Two Critical Radii Dominate Phytoplankton Response to Marine Heatwaves in the South China Sea

Abstract Marine heatwaves (MHWs) are intensifying globally, yet their impact on phytoplankton is typically assessed as a spatially uniform thermal stress, overlooking their internal structure. Here, we investigate the spatial coupling between MHW intensity and chlorophyll‐a (Chl‐a) in the South China Sea using satellite observations (2003–2020). By applying a segmented linear regression model, we statistically identified two robust critical radii (150 km and 380 km) that demarcate distinct biophysical regimes. In warm seasons, Chl‐a anomalies exhibit a distinct “increase‐decrease‐increase” radial pattern, with a significant suppression zone within the core (<150 km). Dynamical analysis reveals that this core suppression is driven by a “double‐lock” mechanism: enhanced thermal stratification coupled with wind‐driven Ekman downwelling. This synergistic barrier physically blocks vertical nutrient supply, creating an oligotrophic core. Conversely, a divergent regime emerges in winter, where Chl‐a exhibits a sustained increase even within the core. We identify anomalous Ekman upwelling in the winter MHW core as the key driver that overrides thermal stratification, maintaining nutrient flux. These findings demonstrate that the biological impact of MHWs is not solely determined by temperature magnitude but by the spatially structured competition between thermal stratification and wind‐driven mixing.