Abstract
The ocean circulation around and over the Seychelles Plateau is
characterized using 35 months of temperature and velocity measurements
and a numerical model of the region. The results here provide the first
documented description of the ocean circulation atop the Seychelles
Plateau. The Seychelles Plateau is an unusually broad
(~200 km), shallow (~50 m) plateau,
dropping off steeply to the abyss. It is situated in a dynamic location
(3.5-5.5S, 54-57$E) in the south-western tropical Indian Ocean where
northwesterly winds are present during austral summer and become
southeasterly in austral winter, following the reversal of the Indian
monsoon winds. Measurements around the Inner Islands, on the Seychelles
Plateau, have been carried out since 2015. Velocity measurements show
that most of the depth-averaged current variance on the Seychelles
Plateau arises from near-inertial oscillations and lower-frequency
variability. Lower-frequency variability encompasses seasonal and
intraseasonal variability, the latter of which includes the effects of
mixed Rossby-gravity waves and mesoscale eddies. A global 0.1-deg
numerical ocean simulation is used in conjunction with these
observations to describe the regional circulation around and on the
Seychelles Plateau. Atop the SP, circulation is dominated by
ageostrophic processes consistent with Ekman dynamics, while around the
SP, both geostrophic and ageostrophic processes are important and vary
seasonally. Stratification responds to the sea surface height semiannual
signal which is due to Ekman pumping-driven upwelling (related to the
Seychelles-Chagos Thermocline Ridge) and the arrival of an annual
downwelling Rossby wave.