Spatio-temporal variability of chlorophyll in the northern Indian Ocean: A biogeochemical argo data perspective

The Arabian Sea (AS) and the Bay of Bengal (BoB) form an integral part of the northern Indian Ocean, exhibiting distinct characteristics despite being in the same latitudinal region. The present study is aimed at assessing the surface and sub-surface chlorophyll-a (chl-a) structure of the AS and the BoB utilizing concurrent Biogeochemical-Argo measurements. As the calibration coefficients require post-processing evaluation to suit the regional waters, where the floats are deployed, two floats each in the AS (WMO ID: 2902093 & 2902118) and the BoB (WMO ID: 2902086 and 2902114) falling in the same latitudinal belt are chosen on a pilot basis for correcting the gain and offset of the derived chl-a. The chl-a derived using scale factor and dark counts provided by the manufacturer is further calibrated using satellite measured chlorophyll and modified chl-a values are obtained. The modified chl-a data obtained by applying the gain and offset was then used to study the variability of chlorophyll, deep chlorophyll maximum (DCM) and factors responsible for its variability in both basins. The present study upheld the existence of a permanent DCM in both the basins. Similarly, the relationship between DCM and mixed layer depth (MLD) is assessed and the DCM is found to vary at an annual mode in BoB and at a quasi-semi annual mode in the AS region. Strong correlation between DCM and depth of 26 degrees C isotherm was also observed in both AS and BoB and the rationale behind it was analyzed. Existence of sub-surface blooms in the pre-monsoon season in the upper 200 m of the AS region is observed. The time-series data from BGC-Argo floats revealed the bio-optical mechanisms and their relationship with the physical processes.