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Earth Institute News

posted 11/29/04

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Reports From the Field

The Anslope Expedition: Cruising the Antarctic

Sunday 28 Nov 2004, 3:15a.m. local time
Dr. Gerd Krahmann aboard the R/V Nathaniel B. Palmer...

strong winds and high seas

Picture 1: In a few moments the back deck will be flooded by one of the waves coming over the railing.

Credit: Alison Criscitiello

74 S 3.7
176 E 12.1
90% ice cover
air temperature -5C
sea surface temperature -1.6C
12kn winds from the south

This is the third report from our cruise. A lot has happened since the last report three weeks ago. We visited the New Zealand port of Timaru to refuel the ship and then headed back south towards Antarctica. During our third crossing the usual strong winds and high seas caused the by now well-known seasickness (see Picture 1). In a few days we are up for crossing number four.

For the first half of our cruise we headed to the Mertz Glacier region, several hundred miles to the west of our original work area, the northwestern Ross Sea. As described, that diversion was due to heavy sea ice concentrations north of that area. This time we went along the original plan and after 6 days of crossing, we steamed into thick and dense sea ice. It took us another 4 days of breaking ice to reach the northern fringes of our work area. Unfortunately that was not all. A storm hit us shortly after we reached the region north of Cape Adare (see Picture 2) where satellite images had shown a Polynya, a region of open water within the sea ice. (Encyclopedia Britannica Online definition of Polynya:

satellite picture of sea ice

Picture 2+3: Satellite pictures of the sea ice in our work area before and after the storm.

Credit: USAP

satellite picture of sea ice

This storm closed the Polynya and was for a day so strong that the captain decided that it was useless to try to steam against the pressure of the ice. So, for more than 24 hours we just let the ship drift with the ice (see Picture 2). Since then the weather has been good to us. No more storms and a few really nice days with sunshine. Not that that meant that anybody was willing to enjoy it for more than a few minutes at a time as the air temperatures were and are still around the freezing point.

Since the storm we have been working around the clock, measuring temperatures, salinities, and currents and sampling the water at so far 50 locations in the northwestern Ross Sea (see other reports from the field and the web pages of the division of Ocean & Climate Physics at LDEO for descriptions of the methods employed

The results are somewhat surprising, in that little if any high salinity shelf water was observed on the slope, currents were not particularly strong, and the Antarctic Slope Front, which separates deep ocean from continental shelf waters, was located well south of the continental shelf break. Indeed, ocean conditions are not markedly different than what was recorded during the earlier late-summer AnSlope cruises in this area. While there are several possible explanations for this situation, including our timing vs. the tidal cycle, a more complete understanding will await analysis of the full AnSlope data sets. It seems increasingly likely however, that these data will challenge our present understanding of bottom water formation.

This week ended with a 24-hr VMP station in progress near the mouth of the Drygalski Trough. The whale and seal observers had a busy time too. Crabeater, Leopard, and Weddell seals as well as Minke, Killer, and probably a Fin whale have been observed. Emperor and Adelie penguins were a frequent sight too (see Picture 3).


Picture 4: Four Emperors staring at their feet.

Credit: Gerd Krahmann

So far 58 sonobuoys have been deployed to monitor underwater sounds. Sonobuoys were originally developed for the navy to listen for acoustic traces of submarines. A number of sonobuoys with an expired shelf life have been handed over to scientists studying marine mammals. Each sonobuoy records underwater sounds with a hydrophone (the underwater version of a microphone) at a preset depth of up to 300 meters and transmits it via a surface antenna as a radio signal to the ship. There the sound is being recorded and stored for later analysis. We found these sounds so intriguing that we developed a software that reduces the often very high level of noise caused by the ship and the motion of ice floes.

We have compiled a 3-minute sample of sounds recorded over a 1-hour period. According to the biologists most, if not all, of these sounds were made by the different species of seals.

Underwater Sounds (MP3) audio icon

A selection of underwater sounds recorded over a 1-hour period in the northwestern Ross Sea. The sounds are likely made by seals. Proper identifications will be made by marine mammal specialists after the cruise.

Credit: Sarah Dolman & Kelly Asmus