The Lagrangian View of South Atlantic Interocean Exchange in a Global Ocean Model Compared with Inverse Model Results

Abstract

Data from a global ocean general circulation model (OCCAM) has been used to investigate the interocean exchange of thermocline and intermediate waters in the South Atlantic Ocean. To resolve the pathways between different ocean basins a Lagrangian particle following technique has been used. The results have been compared with various inverse models and observational studies addressing the interocean exchange in the South Atlantic Ocean. To facilitate the comparison, section-integrated transports in the same density classes and at the same locations as used in the observational studies have been calculated for OCCAM. The flow toward the North Atlantic excluding the Antarctic Bottom Water, is made up for more than 50% of thermocline water. The exact ratio of thermocline to intermediate transport depends on the definition of the water masses. Transport of intermediate water plays a less important role. More than 90% of the flow toward the North Atlantic originates from the Indian Ocean via leakage from the Agulhas Current system. Agulhas leakage into the South Atlantic occurs to 2000-m depth, but transport below 1200 m recirculates within the subtropical gyre and flows back into the Indian Ocean. Several observational studies have indicated a dominant role in the transport toward the North Atlantic for intermediate water or for the direct inflow from Drake Passage. The section-averaged water mass transports in OCCAM are largely in agreement with these observational estimates. Also in OCCAM, the section-integrated transports suggest a minor contribution from Agulhas leakage to the upper branch of interocean exchange in the South Atlantic, in apparent contradiction with the Lagrangian path that was calculated explicitly. The reason for this discrepancy is that at the eastern side of the South Atlantic the net mass flux consists of opposing, and in the thermocline layer nearly compensating, east- and westward flows. In the thermocline layer, part of the westward flow connects with the cross-equatorial flow in the Atlantic, while the eastward flow is partly derived from upwelled intermediate and thermocline water that originates from Drake Passage. The detailed Lagrangian analysis suggests that it is arguable to draw conclusions about the flow pathways from integrated mass fluxes across ocean sections, especially when these contain opposing flows in the same density classes.