Water Mass Properties and Circulation on the west Antarctic Peninsula Continental Shelf in Austral Fall and Winter 2001

John M. Klinck, Eileen E. Hofmann, Robert C. Beardsley, Baris Salihoglu, and Susan Howard

Hydrographic measurements made during the U.S. Southern Ocean Global Ocean Ecosystem Dynamics cruises that took place from April to June and July to September of 2001, provide a description of changes in water mass distributions and circulation patterns in the Marguerite Bay region as a result of seasonal variability and offshore forcing by the southern boundary of the Antarctic Circumpolar Current (ACC). The primary seasonal change in water mass properties is the reduction in Antarctic Surface Water and replacement by a thick Winter Water layer. The primary effect of the ACC is to pump warm (> 1.5ºC), salty (34.65 to 34.7), and nutrient-rich Circumpolar Deep Water (CDW) onto the continental shelf below 200 m at specific sites that correspond to bathymetric features, such as the Marguerite Trough. The CDW was observed to intrude onto the continental shelf, move across shelf, and enter Marguerite Bay. This flow pattern was observed several times suggesting that onshelf intrusions of CDW are a frequent occurrence and that 4 to 6 of these events can occur during a year. A simple heat budget calculation suggests that a CDW intrusion can produce an equivalent surface heat flux to shelf waters of 52 W m^-2 over 60 days. Regions where CDW intrudes onto the shelf are characterized surface waters that are above freezing in winter. Thus, CDW intrusions potentially control heat and salt budgets for the west Antarctic Peninsula region. The hydrographic distributions also show a southwesterly flowing coastal current that enters Marguerite Bay around Adelaide Island and exits around Alexander Island. This current, which may results from seasonal, coastal buoyancy forcing, was present in fall and winter, but was better developed in fall. This current may be part of a larger cyclonic gyre that overlies the northern part of the area surveyed during the two cruises. This gyre provides a retention mechanism for planktonic organisms and a connection between inner and outer shelf waters.

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