Rapid ocean mixing observed by an underwater glider during Typhoon Soulik over tidally dominated and highly stratified waters west of Jeju Island, Korea

Typhoon Soulik formed on August 16, 2018 in the western North Pacific and tracked northwest towards the Yellow Sea, slowing as it passed to the west of Jeju Island on August 22. Here the storm intensity rapidly decreased from a typhoon to a tropical storm before making landfall on the southern Korean Peninsula early the next day. The passage was observed by the Ieodo Ocean Research Station (IORS) operated by the Korea Institute of Ocean Science and Technology (KIOST), Moslpo tidal station operated by the Korea Hydrographic and Oceanographic Agency (KHOA), and an underwater glider RU22 deployed by KIOST and Rutgers University as part of the Korean Ministry of Oceans and Fisheries (MOF)/NOAA Joint Program Agreement (JPA). The glider was deployed over the Yellow Sea trough west of Jeju Island and east of IORS from August 15 to 25. During this mission, the eye of Typhoon Soulik passed within about 10 km of the glider on August 22 at 2100 UTC. The IORS measured a minimum pressure of 963 hPa, maximum wind speed of 43 m/s, and maximum significant wave height of 7.2 m.

The Yellow Sea is highly stratified during the summer months with warm surface water, typically warmer than 25C and an extensive area of cold bottom water less than 15C. Similar to hurricanes along the US east coast this highly stratified region is frequently impacted by typhoons in summer. Unlike the US east coast, strong tidal currents off the Korean Peninsula often reach 1.2 m/s with maximum tidal ranges of 3 m. Recent studies off the US east coast have identified the essential ocean processes responsible for ahead of eye center cooling and its feedback on rapid storm deintensification.

During Typhoon Soulik, the glider observed rapid cooling. Surface temperatures dropped 11C, bottom temperature increased by 3C, and a dramatic deepening of the pycnocline from 30 to 70 meters depth. This rapid mixing coincided with a tidal transition after hours of sustained winds and waves with limited mixing and cooling. In this study we present a preliminary analysis of Typhoon Soulik and its interaction with this tidally dominated and highly stratified coastal ocean.