Monsoonal Variability in the Mesoscale Coupling of Wind and SST in the Arabian Sea

Abstract

Satellite observations over the Arabian Sea (AS) have revealed a strong signature of air-sea interaction during the Indian Ocean monsoon. In this study, satellite observations of ocean surface winds from the SCATSAT scatterometer are used to study the monsoonal variability in wind and SST (WS) coupling over the AS at the oceanic mesoscale for the period 2017 to 2019. The study investigates the role of critical parameters including SST gradient, wind speed steadiness and wind directional steadiness with respect to coupling during the winter and summer monsoons. Mesoscale WS coupling is stronger in the summer monsoon. A seasonally varying threshold of SST gradient required for the generation of WS coupling is clearly visible. This threshold is higher during the summer monsoon (0.8 °C/100 km) than during the winter monsoon (0.4 °C/100 km). Results reveal the need for a strong background wind field along with SST gradient for the generation of strong WS coupling. While a strong SST gradient ([ 0.8 °C/100 km) is sufficient to overcome the adverse influence of background wind conditions, strong support from the stable background wind fields is necessary in the regions with a relatively weak SST gradient ([ 0.4 °C/100 km) for the generation of coupling. The observed coupling between the SST and wind is quantified using a coupling coefficient. The coupling between wind stress divergence and downwind SST gradient is more prominent. It is observed that large warming of seawater (SST anomaly [ 1 °C) suppressed the positive linear coupling between anomalies of wind speed and SST in the western AS during the summer monsoon.