Unveiling the Recent Changes in Barrier Layer Dynamics over the Arabian Sea Mini Warm Pool

Abstract

The reversal of monsoon wind and restriction of further northward oceanic heat transport makes the Indian Ocean unique compared to the other two tropical oceans. The circulation over the north Indian Ocean (NIO) also reverses following this change in the wind pattern. Two basins of NIO, i.e., the Bay of Bengal and the Arabian Sea, witness distinct physical and dynamical properties in response to this wind pattern and freshwater influx, although they lie within the same latitudinal band. The focus of this study is over the south-eastern Arabian Sea (SEAS) (7.5 - 12.5°N, 72.5 - 76.5°E), where sea surface temperature (SST) of more than 29.5°C is observed during late April and early May. This warm temperature over the SEAS is associated with the formation of a monsoon onset vortex that influences the onset of the Indian Summer Monsoon. Previous studies have suggested that high SST over the SEAS is independent of the tropical Indian Ocean warm pool. This high SST region is referred to as the Arabian Sea Mini Warm Pool (ASMWP). The development of ASMWP starts in November when the coastal Kelvin wave packets initiate the formation of an equatorward flowing boundary current along the east coast of India, East Indian Coastal Current (EICC). EICC transports the low saline Bay of Bengal water to the SEAS, resulting in a strong haline stratification which leads to the formation of a barrier layer. Once this layer forms, it restricts the vertical mixing of water in the mixed layer with the thermocline water. The objective of this study is to observe the recent change in the dynamics of this barrier layer thickness (BLT) over SEAS. Using reanalysis data from Copernicus Marine Services, the seasonal and yearly evolution of BLT is analyzed from 1993 to 2018. This study calculates the isothermal layer depth (ILD) based on the 1°C temperature criteria. The density change is computed following this temperature change which is used to calculate mixed layer depth (MLD). The monthly climatology suggests the presence of thick BLT (i.e., ILD - MLD) over SEAS from December to February, although some remnant is present in March. A seasonal average (December - February) of BLT suggests a significant increasing trend from 1993 to 2018. Although the MLD is not showing any significant changes, the ILD is witnessing a substantial increase over these years. The effect of the ILD increase is also reflected in the stratification and heat content. Using geostrophic eddy kinetic energy, the energetics of the EICC in October-November are noticed in three different regions along the southeast coast of India and south of Sri Lanka. The influence of local forcings on the dynamics of the BLT is investigated to understand the mechanism behind this evolution of BLT and its role in ASMWP variability.