Report of Activities on the RVIB N.B. Palmer Cruise 02-04

4 August 2002

 

The long transit from Punta Arenas, Chile to the first work site in Crystal Sound was nearly over by the end of 4 August. During the night, the N.B. Palmer steamed along the Bransfield Strait and into the northern end of the Gerlache Strait, while the physical oceanography group continued the XBT survey of the area. A sliver of a waning moon was setting in the northwest just as the first light of dawn, which took place around 0800, allowed the mountain ranges on both sides of the strait to become visible. For the most part, the transit was ice free allowing the ship to maintain a normal cruising speed of 10 to 11 kts.

 

The morning was an extraordinary opportunity to see the inner passageway (Gerlache Strait) in all its splendor. Clear skies for most of the morning provided superb views of the snow covered peaks and mountain glaciers that were sparkling in the bright sunlight. Broken gray clouds were draped over some of the peaks adding an extra dimension of texture to an already wondrous scene. Although the sun was out, winds of around 25 kts out of the west kept the wind chill well down into minus numbers and for the many in the scientific party taking pictures, being out on deck was a bit brutal.

 

The weather in this part of the world is also extraordinary in that it changes rather rapidly. Early in the morning the air temperature was around -1.9C and the sea temperature was -1.268C. The barometer remained fairly high at 1006.1 mb. By early afternoon, low clouds moved in and the mountain peaks were obscured. As we moved westward along the Bismark Strait in the late afternoon (1800) toward the open water of the Western Antarctic Continental Shelf, the barometric pressure, which had begun to climb earlier in the day reached a remarkable 1017 mb, while the air temperature dropped to -8.6C and the winds dropped to less than 5 kts.

 

Around 1830, we made the turn to head south towards the entrance to Crystal Sound. Water temperatures of -1.764C (approaching the freezing point), were substantially colder than in the inner passage. Within an hour of making the turn, we ran out of open water and were back into the pack ice and the consequent slower ship's speeds (6 to 7 kts)

 

We should reach Crystal Sound by mid-morning on the 5th of August where we will rendezvous with the L.M. Gould for a transfer of equipment and supplies, and two to three days of joint science operations. This will provide the investigators on the Palmer their first opportunity to deploy the over-the-side equipment and to begin collecting krill and other species for experimental work. The Crystal Sound area has on past cruises been a hot spot for krill and their associated predators (seals, penguins, sea birds, etc.), and a part of the work will involve a study of the hydrographic setting to provide some insight into why this is so.

 

CTD Group report (Eileen Hofmann, Bob Beardsley, Baris Salihoglu, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

After completing the XBT section across Drake Passage, the CTD group did two additional XBT sections during the transit to Crystal Sound. The first was along a transect that extended from Drake Passage through Boyd Strait and into Bransfield Strait. The second extended along the axis of Gerlache Strait. The section along the Gerlache Strait is a repeat of a section that was done in April 1993, which is described in Smith et al., (1999, Deep-Sea Research I, Vol. 46, 925-949). Comparison of this section with the new one will allow us to determine the consistency of the water mass distribution in Gerlache Strait.

 

The weather and sea state conditions during both XBT transects were calm, which allowed us to deploy T-7 probes (nominal depth of 760 m) over the ship's stern from the main deck. The bottom depth at most of the XBT drop locations was shallower or within a few 10s of meters of the maximum depth for the XBT probes, which provided coverage of most of the water column. The XBTs were dropped at 10-nm intervals and nine probes were dropped along each section.

 

Sea ice was minimal over most of the two transects. The ship moved through patches of sea ice that were separated by wide areas of open water. Whenever possible, we waited for open water to deploy the XBTs. The only problems with sea ice occurred at the first four XBT stations in Gerlache Strait when we encountered an extensive area of slushy sea ice. At these sites, the XBT wire became entangled in the sea ice and broke before the probe hit the bottom. The XBTs that broke at shallow depths were repeated.

 

The vertical temperature distribution along the first XBT section shows clearly the intrusion of Circumpolar Deep Water from Drake Passage through Boyd Strait and into Bransfield Strait. Temperatures below 200 m were greater than 1.0C, and reached a maximum of about 1.23C at 650 m to 700 m about mid-way through Boyd Strait. At the Drake Passage end of the section, there is a strong temperature front that likely separates the shallower coastal/shelf waters surrounding the South Shetland Islands from the deeper waters of Bransfield Strait.

 

At the end of the section at about -63 45.165′S; -61 21.505′W, a front separating the Circumpolar Deep Water and the Bransfield Strait water was encountered. This is the point at which the Circumpolar Deep Water turns northeast and flows along the southern flank of the South Shetland Islands. Our XBT transect continued further south into Bransfield Strait, so Circumpolar Deep Water was not present at subsequent stations. Rather, the temperature profiles showed vertical distributions that are characteristic of Bransfield Strait water.

 

Temperatures above 200 m were less than 0C and reached a minimum of about -1.75C, producing a thick Winter Water layer. It is interesting that surface temperatures were above freezing over the entire section. This is in contrast to the surface waters in southern Drake Passage which were all below freezing. This likely contributed to the lack of sea ice in the southern Bransfield Strait. Why these waters remain above freezing in winter is a topic for further investigation.

 

The vertical temperature distribution constructed from the XBT section along the Gerlache Strait showed a thick (about 100 m) Winter Water layer. However, temperatures in this layer were all above freezing, which is consistent with the large regions of open water encountered during the transit of Gerlache Strait.

 

At -64 32.920′S; -62 33.699′W near the opening between Brabant Island and Anvers Island, Circumpolar Deep Water flows into the Gerlache Strait. This appears in the vertical temperature section as a bolus of warm water (above 0C) at about 200 m, which extends north-south along the Strait for about 25 km. This feature was described in Smith et al., (1999). In the section done during NBP02-04, the temperatures associated with the inflow water are lower than seen in April 1993, which may result from mixing of this water with the colder surface waters that prevail in winter. It may be that the input and subsequent mixing of this warmer water is what keeps the surface waters of Gerlache Strait above the freezing point in winter. The surface waters above the point where this water enters the Strait were the warmest encountered along the section.

 

At about the mid-point of the Gerlache Strait section, the -0.2C isotherm intersects the bottom and forms a temperature front between the warmer Circumpolar Deep Water entering the Gerlache Strait from the southern end near Anvers Island and the colder waters that flow out of the Bransfield Strait at depth to the north. This front is similar to that described in Smith et al. (1999).

 

The final XBT was dropped at the southern end of Gerlache Strait around 1230 on 4 August. The hydrographic group is now preparing for the CTD casts and related activities that will take place when we reach Crystal Sound in about one day from now.

 

Marine Mammal report (Chico Viddi)

Seven hours of marine mammal observation effort were performed on 3 August, bringing the cumulative total number of hours for the first four days of the cruise to 13.6 hours. Early in the morning while transiting the Drake Passage, the mate in the bridge saw a small whale right in front of the vessel. Unfortunately, I was just taking the ice off of the windows, so the sighting was missed. A male elephant seal (Mirounga leonina) was observed on an ice floe at about 1005 at 32 to port and 450 meters from the vessel (-61 17.0′S; -62 52.91′W). Another elephant seal was observed (possible a female or juvenile) about a mile ahead, also on an ice floe. Finally, one and half hours later, another seal (not identified) was seen at about 0.8 nm on the port side from the vessel. We had a beautiful day, with spectacular icebergs and weather conditions.

 

Antarctica gave us an incredible day (5 August)sunny morning with pink, golden, and blue colors in a perfect sunrise. Today's weather conditions were excellent and the cumulative hours of marine mammal observations to date now total 21.5, of which 7.8 hr were done today. The most frequent marine mammal species seen today was the Antarctic fur seal (Arctocephalus gazella), which was first sighted at 0856 (-64 30.17′S; -62 21.91′W). Half an hour later (0924), a few more sightings were made bring the count up to more than 40 fur seals. The last sighting was done at 0938 (-64 33.45′S; 62 36.65′W). Most of the fur seals were on ice floes, except for one sighting in which about ten fur seals were swimming less than 50 m from the ship. Three crabeater seals (Lobodon carcinophagus) were sighted three different times, at 1132 (-64 46.75′S; 63 08.35′W), 1208 (64 51.69′S; 63 14.11′W) and 1259 (64 58.28′S; 63 25.34′W). Finally, two like humpback whales (Megaptera novaeangliae) were seen at 3.6 miles, 175 to starboard (-64 53.15′S; 64 08.37′W). No confirmation on species identification was made.

 

ADCP/OPC/MOCNESS Studies (Ryan Dorland)

The vessel-mounted Acoustic Doppler Current Profiler (153 kHz VM-ADCP, RD Instruments) on the NB Palmer measures currents over a depth range from 31 m to around 300 m, sometimes less depending on weather and ice conditions. ADCP Current data are processed through CODAS (Common Oceanographic Data Access System) software developed by Eric Firing at the University of Hawaii. Both the Palmer and Gould are set up with an automated processing system, which includes heading corrections from an Ashtech ADU-2 system, and some editing based on cruise-wide statistics.

 

Two data sets are produced daily, one with 15-min averages over the previous 36 hours, and another of one-hour averages for the entire cruise. Figures from these sets are available on the ship intranet at http://peale/ adcp/figures/index.html. In addition to these sets and figures, a 5-min high resolution data set for the entire cruise is currently being maintained and figures of interest will be posted daily. These data sets and figures are a first pass and should not be considered a final product.

 

The ADCP measured southeastern currents of over 150 cm/s near the South American continental shelf-break. From -56S to -57 45.0′S, currents were primarily SSE, measuring up to 50 cm/s near the surface and diminishing to around 20 cm/s at 200 m. A 10 cm/s SW flow was present around -58S. Between -58 30.0′S and -59 30.0′S, the currents were predominately eastward reaching 30 cm/s uniform with depth. Around -59 30.0′S, the five-minute ensemble averages were lost, coinciding with a loss of pcode navigation data. Further processing and analysis is required to determine the exact cause of the missing measurements. Ensemble data resumed around -61 30.0′S, but remains sporadic due to icy conditions. Single-ping data continued to be collected despite the lapse in the ensemble current data.

 

Current Position and conditions

We are currently steaming into Crystal Sound in 9/10 or 10/10 pack ice and our position on 5 August at 1114 is -66 30.639′S; 67 33.759′W. The air temperature is -5.3C and the sea temperature is -1.833C. Winds are out of NE (045) at 8 to 10 kts and the barometer is at 1018.6 mb. Skies are cloudy.

 

 

Cheers, Peter