Mary Tobin

Reports from the Field

The Antarctic AnSlope Expedition,
February 25 through April 11, 2003

March 4, 2003

AnSlope cruise, March 4 2003, from Arnold L. Gordon, Chief Scientist
Aboard the R/V NATHANIEL B. PALMER, Ross Sea, Antarctica...

En-route from McMurdo, Dr. Gordon's vessel passed through heavy sea ice, with ice thickness ranging between ten centimeters to two meters.

The AnSlope cruise has been underway for about a week. We left McMurdo Sound on 25 February and are presently in the western margins of the Ross Sea, near 72S, where we are measuring the characteristics of the ocean layering: the change with depth of temperature, salinity, oxygen, and varied chemical tracers from the sea surface to the sea floor. With these data we can decipher the processes that project the extreme climate conditions of the Antarctic coastal region onto the World Ocean. The primary aspect of this 'projection' is the injection of very cold, dense water into the lower kilometer of the World Ocean, contributing to ocean overturning.

At present we are in the middle of the task of setting out ten current meter moorings. The instruments attach to a cable and measure currents, temperature, and salinity for one year. This time series supplements the snapshot, high-resolution, spatial views of ocean layering and currents we obtain during the cruise. With the AnSlope data we will investigate the exchange of water between the deep ocean to the north and the cold dense waters within the Ross Sea to the south, addressing the primary objective of the AnSlope expedition. Next year, AnSlope cruise 3 will recover the moorings. In between, AnSlope cruise 2 will provide another glimpse of the oceanographic conditions within the AnSlope working area, as the current meters continue to gather data.

Dr. Gordon reports that the penguins are disinterested in his work.

En-route from McMurdo we passed through heavy sea ice conditions, nearly full ice cover, with thickness from 10 centimeters of the newly formed ice to 2 meters of ice that survived the summer melting season. Luckily the ice was not under compression from neighboring ice floes, so the ship was able to push floes aside and make progress on average at 6 knots.  We passed within a few 1000 feet of the giant iceberg called C-19, about the size of New Jersey. What a view! It's some 30 meters above the sea surface, balanced by some 250 meters of draft below the sea surface. As we passed through the ice we encountered many Weddell Crabeater Seals. Our attempts to free a mooring stuck on an ice floe to allow it to sink to the sea floor and begin its one-year recording period were curiously watched by  a playful seal bobbing its head in and out of the water.  There are penguins galore, but they did not take notice of us.


1. We are finding signs of strong mixing at the boundary between the Ross Sea and deep ocean to the north. This mixing blends with differing waters and may be key in determining the nature of shelf-slope exchange and descending dense water plume waters. Within this active mixing regime, probably driven by the tides, deep-water properties are mixed towards the sea surface. As the deep water is relatively warm - that is about 0C, whereas the freezing point of sea water is about -1.9C - its mixing to the sea surface prohibits ice formation, despite the air temperature of -10C. Surface water temperature was about -1.65C, well above freezing. During our work more than 10 miles north and south of the front, we observed newly formed sea ice, but not at the front. The mixing at the front seems to be a 'sea surface hot' zone, and a place where waters of different properties mix, perhaps influencing the nature of the exchange between shelf and slope.

2. Dense shelf water from at least two sources contribute to descending plumes in the western Ross Sea. These two types of water, one of cold low salinity water, the other also cold but saltier, may interact with each other to allow more dense water to reach the deep ocean without significant loss of their surface properties during descent. This concept needs to be fully investigated with the AnSlope data set and by model studies.

  next report (March 11, 2003) -->


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