Seasonal variability in the vertical current structure and kinetic energy in the Central Indian Ocean Basin

Seasonal variability in the currents at the subsurface (450–670 m), intermediate (1150–1370 m), deeper (3450–3670 m) and near-bottom (4270–5100 m) depths at three mooring stations in the Central Indian Ocean Basin (CIOB) were documented in 1995–1997. The current-meter moorings were deployed thrice at station 1 (MS-1, MS-1A and MS-1B) in the northern basin (∼10°S), and twice at stations 2 (MS-2 and MS-2A) and 3 (MS-3 and MS-3A) in the southern basin (∼15°S). In the northern basin, seasonal variability in the currents at 500 m and 1200 m depth is due to the seasonal north–south shift in the westward-flowing South Equatorial Current (SEC). Low-frequency (30–60 day) oscillation, superimposed on high-frequency (inertial, semi-diurnal and diurnal) fluctuations, is noticed at all the depths including near-bottom depth (5100 m) where the U-component of current was dominant. The variability of the currents at the intermediate depth range (1150–1300 m), encompassing the three stations of 500 km apart, indicates the existence of anticyclonic gyral circulation. The depth variation of kinetic energy (KE) emphasises the bottom intensification of currents with minimum KE at deeper depths followed by relatively higher KE at abyssal depths. In the northern basin, the total KE exhibits seasonal variation; it is high during southern spring–summer and low during southern fall–winter. The eddy KE constitutes a larger part of the total KE at synoptic scale (12–15 day) fluctuations in the northern basin and meso-scale (inertial, semi-diurnal and diurnal) oscillations in the southern basin.