Depth- and season-resolved characteristics and drivers of marine heatwaves in the East Sea (Sea of Japan).

Marine heatwaves (MHWs)-events of anomalously high ocean temperatures-are intensifying globally, yet most studies have focused on summer surface events, overlooking their vertical structure and seasonal variability. Here, we present a depth- and season-resolved assessment of MHWs in the East Sea (Sea of Japan) using in-situ observations (2000-2016) alongside multi-decadal reanalysis products (1982-2021). The results reveal pronounced vertical asynchrony: MHWs frequently occur in either the upper (5 m) or lower (60 m) layers, but rarely in both simultaneously. Unlike summer upper-layer MHWs driven by high air temperatures and strong stratification, winter lower-layer MHWs occur when strong winds deepen the mixed layer and push warm surface water downward. In addition, summer MHWs in the lower layer are strongly associated with the northward intrusion of the East Korea Warm Current (EKWC), with interannual variations in its path and intensity modulating subsurface temperature extremes. Long-term trends show that the frequency, duration, and intensity of surface MHWs around the Korean Peninsula have all increased since 1982, with the steepest trends occurring in the East Sea, particularly after 2017. These findings underscore the importance of considering both vertical and seasonal dynamics in MHW assessments and highlight the combined roles of stratification, wind-driven mixing, and warm-current advection in generating thermal extremes in marginal seas.