Environmental controls on chlorophyll-a and dissolved oxygen variability in contrasting Basins of the Northern Indian ocean.

This study examines the contrasting environmental processes driving spatio-temporal variations in chlorophyll-a (Chl-a) and dissolved oxygen (DO) across latitudes in the north Indian Ocean, focusing on the roles of mesoscale eddies and Rossby waves, using biogeochemical Argo profiles and outputs from a high-resolution coupled ocean-ecosystem model. The model effectively reproduces the observed upper-ocean variability of temperature, salinity, Chl-a, and DO on seasonal to interannual timescales. Analysis of biogeochemical Argo profiles reveals that anticyclonic eddies (ACEs) deepen the oxycline, facilitating DO ventilation into deeper waters. The extent of this deepening varies linearly with eddy strength, with ACE-induced DO ventilation being more pronounced in the Arabian Sea (AS) than in the Bay of Bengal (BoB). While the model captures DO ventilation reasonably well, except for a few mild eddies. Nevertheless, mesoscale eddies contribute to DO replenishment in deeper waters across both basins. Chl-a and deep chlorophyll maximum (DCM) depth variability exhibit distinct regional patterns. ACEs cause a rapid downward movement of Chl-a, followed by immediate shoaling, creating a vertically homogeneous Chl-a column with unusually high surface concentrations across most of the AS, but this does not occur in the BoB. The DCM depth in the AS shoals progressively northward from the equator due to regional physical processes, whereas in the BoB, mesoscale eddies play a dominant role in controlling its variability. Rossby wave propagation also plays a crucial role in deeper oxygen ventilation, with these waves occurring more frequently in the BoB. In the AS, thermocline, oxycline, and DCM depth variability are influenced by Rossby waves propagating across different latitudinal bands. In contrast, in the BoB, Rossby wave effects are largely confined to the southern region, while eddy activity dominates variability in the central and northern regions.