Cold-air outbreaks amplify ocean cooling and primary productivity in the North Arabian Sea
Abstract
Cold waves (CWs) are periods of extremely low temperatures affecting north India during winter. This study investigates the response of the North Arabian Sea (NAS) to CWs and the associated processes during winter season from 1982–2020. Strong northerly to northwesterly winds associated with eastward-propagating western disturbances reach the Arabian Sea approximately 6–4 d before CW peaks over north India, initiating pronounced sea surface temperature (SST) cooling. SST anomalies decrease by more than 0.5 °C, with maximum cooling occurring 2–4 d after CW onset and persisting for several days. The cooling is primarily driven by enhanced surface heat loss, with latent heat flux anomalies exceeding −80 W m−2, associated with the intrusion of cold, dry continental air and enhanced evaporation. Mixed-layer heat-budget analysis confirms the dominant contribution of surface heat flux. The surface cooling and convective mixing deepen the mixed layer by more than 15 m over the northwestern Arabian Sea, promoting nutrient entrainment and a delayed biological response. Chlorophyll-a concentrations increase after day 4 and exhibit widespread positive anomalies exceeding 0.5 mg m−3 between days 15 and 25, particularly north of 12° N, linked to convective deepening of the mixed layer and nutrient entrainment. This adds to the predictive values of the biological activity over the North Arabian Sea and surplus fish landings. Interannually, CW frequency and duration exhibit substantial variability but no statistically significant long-term trends. Although La Niña years show a descriptive tendency toward more frequent and persistent CW activity, neither El Niño Southern Oscillation nor the Indian Ocean Dipole produces statistically significant differences in CW frequency, duration, or intensity. These results demonstrate a coupled atmosphere-ocean-biological response to winter CWs, linking synoptic-scale cold-air intrusions to ocean cooling, upper-ocean mixing, and enhanced biological productivity in the NAS.