymetric data between transect lines along the continental shelf break, the decision was made to continue steaming along the shelf break until reaching station #13 on transect line 3. This line has special interest because it cuts across the northern side of a large clock-wise flowing eddy that, based on data collected earlier in the cruise, seems to be transporting oceanic water onto the shelf. At the shore end of the line, station #16 appears to have water with some oceanic properties. Work along this line to confirm the presence of that water type and to provide solid evidence of the impact that the eddy is having on this portion of the study area, is planned for the next two to three days.
Work was finished up in the early hours of 23 August at station #71 and described in the daily report for 22 August. A CTD cast to 800 m and a phytoplankton net tow were done at station #66, the last of the grid stations to be sampled for the first time this cruise. While sea beaming along the shelf break on the transit to the northeast, XBTs were dropped at stations #48, 46, and at the half-way points between the stations in order to define the hydrographic structure along the shelf margin.
During 23 August, skies were partly to mostly cloudy, but visibility was good. Winds were mostly in the 20 to 25 kt range out of the southwest as the region was under the influence of an approaching high pressure system. It remained cold, however, with temperatures remaining below -15°C. The ice pack conditions (see description by Friedlaender below) were no longer an obstacle to steaming and the Palmer easily made 6 kts during transit between station locations.
John Klinck reports that on 23 August, the CTD group did 2 CTD casts and 2 XBT launches in the transit along the shelf break going north.
Station #71 (785 m; CTD) has a rather deep mixed layer (140 m) which is typical of stations off the shelf. There is some temperature gradient in the mixed layer, although there is no salinity structure. Several strong temperature reversals occur between 200 and 240 m just above the temperature maximum (1.8° C) at 300 m. The oxygen minimum extends between 260 and 300 m.
Station #66 (1210 m; CTD) had a cast to 1000 m, because the PAR sensor stayed on. The mixed layer extends to 140 m with some density gradient. The pycnocline is fairly smooth and abrupt with some indication of layering between 250 and 300 m. The temperature maximum is somewhat deep (370 m) at which there is a local oxygen minimum. At 470 m is an even lower oxygen minimum with an associated local temperature maximum. One anomaly on this cast was a bag of delicately painted coffee cups that was observed on the rosette after the cast. An extensive investigation did not reveal the mechanism by which this bag came to be attached to the rosette, nor how it escaped the attention of the Baltic Room crew before the cast. The shrunken cups disappeared, so no evidence remains of the anomaly.
Station #48 (408 m; XBT) has a mixed layer to about 100 m with nearly uniform temperature. A steppy thermocline extends to about 280 m at which is the temperature maximum (1.7°C). Warm water is observed to the bottom.
Station #48 to 46 (1216 m; XBT) has a mixed layer to about 80 m. The bottom of the thermocline (200 to 300 m) has a number of layers with temperature reversals. The temperature maximum (1.85°C) is at about 350 m depth.
Ari Friedlaender reported that marine mammal observations began this morning at 0830 (68° 03.669S, 74° 52.01W) as the Palmer steamed towards station #66. Observations ceased from 1140-1350 while at station, and then continued on until losing light and visibility at 1500 (67° 28.488S, 73° 48.237) while approaching station #48. Sighting conditions were moderate to good with overcast skies and visibility at 1.5-2 nm. Ice conditions were 9/10 concentration of medium and large floes with some broken rubble and smaller cakes as well. Open water in thin leads were persistent throughout the day. The floes were much flatter, and it was obvious that there was not nearly as much pressure on the ice as further south or east inside Marguerite Bay. While no cetaceans were sighted, 4 crabeater seals and 3 Weddell seals were seen (See sonobuoy report for description of Weddell seal recordings).
Ana Sirovic reported that on 23 August the first sonobuoy that was deployed failed immediately. The signal from the second buoy was monitored for 1 hour 8 minutes (range for this buoy was 6.5 nm). This buoy was deployed 8.5 nm before station 48. Weddell seal calls could be heard on it for approximately 15 minutes. These were mostly upsweeps and downsweeps over a big frequency range (top usually at approximately 1.2 kHz and bottom at 500 Hz).
Chris Ribic and Erik Chapman report that on 23 August (JD235), they surveyed for 6 hours between stations #71, 66 and 48. The transit was parallel to the shelf break. Few birds were seen in this area despite there being more open water than has been seen in days. Overall, birds have been seen in low abundance throughout the study area. Adélie Penguins, in particular, have been in low numbers, and Chris and Erik have yet to find an area where significant numbers of birds are concentrated. Previous studies have found Adélies in large groups of 100's of birds along the sea ice edge during winter on the Antarctic Peninsula. It will be interesting to see if when we cross the ice edge on the way to Palmer or on our way north whether we observe this phenomena. This may also be an interesting area to investigate other biological processes that may be occurring at the ice edge. A summary of the daytime observations is the following:
Primary Ice: Vast floes, mostly at 8/10ths concentration
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, and S. Gallager):
BIOMAPER-II was not deployed during 23 August, but the electronic problems were identified and fixed, and the towed body is again ready for use.
There were no 1-m2 MOCNESS tows taken other than the one
described in the daily report for 22 August at the end of station #71.