Enhanced Atmospheric Pollutants Strengthened Winter Convective Mixing and Phytoplankton Blooms in the Northern Arabian Sea

Abstract

The recent increase in atmospheric pollutants (aerosols) over the northern Indian Ocean due to an increase in industrial activity in South and Southeast Asia decreased air temperature (AT) leading to the strengthening of winter convective mixing and promotion of phytoplankton blooms in the northern Arabian Sea (AS).To test this hypothesis, the variability in meteorological and surface ocean properties was analyzed for a period of 4 decades (1979-2018) in the northern AS.The total and anthropogenic aerosol optical depth (AOD) increased at a rate of 0.05 and 0.07 per decade respectively in the recent decades associated with a decrease in AT over the northern AS during winter.The estimated decrease in AT due to increasing AOD accounts for 0.18�C over the AS and it is close to that of the mean decrease in AT from 1994 to 2018 of 0.25�C.This increased the difference in temperature between the sea surface and atmosphere and enhanced specific humidity in the atmosphere resulting in the strengthening of convective mixing in the northern AS as evidenced by the deepening of the depth of 20�C isotherm in the recent decades during winter.Enhanced nutrient concentration and phytoplankton biomass were observed in the photic zone during winter further confirming that convective mixing strengthened in the recent decades in the northern AS than hitherto hypothesized as weakening due to the warming of the Eurasian landmass.The enhanced nutrients supported diatoms blooms, which are prey for Noctiluca scintillans to initiate the bloom in the AS in the recent decades.