Observation of intensification of western boundary current in summer of 2021

Due to climate change, the occurrence of extreme events such as typhoons, marine heat waves, storminess, and cold waves is increasing in many regions, and these events could dramatically change with significant impacts on the marine environment (e.g., ocean circulation). The East Korea Warm Current (EKWC) has been recognized to flow along the western boundary current of the East Sea (Japan Sea). To examine the variations of EKWC, six bottom-mounted current profiler moorings were operated off the east coast of Korea (the Hupo Bank and Wangdolcho) since June 2021. The observed mean current speed and their principal axis were 0.40 m/s and 58^o (counterclockwise from the east) implying northeastward EKWC over the region. In August 2021, an unprecedentedly strong surface current was observed with a maximum of 1.89 m/s and observed currents showed similar variability at mooring sites and several depths. This strong current lasted for about a month, and then rapidly disappeared within a few days. At that time, the geostrophic currents based on satellite-altimetry has a strong current pattern with the Inertial Boundary Current pattern, which is one of the EKWC patterns that flows strongly northward currents closer coast. In addition, the highest mean speed of the EKWC near the mooring sites from 1993 was found in August 2021. The high-speed period was similar to the period of the North Pacific marine heat waves that were already reported, and the low-speed period was related to typhoon passage. This study reported the results of observed EKWC for two years from 2021 and the unprecedentedly enhanced EKWC in August 2021. In particular, it can be a case in which the rapid changes of western boundary currents interact with extreme events such as marine heatwaves and typhoons.