Role of physical processes in chlorophyll distribution in the western tropical Indian Ocean

Abstract Physical control of the chlorophyll a (chl a ) distribution in the western tropical Indian Ocean (WTIO, 8°N to 18°S along 65°E) was studied during the 2008 winter monsoon (WM) and the 2009 summer monsoon (SM). During both seasons, a prominent deep chlorophyll maximum (DCM, 0.3–0.5 mg m − 3 ) was observed at all stations between 8°N and 10°S in the depth range of 50–75 m, but south of 10°S, this phenomenon was observed as deeper (~ 120 m) and relatively weak (0.15–0.3 mg m − 3 ). During the SM, in addition to seasonal forcing, eddies and a freshened surface layer also played major roles in controlling the DCM and the surface chl a concentrations in the southern Arabian Sea and the equatorial Indian Ocean. During the WM, surface freshening controlled the chl a distribution in the Seychelles Chagos Thermocline Ridge (SCTR, 5°S–10°S) region by modulating the static stability and mixed layer depth. It appears that the surface freshening in this region is associated with the core of the South Equatorial Current. South of the SCTR, the chl a distribution was predominantly determined by the anti-cyclonic eddies in both seasons. The spatial patterns of the Sea Level Anomaly (SLA) followed most of the thermocline features observed during the study period.