Pathlines reveal cross-equatorial gyres of the Indian Ocean driven by seasonally reversing monsoon winds

Thursday 23 August 2018 - 14:00 to 15:00
NOC Southampton - Node Room (074/02) (Waterfront Campus).
Dr Lisa Beal, University of Miami
Interior, cross-equatorial transports are unique and significant phenomena of the Indian Ocean, fluxing heat and salt from the Northern to the Southern Hemisphere. We use real and simulated surface drifter trajectories to reveal three cross-equatorial gyres in the northern Indian Ocean that are driven by the seasonally reversing monsoon winds. Simulated trajectories are computed by advecting hundreds of thousands of particles through a monthly drifter climatology. We then depict the gyres using probabilistic pathlines, defined as the most common, or probable, trajectories that cross the equator within the Somali Current. 
Northward pathlines through the Somali Current in boreal spring and summer split into two gyres: A one-year loop contained west of the Maldives, described as a wind-driven overshoot of the Southern Hemisphere equatorial gyre; and a two-year figure-of-eight, encompassing the Great Whirl and reaching the eastern boundary via the Wyrtki Jet, to return westward within the North Monsoon Current. Both these gyres cross back into the Southern Hemisphere, driven by southward Ekman drift during the southwest monsoon. Southward pathlines through the Somali Current in boreal winter originate in the dissipating Great Whirl and feed eastward into the South Equatorial Counter Current. 85% of all simulated trajectories participate in these gyres, with some leakage into the Bay of Bengal (14%) and Mozambique Channel (8%).
A striking result of our analysis is the lack of pathlines linking the Somali Current and upwelling regions with the Arabian Basin, in contrast to the continuous coastal current that seasonal streamlines suggest. Geostrophic pathlines computed using a satellite altimeter climatology reveal slow, eddying connectivity between these regions below the surface.
Seminar category: 
POETS Corner