The physical and biogeochemical structure of eastward flows and eddies in the southeast Indian Ocean

A striking feature of the South Indian Ocean circulation is near-surface eastward flows that extend from Madagascar to Australia. The eastward flow concentrates into jets that have recently been described as branches of the South Indian Countercurrent (SICC), the northern branch of which feeds into the Eastern Gyral Current (EGC). The eastward flows are remarkable because they flow against the Sverdrup gyre circulation and against the wind-driven surface Ekman flow. There are near-surface eastward flows in the subtropics of other ocean basins, but none so strong, and none extending all the way to the eastern boundary, as in the South Indian Ocean.

Recent work has provided a detailed description of the observed 3-dimensional vertical structure of near-surface eastward flows and their seasonal variability from an exploration of gridded climatologies, altimetry and mean dynamic topography (Fig. 1). We complement this broad-scale view with a detailed description of the physical and biogeochemical structure of the southern SICC, the strongest jet, from hydrographic sections along 100E and 105E conducted in 2012 and 2013. Although the eddies that are ubiquitous in the region obscure the weaker eastward flows, we have quantified the volume and property transport of the near-surface flows that feed into the eastern boundary current, the Leeuwin. At deeper levels westward flow carries Southern Ocean watermasses into the interior of the Indian Ocean.