Seasonal variability of the observed barrier layer in the Arabian Sea

Abstract

The formation mechanisms of the barrier layer (BL) and its seasonal variability in the Arabian Sea (AS) are studied using a comprehensive dataset of temperature and salinity profiles from Argo and other archives for the AS. Relatively thick BL of 20-60 m with large spatial extent is found in the central-southwestern AS (CSWAS), the convergence zone of the monsoon wind, during the peak summer monsoon (July-August) and in the southeastern AS (SEAS) and northeastern AS (NEAS) during the winter (January-February). Although the BL in the SEAS has been reported before, the observed thick BL in the central-southwestern AS during the peak summer monsoon and in the northeastern AS during late winter are the new findings of this study. The seasonal variability of BL thickness (BLT) is closely related to the processes that occur during summer and winter monsoons. During both seasons, the Ekman processes and the distribution of low-salinity waters in the surface layer show a dominant influence on the observed BLT distributions. In addition, Kelvin and Rossby waves also modulate the observed BL thickness in the AS. The relatively low salinity surface water overlying the Arabian Sea high-salinity water (ASHSW) provides an ideal ground for strong haline stratification in the CSWAS (during summer monsoon) and in NEAS (during winter monsoon). During summer, northward advection of equatorial low-salinity water by the Somali Current and the offshore advection of low-salinity water from the upwelling region facilitate the salinity stratification that is necessary to develop the observed BL in the CSWAS. In the SEAS, during winter, the winter monsoon current (WMC) carries less saline water over relatively high salinity ambient water to form the observed BL there. The winter West India Coastal Current (WICC) transports the low-salinity water from the SEAS to the NEAS, where it lies over the subducted ASHSW leading to strong haline stratification. Ekman pumping together with the downwelling Kelvin wave in the NEAS deepen the thermocline to cause the observed thick BL in the NEAS.