Differential Response in Chlorophyll-a Concentration to Seasonal Forcings along the West Coast of India

The chlorophyll-a (Chl-a) concentration, an indicator of biomass that is a conduit for fixation of atmospheric carbon dioxide, is analyzed using Ocean Colour Climate Change Initiative (OC-CCI) 4-km eight-day resolution data along the west coast of India. A peak of Chl-a blooms during the southwest monsoon (SWM) along the west coast of India and during northeast monsoon (NEM) in northern regions is observed. The blooms start as early as the end of April in southern regions and spread northward. The Fourier transformation and wavelet analysis explicitly reveals the annual and seasonal variability of these blooms and aids in grouping the regions into three zones. The blooms are strong in the southern regions and occur during SWM, whereas in the northern regions, they occur during SWM as well as NEM. The Indian Ocean Dipole (IOD) and Niño3.4 is negatively correlated with Chl-a concentration in southern regions only during a few months. Similarly, only for a few months do the northern regions show negative correlations between Niño3.4 and Chl-a concentration. The Chl-a blooms are positively correlated with concurrent Ekman mass transport and precipitation and with a lag of four to seven steps of eight-day cycles in southern regions, whereas the northern regions are positively correlated with concurrent precipitation only. The time of onset and end of blooms and the time span for their northward spread during SWM and southward spread during NEM vary from year to year. The different onset and end times of blooms and varying periods of blooms are vital for policy decisions on regulating fishing activity and establishing a ban period along the west coast of India for sustainable utilization of fishery resources.