Investigation of the biophysical processes over the oligotrophic waters of South Indian Ocean subtropical gyre, triggered by cyclone Edzani

Abstract

The biophysical effects of a storm in the most oligotrophic waters of the South Indian Ocean (SIO) subtropical gyre have been investigated by conjunctive analyses using space-borne sensors and in situ observations. The most oligotrophic waters of the SIO are identified using more than 8-years of chlorophyll-a images derived from Aqua-Moderate Resolution Imaging Spectroradiometer (Aqua-MODIS). Earlier studies revealed that the source of oceanic primary production enhancement in these oligotrophic waters has remained inconclusive. However, the present study succeeded in attributing the cyclone, named Edzani, which passed over these waters and to be responsible for enriching the chlorophyll-a pigment, lowering of sea surface temperature (SST) and deepening of mixed layer. Analyses of MODIS Chlorophyll-a and SST images during the cyclone and pre-cyclone period shows lowering of SST values up to 2.23°C and chlorophyll-a enrichment up to 0.062mg/m3 from the pre-storm values along the cyclone track. Argo floats in the region recorded 10m deepening of mixed layer with an average mixed layer cooling of ∼1.34°C and 0.14‰ increase in salinity. These changes controlled by the physical processes have been examined using wind stress, wind stress curl and upwelling velocity derived from the new Advanced Scatterometer (ASCAT). The results provide a significant evidence to suggest that the frequent storms could possibly modify the prevailing oligotrophic conditions of the SIO subtropical gyres into a relatively productive environment, thus leading to regulate the global carbon cycle which is an essential component of climate change.