Asymmetric effects of Indian Ocean dipole on surface chlorophyll variability in the Indian Ocean

The Indian Ocean Dipole (IOD) is a dominant climate mode in the Indian Ocean, characterized by oscillating sea surface temperature anomalies between the western and southeastern equatorial regions. The IOD influences surface circulation, affecting coastal upwelling (downwelling), thereby increasing (decreasing) surface nutrients. These changes alter surface phytoplankton populations, measurable through their photosynthetic pigment, chlorophyll-a. The positive (pIOD) and negative (nIOD) phases of the IOD affect surface chlorophyll differently, particularly along the west of Sumatra (WS) and southwest of the Indian subcontinent (SWI). While changes in surface chlorophyll occur oppositely during each IOD phase, it is unclear how the intensity of surface chlorophyll changes during the positive and negative IOD phases. Given its importance, we investigate this question using observational data and historical simulations from a Coupled Model Intercomparison Project Phase 6 (CMIP6) model. Our findings reveal that although the chlorophyll response follows a dipole pattern, the pIOD phase exerts a dominant influence, with chlorophyll intensity during pIOD exceeding that during nIOD. Specifically, during pIOD, chlorophyll concentrations increase in the WS region and decrease in the SWI region, showing pronounced asymmetry in the intensity of IOD-induced chlorophyll changes. This disparity is driven by anomalous southeasterly winds during pIOD, which enhance nutrient upwelling in the WS and suppress it in the SWI, leading to variations in surface chlorophyll blooms. Thus, our results emphasize the asymmetric nature of IOD-induced changes in surface chlorophyll in the Indian Ocean, with important implications for marine food webs, fisheries, ocean biogeochemical cycles, and global climate systems.