A biological dipole variability in the Indian Ocean

The Indian Ocean Dipole (IOD) is a prominent mode of climate variability in the tropical Indian Ocean (IO). It exerts a significant influence on biological activities in this region. To elucidate the biological response to the IOD, previous research has introduced the biological dipole mode index (BDMI). However, the delineation of the region by the BDMI has limitations in capturing IOD-induced chlorophyll variations in the IO. By analyzing observational data and historical simulations from a Coupled Model Intercomparison Project model, this study shows that chlorophyll anomalies in the IO exhibit a dipole pattern in response to IOD. During the developing and mature phases of the positive IOD, we observe a substantial decrease in chlorophyll in the south-southwest of India, contrasting with a pronounced increase in the southeast of the IO. This response is attributed to anomalous southeasterly winds induced by IOD, which enhance nutrient upwelling in the southeastern IO and suppress it in the south-southwest of India, resulting in corresponding changes in surface chlorophyll blooms. Based on this finding, we propose a new biological dipole index that more robustly explains the surface chlorophyll response to IOD in the tropical IO. This study highlights the profound influence of IOD on oceanic chlorophyll and underscores the importance of a more comprehensive understanding of the associated biophysical interactions.