Eight Year Timeseries of Weddell Sea Bottom Water Outflow

Two moorings positioned south of the South Orkney Islands have provided a nearly 8-year record of the currents and thermohaline stratification within the lower 500 m of the outflow of dense Weddell AABW, revealing significant seasonal and interannual variability during the measurement period (April 1999 - February 2007). At mooring M3 (4565 m depth), an annual pulse of the coldest bottom water is evident in the May-July period, though the precise timing and duration varies with year. Intensification of the near bottom stratification is observed as the bottom water attains its coldest values. The coldest bottom events occurred in 1999 and 2002, while in 2000 it was absent. At bottom temperatures <-0.8°C the salinity fluctuations produce a 'fan-like' appearance in T/S space suggesting a varied source of dense shelf water. The coldest bottom water <-1.0°C is relatively salty indicating a source in the southwest Weddell Sea, about 1300 km along isobaths to the mooring site. The typical bottom speed at M3 of 10-15 cm/sec implies shelf water export of deep and bottom water components during the austral summer. A record at a second mooring (M2) at 3059 m depth displays a much reduced annual cycle, but it too records a relatively warm period in 2000. Correlations of the M3 time series with NINO3.4 and SAM suggest that these indices lead M3 on the order of 14-20 months, implying a likely relationship between the water mass and surface forcing. Both M3 and M2 were reinstalled in March 2007.