Seasonal to interannual variability from XBT Measurements across Drake Passage

In September 1996, year-round monitoring of the upper-ocean temperature variability in Drake Passage was initiated via a high resolution XBT sampling program on the U.S. supply vessel to Palmer Station, Antarctica. A total of 16 transects are available to date, sampling approximately bi-monthly. Typically 65 XBTs are dropped per cruise, with station spacing of about 6-10 km when crossing the Subantarctic and Polar Fronts, and 10-15 km elsewhere. Some cruises also included sparse sampling with XCTDs, that provide both temperature and salinity information. Both the XBTs and XCTDs sample to around 800 m, providing a high density record of subsurface variability along the track. Few observational measurements are available for Drake Passage during the harsh winter months, and the XBT/XCTD program provides a fascinating insight into the evolution of the surface layer thermal variability over the three year period. The paper will focus on describing the temperature structure in Drake Passage in general terms of frontal positions and the dominant features between and within each transect. For example, during the September (winter) survey of each year of the program, a cold core feature is found near 56S, which may be either a meander of eddy of the eastward jet associated with the Polar Front. Preliminary estimates suggest that this feature tends to reduce the heat content of the region north of the Polar Front, but south of the Subantarctic Front, in the subsequent survey (typically November) when air-sea interaction indicate the water column should be warming. Comparisons of the summer sections from this recent XBT program with the historical data provided by the 1976-77 ISOS XBT sections will also be discussed, in terms of any longer time changes evident in the section across Drake Passage.