Types of the evolution and dynamics of marine heatwaves in the East Sea (Japan Sea)

Extreme oceanic conditions known as marine heatwaves (MHWs) are characterized by high seawater temperature (beyond the 90th percentile threshold) that has a negative impact on marine ecosystems and humanity. The East Sea (Japan Sea), a semi-enclosed deep basin connected to the outside seas/ocean by shallow and narrow straits, has recently experienced the most rapid upper ocean warming in the world seas. Characteristics and evolution of MHWs in the East Sea, including their subsurface evolution, need to be better understood as still poorly understood in spite of recent reports on their increasing frequency and severity. Here, the sea’s MHWs retrieved from ocean reanalysis/model data (Simple Ocean Data Assimilation (SODA), HYbrid Coordinate Ocean Model (HYCOM), Global Ocean Reanalysis product (GLORYS) and Estimating the Circulation and Climate of the Ocean (ECCOVr4)) were initially compared to those from a long-term (from 2000 to 2014) time-series observations conducted near the east coast of Korea using a surface mooring named the East Sea Real-Time Ocean Buoy (ESROB). Then, a rising frequency of annual mean and summer (JJA) MHWs from 1982 to 2019 throughout the entire East Sea was characterized using the SODA, HYCOM, GLORYS, and ECCOVr4, yielding a maximum increasing rate of 0.45 occurrences per decade. Using unsupervised machine learning clustering techniques (K-mean and Hierarchical), three different types of MHW evolutions were identified — subsurface to surface evolution (Type-A), surface evolution (Type-B), and surface to subsurface evolution (Type-C) in six sub-regions — three in the west and three in the east. This study discusses possible explanations for these types of MHW evolution in the selected sub-regions of the sea, providing better understanding of surface and subsurface MHWs in the most rapidly warming marginal sea.