Mechanisms controlling the seasonality and formation of subsurface chlorophyll maximum and oxygen-depleted waters in the east-central equatorial Indian Ocean

The time-series observations of temperature, salinity, chlorophyll-a (chl-a), particle back scattering at 700 nm (b${}_{\text{bp}}$) and dissolved oxygen (DO) obtained from 2029 profiles of eleven autonomous biogeochemical (BGC) Argo profiling floats deployed in the east-central Equatorial Indian Ocean (EEIO) have been used to study the seasonal variability of chl-a and DO across the water column and to identify the mechanisms governing their variability. EEIO experiences surface oligotrophic condition throughout the year and exhibit a strong ($\sim$ 0.8 mg m⁻³) subsurface chlorophyll maximum (SCM) except during summer monsoon when chl-a concentration at SCM is weak ($\sim$ 0.65 mg m⁻³). The SCM is a persistent feature of the EEIO occurring at around 60 m. Similar to chl-a, the concentration of b${}_{\text{bp}}$ is low within the mixed layer, while maximum values are associated with the SCM. A coupled ocean-ecosystem model was employed to investigate nitrification, the uptake of nitrate (NO${}_3$) by phytoplankton (uptake), and the physical processes that maintain the availability of NO${}_3$ in the SCM region. We find that, among the biological processes, uptake dominates over nitrification in the SCM, as nitrification is inhibited by surface light, and the nitrification maximum occurs just below the SCM. The NO${}_3$ budget analysis also revealed that vertical advection from the nitrification maximum facilitates the availability of NO${}_3$ in the SCM region. Further, the vertical distribution of DO shows a narrow DO-depleted region in the upper 200 m of the water column between the depth of 100–200 m, which lies just below the Arabian Sea High Salinity Water and another wider and deep DO-depleted region between the 600–1200 m depth. The upper narrow DO-depleted water, driven by volume transport across 4°N, extends southward from the Bay of Bengal, reaching its maximum extent during January–February and August.