Marine Heatwaves Have Increased in Frequency, Duration, and Depth Across Southeast Asia

Abstract Southeast Asia supports high marine biodiversity that is increasingly exposed to intensifying marine heatwaves (MHWs). Despite the ecological and socio‐economic consequences of thermal anomalies, the regional spatio‐temporal dynamics and vertical structure of MHWs remain poorly constrained. Here, we use satellite‐derived surface and reanalysis subsurface temperature products to quantify: (a) regional and seasonal variability in MHW characteristics; (b) the associated physical drivers of MHWs; and (c) the vertical extent of ocean warming. MHW frequency increased from <2 events yr −1 in the 1980s to 5 events yr −1 by the 2010s, with a sharp escalation post‐2003 (108% increase in frequency; 63% increase in duration). MHWs occurred most frequently during the southwest monsoon (June–September) but exhibited maximum duration and intensity during the northeast monsoon (December–March). Periods of enhanced incoming shortwave radiation and reduced cloud cover dominated surface net heat flux anomalies during MHWs. The strongest events in 1998 and 2015–2016 were associated with concurrent El Niño and positive Indian Ocean Dipole phases. Surface‐intensified warming extended to ∼25 m; however, regions of the tropical western Pacific Ocean and South China Sea exhibited subsurface‐intensified MHWs, with temperature anomalies increasing with depth. These results provide a comprehensive characterization of historical MHW behavior and their depth signatures in Southeast Asia, informing prediction efforts and assessments of ecosystem vulnerability.