Seasonal to interannual upper-ocean variability in the Drake Passage

Year-round monitoring of the upper-ocean temperature variability in Drake Passage has been undertaken since September 1996 through repeat expendable bathythermograph (XBT) surveys. The closely spaced measurements (6-15 km apart) provide the first multi-year time series for examining seasonal to interannual variability in this Southern Ocean choke point. While the temperature sections reveal the seasonal variability in water mass formation of the upper layer, there was no seasonal signal evident below 200 m. Similarly, there was little seasonal cycle evident in the position of the Subantarctic Front, the Polar Front and the Southern Antarctic Circumpolar Current Front associated with the Antarctic Circumpolar Current (ACC) in Drake Passage. Mesoscale eddy features are readily identifiable in the XBT sections and in some sparse salinity sections, as distinct alternating bands separated by near-vertical isotherms of cold and warm core temperatures. The eddies can also be tracked in concurrent maps of altimetric sea surface height, with time scales of similar to35 days and diameters of 50-100 km, following a north to north-east trajectory with the main path of ACC flow through Drake Passage. Both the XBT and the altimetric data suggest the eddies are mainly confined to the Antarctic Polar Frontal Zone. To determine transport, an empirical relationship is derived between upper ocean XBT temperature and a baroclinic mass transport function from historical CTD casts collected in the Drake Passage. While in the temporal mean the strongest eastward transport is associated with the three major fronts in the ACC, the individual cruises strongly suggest-a banded nature to the flow through the passage. Some, although not all, of the eastward and westward bands of transport can be attributed to the presence of eddies. The high spatial resolution of the XBT measurements is more capable of distinguishing these counterflows than the typical 50 km resolution of historical hydrographic sections across Drake Passage. Commensurate with the position of the fronts, no real seasonal signal in Drake Passage transport is discernible, although there is substantial variability on interannual time scales. The Drake Passage XBT transport time series is strongly correlated to both zonal wind stress and wind stress curt in the southeast Pacific Ocean.