3-D Export Pathways from the Southern Ocean

The Southern Ocean has been identified as an important region for the sequestration of heat, carbon dioxide and other tracers and, more generally, for communication between the atmosphere and the subsurface ocean. This communication is accomplished through the subduction and upwelling of different water masses such as circumpolar deep water, mode water and intermediate water. This circulation is understood in a circumpolar-averaged sense, but the detailed 3-D pathways of these different water masses remains poorly known.

In the study reported here we map out and quantify transport through a Lagrangian approach in which large ensembles of particles (typically 10⁵-10⁶) are advected over periods of 10-20 years using offline 3-D velocity fields. The latter are taken either from the Southern Ocean State Estimate (SOSE) or from 20+ year integrations of the MITgcm at 5-day time resolution and at 1/6 degree spatial resolution.

We first consider possible definitions of the Antarctic Circumpolar Current (ACC) northern boundary including Lagrangian-based definitions. We then use these definitions to consider 'permanent' transport of particles between the ACC and the subtropical gyres. We show that this transport occurs in localised regions and investigate the dependence of these regions on the vertical level of particle release. We compare these results with the total transport, i.e. including transport which does not result in the particles staying in the subtropical gyres, and use this comparison to identify regions of 'efficient' transport from the ACC northwards and examine the upstream pathways within the ACC that lead to such export.

We examine and compare the above for the cases where the particles are advected in time mean and time evolving velocity fields, and also for particles advected by isopycnal velocity fields rather than 3-D velocity fields.