Bottom Temperatures in Drake Passage

Abstract

ong time series of bottom temperatures in the Southern Ocean are rare. The cDrake array with over 40 current- and pressure-recording inverted echo sounders, moored across Drake Passage to monitor the Antarctic Circumpolar Current (ACC) variability and transport, measured temperature at 1 and 50 m above the seafloor at depths ≥ 3500 m and at the southern continental margin. The 4-yr dataset provided an opportunity to examine the temporal and spatial scales of bottom temperature variability. High variability was observed; ranges were 0.5°?0.9°C in the northern passage and 0.3°?0.6°C in the southern passage. Standard deviations in the two regions were 0.1°?0.15°C and <0.05°C, respectively. Meandering of the ACC with its deep-reaching thermocline accounted for up to 50% of the observed bottom temperature variance. Northern passage temperatures, spaced less than 40 km apart, were correlated with each other, while those in the southern portion, separated by 60?70 km, were not. A gap in the West Scotia Ridge provided a deep passageway for cold water to reach the northern passage from the southern basin; an extreme event during February 2008 brought bottom waters with in situ temperatures below 0.38°C as far north as 57°S. Strong vertical temperature gradients between 1 and 50 m above the bottom occurred intermittently due to intrusions associated with deep eddy circulations arising beneath the meandering jet and to flow over steep topography, permitting the generation of internal waves. High variability in temperature on interannual time scales requires record lengths of 13?17 yr to estimate long-term trends reliably.