The Influence of Monsoon Variability on the Circulation of the Near-Surface Indian Ocean and the Depth-Integrated Chlorophyll

The Indian Ocean experiences a strong semiannual reversal of monsoon winds, which determines the weather and climate of Asia, including freshwater fluxes between the atmosphere, land, and ocean. Studies and model projections suggest that the timing and intensity of the seasonal monsoon have already started to change, with more dramatic changes likely in the future. However, the impact of these changes on the Indian Ocean circulation system, including the inter-basin salt/freshwater transport between the Bay of Bengal and the Arabian Sea, remains unclear. To better understand how monsoon variability affects Indian Ocean circulation patterns, a Regional Ocean Modeling System simulation of the Indian Ocean has been developed, incorporating riverine freshwater fluxes. The model focuses on five possible monsoon scenarios: early arrival of monsoon, late arrival of monsoon, strong monsoon, weak monsoon, and normal monsoon. The results show that the most prominent changes occur when the timing of the monsoon shifts, rather than its intensity. The net annual westward transport of the monsoon current south of Sri Lanka increases when the monsoon arrives early and decreases significantly during a late monsoon. Understanding how boundary currents respond to shifts in monsoon timing and strength is crucial for predicting future changes in marine productivity, as these currents regulate upwelling intensity and nutrient advection. To assess how marine productivity has already been affected in this region, Biogeochemical-Argo data from 2013 to 2022, comprising over 9,000 individual profiles, were used to examine regional patterns and trends in depth-integrated chlorophyll and stratification. Unlike satellites, which are limited to surface measurements and affected by clouds and turbidity, Biogeochemical-Argo data provide cloud-independent chlorophyll profiles from the surface to a depth of 2000 meters. The analysis reveals that water column stratification is increasing most rapidly in the western Bay of Bengal. Depth-integrated chlorophyll concentrations are increasing across large areas of the Bay of Bengal and the western Arabian Sea. Climate modes show a weak negative correlation with depth-integrated chlorophyll in the northeastern Bay of Bengal. The results suggest that depth-integrated chlorophyll is not strongly or consistently correlated with stratification, indicating that projected increases in stratification due to ocean warming may not necessarily drive concurrent reductions in primary production.