The impact of the broad-shallow Seychelles Plateau into the Southwestern Tropical Indian Ocean circulation.

Alma CAROLINA Castillo-Trujillo, Scripps Institution of Oceanography, Oceanography, La Jolla, CA, United States; University of Hawaii at Manoa, Honolulu, CA, United States, Sarah N Giddings, Scripps Institution of Oceanography, La Jolla, CA, United States, Geno R Pawlak, University of California San Diego, La Jolla, CA, United States and Isabella Beatriz Arzeno-Soltero, University of California Irvine, Civil and Environmental Engineering, Irvine, CA, United States
This study examines the interactions between the large scale circulation and the broad (150 - 300 km wide), shallow (50 - 200 m) Seychelles Plateau located in the southwestern tropical Indian ocean (STIO).

The STIO is tightly coupled to the Indian monsoon because of strong air-sea feedbacks as a result of the shallow Seychelles-Chagos thermocline ridge produced by the annual cycle of the wind stress curl. Biologically, because of this shallow thermocline, the STIO is also a highly productive area and therefore important for regional fisheries. The large-scale surface circulation, namely the geostrophic South Equatorial Current and South Equatorial Counter Current, along with the monsoonal-driven conditions, appears to also drive complex local and remote ocean dynamical responses atop and off-shore of the Plateau, that could, in turn, be influenced by its unique topography, though these connections are poorly understood.

Observations atop the Plateau, in the Republic of Seychelles, and a high resolution (~500 m) regional numerical model are used to study the interactions between the shallow topography and the low-frequency circulation. Acoustic Doppler current profiler (ADCP) velocity and temperature sensors reveal coastal dynamics dominated by the large scale monsoonal patterns. Atop the Plateau, both model and observations show swift zonal currents ranging from 0.5 to 1.5 ms-1 with high spatial variability not captured by lower resolution models. Around the Plateau flanks, the model shows flow acceleration and a rich submesoscale vorticity field with O(1) Rossby number, suggestive of a cascade of energy to smaller scales and the existence of sharp thermal fronts. Moreover, EKE weakening southwest of the Plateau observed in the model and observations is suggestive of a wake pattern and of the reduction of the westward propagating sea surface height (SSH) anomalies downstream of the Plateau. These patterns have a strong impact on the low-frequency evolution of surface tracers such as temperature and salinity. By highlighting the dominant dynamics, these results can be used to improve the STIO circulation forecasts and numerical models with steep topography, as well as aid Seychelles fisheries management and navigation planning in this strategically important, little studied region.