Asymmetric basin-wide responses of coastal phytoplankton biomass to the Indian Ocean dipole.

Frequent Indian Ocean Dipole (IOD) events exert substantial ecological pressure on marine ecosystems of the Indian Ocean. However, how coastal ecosystems respond to IOD events remains unknown. Here, combining field observations, remote sensing, and model-derived product, we reveal a basin-scale response of surface and subsurface phytoplankton along the vast eastern Indian Ocean coastline; further, the response is asymmetric between IOD phases, with a 27.0 ± 17.0% increase during positive IOD but only a 13.1 ± 5.2% decrease during negative IOD. Upwelling Kelvin waves (KWs) generated by equatorial easterly wind anomalies during positive IOD raise the thermocline and mixing intensity, increase nutrient availability, and phytoplankton biomass. In contrast, downwelling KWs generated by equatorial westerly wind anomalies during negative IOD further constrain the oligotrophy-limited phytoplankton baseline, resulting in a substantially weaker biological response. Within the KWs framework, underwater topography enhances regional sensitivity to KWs forcing, thereby amplifying the asymmetric phytoplankton biomass responses. Our findings imply that the projected increase in extreme positive IOD events will likely enhance primary production, with possible implications for regional fisheries.