Intensified Responses of Phytoplankton to the Indian Ocean Dipole Under Greenhouse Warming

Abstract

Abstract The Indian Ocean Dipole (IOD) has been proposed to be a key driver of biological processes in the Indian Ocean (IO) in the present climate. Given the expected influence of global warming on both the properties of the IOD and the biogeochemistry within the IO, a key question arises: How will the relationship between the IOD and chlorophyll evolve in a warming climate? Here, utilizing simulations from the Coupled Model Intercomparison Project (CMIP) Phase 6 Earth System models, our findings reveal a notable intensification in the IOD-chlorophyll relationship under greenhouse warming. This intensification is linked to an increase in phytoplankton biomass during the June to November period of positive IOD years in the southeastern IO (SEIO). Interestingly, our analysis indicates a substantial rise in IOD-induced chlorophyll levels in a warming climate, despite a marked decrease in IOD-induced upwelling in the SEIO. The shallower thermocline leads to an increase in the mean nutrient concentration in the subsurface layer, thereby facilitating an enhanced anomalous nutrient supply to the surface layer, which contributes to increased phytoplankton biomass. Our study highlights the consequential effects of IOD on chlorophyll dynamics and underscores the need for improvement of Earth System Models to resolve our understanding of biophysical interactions in the IO in response to global warming.