Subsurface evolution of three types of surface marine heatwaves over the East Sea (Japan Sea)

Extremely high seawater temperatures (beyond the 90th percentile threshold) are a key characteristic of marine heatwaves (MHWs), which adversely affect human and marine ecosystems. It is important to characterize the MHWs in the East Sea (Japan Sea), a semi-enclosed deep basin connected to the outside seas/ocean by shallow and narrow straits, where the most rapid upper ocean warming has been reported in recent decades. Despite several reports on their rising frequency and severity, the characteristics, and underlying dynamics of East Sea MHWs (ESMHWs), particularly their subsurface evolution, remain poorly understood. In this study, we aimed to understand the subsurface evolution patterns of ESMHWs and the underlying mechanisms. Here, summer (June, July, and August) ESMHWs from four ocean reanalysis products were characterized after being verified against those from long-term (2000–2015) time-series observations near the east coast of Korea. We found the HYCOM reanalysis products is the best match with the observation, showing consistent evolution patterns with the ESMHWs in the observation, yielding high correlation coefficients (ranging from 0.7 to 0.9 at the surface), and low root-mean-square errors (3°C). Therefore, the HYCOM reanalysis is further used to characterize the subsurface evolution of surface ESMHWs. A rising frequency of annual mean and summer ESMHWs across the whole East Sea, with a maximum increasing rate of 1–2 events in 40 years and mean cumulative intensity of 5–9°C days decade^-1. Three types (Types-A, -B, and -C) of subsurface evolution of ESMHW were commonly found in six sub-regions (where the increasing ESMHWs trends are maximum)—three in the western and three in the eastern parts. The three types are Type-A, which is surface-confined and short-lived with anomalous warm surface temperatures and cold subsurface temperatures; Type-B, characterized by warm anomalies propagating from the surface to the upper several meters with no subsurface cold anomalies; and Type-C, exhibiting anomalous warm temperatures at the upper 200 m. Although increased shortwave radiation, decreased wind speed, and thinned mixed layer commonly contribute to the formation of all three types, the relative role of the heat penetration into the layer below the mixed layer and dissipation mechanisms significantly differ among the types. Common ESMHW evolution features include mixed layer deepening, heat release to the atmosphere, and interactions between the surface mixed layer and deeper layers, while horizontal advection played a significant role only during the Type-C ESMHW events. The long-term trends in frequency, cumulative intensity, and duration of ESMHWs were discussed in terms of those in each type ESMHWs metrics along with long-term surface warming, subsurface cooling, and weakening currents in the six sub-regions. This study provides significant insights into surface and subsurface evolution of MHWs in this and other marginal seas in warming climate conditions.