Variability of the thermal front and its relationship with Chlorophyll-a in the north Bay of Bengal

Abstract

The spatial and temporal variability of the thermal fronts and their relationship with chlorophyll-a in the north Bay of Bengal (BOB) was studied using time series of MODIS daily Sea Surface Temperature (SST) and weekly chlorophyll-a concentration to investigate the locations of intense fish catch activities in the continental shelf and slope as inferred from NOAA Visible Infrared Imaging Radiometer Suite (VIIRS) nighttime boat detection products. An edge-detection algorithm operated on each SST image to identify thermal fronts in which a gradient of 0.3 °C/pixel (∼0.075 °C/km) was chosen following the sensitivity analysis. These thermal fronts at the daily scale were aggregated weekly to estimate the thermal front density. A least square-based harmonic analysis procedure was adopted to estimate mean climatology and seasonal harmonics of thermal front density and chlorophyll-a data sets. It shows persistent and coherent spatial features between the datasets in their climatology and seasonal cycles. They exhibit distinct locations of maxima co-occurring between the 10 and 50 m isobaths in the continental shelf and slope region coincided with the areas of the VIIRS mapped fishing vessels cluster sites. The annual frequency dominates the seasonal cycles of TFD with a positive phase during November–March (winter and spring seasons) and peaks in January. Mean (residual) circulation, as simulated from a regional ocean model, shows coherent spatial features congruent to the persistent thermal fronts and steered by the bathymetry. These locations with a high probability of thermal fronts and high concentrations of Chlorophyll-a can be considered as the perennial potential fishing zone, with the winter season being a more favorable period for fishing.