Marine heatwaves (MHWs), extreme warm water events, significantly impact ecosystems. While surface MHWs are well-studied using satellite data, their vertical structure remains unclear. This study analyzes three ocean reanalysis datasets to identify MHWs in the epipelagic zone (0–200 m) using a four-dimensional spatio-temporal framework. Results show that about half of MHW events have continuous surface signals, while approximately one-third are entirely subsurface with no sea surface temperature (SST) imprint. The annual number of subsurface MHWs has significantly increased in the past three decades, correlating with subsurface warming. These findings highlight the limitations of SST-only MHW detection and the need for subsurface observations.

Di Sun, Furong Li, Zhao Jing, Shijian Hu, Bohai Zhang