Revisiting Antarctic Circumpolar Current Transport Estimates
Wednesday, 17 December 2014
The Antarctic Circumpolar Current (ACC) is the world’s strongest current, and acts as a conduit that transports water between Atlantic, Pacific, and Indian Oceans. ACC transport is a key metric used to evaluate the accuracy of ocean and climate models. The canonical transport of 134 Sv through Drake Passage, the narrowest choke point of the ACC, derives from a year-long experiment conducted in 1979 (DRAKE79). Recent studies suggest that this historical value may be biased low by as much as 20%. DRAKE79 transport estimates resulted from a complicated synthesis of historical data and in-situ measurements, and relied heavily on the outcome of referencing three hydrographic sections with directly measured currents. This study focuses on evaluating DRAKE79’s geostrophic referencing technique. We hypothesized that the horizontal spacing and temporal averaging of current meters led to a bias in the historical estimate. Southern Ocean State Estimate (SOSE) was used as a test bed to evaluate DRAKE79 methods. A mean AAC transport of 181.5 ± 17.6 Sv was obtained by applying DRAKE79 methods to 2005 SOSE output. This value is greater than SOSE’s “true” geostrophic transport (153.0 ± 5.7) by 29 Sv. This difference resulted primarily from linear interpolation between two lost moorings; however the horizontal spacing of the current meters did not resolve the narrow jets of the ACC regardless of mooring loss. Within SOSE, geostrophic transports referenced with velocities at all mooring locations resulted in a mean transport of 161.0 ± 10.2 Sv. Temporal smoothing of the reference velocities, using up to a 20-day running mean, had minimal impact on the mean transport estimate. A next step would evaluate whether the mooring positions should be modified within SOSE to capture the same circulation features as DRAKE79.