Pierre L'Hegaret (OCE department, RSMAS, Miami, Florida)
Evolving oceanic pathways through the Somali Current (in the western Indian Ocean)
We use real and synthetic drifters to reveal the seasonally-evolving pathways of the Somali Current in the western Indian Ocean on a monthly time scale. The basins in the northern Indian Ocean produce and are crossed by different water masses. Connectivities of the currents give indications on the thermohaline interactions, ventilation and heat content. Usually an Eulerian approach is used to describe the Indian Ocean circulation, linked to sparse observation. Nevertheless, Eulerian temporal averages can create artefacts of currents and connectivies in this region influenced by the monsoon reversal and thus can not describe the real exchange pathways. Hence, we simulate hundreds of thousands of synthetic drifters in both an absolute velocity field (from drifters) and in a geostrophic velocity field (from satellite measurements), in order to create statistical robustness, longer trajectories, and a separation of the Ekman and geostrophic components of the flow.
The simulated Somali Current, as the observed one, drives most of the exchanges between hemispheres, reverses seasonally, splits into coastal and offshore components during the southwest monsoon, and drives most of the exchanges between hemisphere in the western tropical Indian Ocean. Through this annual cycle, connections with other seasonally sustained currents form three different equatorial gyres at different periods, taking 9 months, one year and two years to loop back to the Somali Current and representing 70% of our synthetic trajectories. We are able to further asses the main mechanisms driving the connectivities between the currents. The equatorial gyre is sustained by geostrophy year-round, but the wind-induced circulation interrupts its northern branch in late southwest monsoon, allowing drifters to cross the equator southward. Each of the Somali Current branches during this season is also linked to different mechanisms : the coastal pathway joining the Great Whirl is geostrophic, whereas the offshore ejection is wind-driven.
A striking observation of our analysis, is the lack of surface connectivity between the Somali Basin and the northeastern Arabian Sea throughout the year. The exchanges between these basins, supposed to radiate from the Great Whirl, are blocked by westward propagating Rossby waves modifying the sea surface elevation, creating a frontier, and advecting the drifters from the Great Whirl to the equator eastward.