Stratification and wind changes shape future Indian Ocean chlorophyll

Abstract Large uncertainties in projected future changes of Indian Ocean chlorophyll across Earth System Models are largely attributed to differences in biogeochemical process representation. We show, however, that large biases in the historical physical state of the ocean and inter-model differences in future climate also play a substantial role. Using a bias-corrected ocean-only model forced by air-sea flux anomalies from multiple CMIP6 models, we find that correcting historical physical state biases amplify projected summer surface chlorophyll (SChl) changes by up to a factor of two and improves inter-model consistency. Heat flux driven enhanced upper ocean stratification, consistently reduces SChl by −10% to −40%, depending on the upwelling region. Wind-driven nutricline changes dominate responses in SChl, off southern India (−45% wind effect versus −30% from stratification) and Sumatra (+25% versus −12%). These results highlight the need to improve the representation of monsoonal winds and their future evolution to better constrain biogeochemical projections.