3-D Transport Pathways of Climatically Important Tracers in the Southern Ocean

The subduction of water masses, i.e. bottom water, mode water and intermediate water, is central to the sequestration and global redistribution of heat, carbon dioxide and other tracers. The Southern Ocean has been identified as a hot-spot for such subduction and in this study our aim is to identify the mode and intermediate water export pathways from the Antarctic Circumpolar Current (ACC) into the subtropical gyres, using a newly available observation assimilating model, the Southern Ocean State Estimate (SOSE).

Three hundred thousand Lagrangian particles were initialised at 700m depth, chosen to be below the mixed layer, with a uniform horizontal distribution every 1/6 of a degree between 65 and 35 South, and advected for 20 years in the 5-day averaged 3-D velocity fields available from SOSE. Approximately 55% of the floats left the ACC within 20 years, of which 20% entered the Pacific Ocean, 16% entered the Indian Ocean and 13% entered the Atlantic Ocean. Approximately 6% entered the region south of the ACC. Although some transport between the ocean basins and some transport from the ocean basins back to the ACC was observed, over the 20 years of simulation approximately 80% of floats which entered the Pacific, Indian and Atlantic oceans did not leave it.

We identify possible pathways from the ACC into the Pacific Ocean, and show that these may be influenced by the topographic steering, notably the Campbell Plateau that strongly deflects the ACC equatorwards. Evidence for this is the high concentration of particles which enter into the Pacific ocean within the first 5 years of the simulation initialised at (120-150)E (50-52)S. We examined the sensitivity of these results to various definitions for the Southern Ocean, ACC and the neighbouring gyres, including partitioning by sea surface height.