Rapid Ocean Destratification by Typhoon Soulik over the Highly Stratified Waters of West Jeju Island, Korea

Abstract

Global warming is affecting ocean disasters by intensifying typhoons, which present a threat to people living in coastal cities. The increase of intensity has been observed in the typhoons approaching the southwestern Korean Peninsula; this increase can be attributed to the increasing water temperature and strong stratification of the Yellow Sea in summer. Typhoon Soulik was formed on August 16, 2018 in the western North Pacific and tracked northwestward toward the Yellow Sea, slowing down as it moved towards the highly stratified coastal waters of western Jeju Island on August 23. However, its intensity rapidly decreased from a typhoon to a tropical storm before it landed on the southwestern Korean Peninsula. Typhoon Soulik was observed by the Korea Institute of Ocean Science and Technology's (KIOST) Ieodo Ocean Research Station (IORS), the Korea Hydrographic and Oceanographic Agency's (KHOA) Moslpo tidal station, and an underwater glider deployed by KIOST and Rutgers University as a part of the Joint Program Agreement (JPA) by the Korean Ministry of Oceans and Fisheries (MOF) and National Oceanic and Atmospheric Administration (NOAA). The glider was deployed over the Yellow Sea Trough, west of Jeju Island near IORS, from August 15 to 25, 2018; during this time, the eye of typhoon Soulik travelled within 26 km of the glider track on August 23 at 04:37 Korea Standard Time (KST). The glider also observed a rapid ocean cooling where the surface temperature dropped from 27.5 degrees C to 16.5 degrees C while the bottom temperature increased from 11 degrees C to 14 degrees C, and the pycnocline deepened from 30 to 60 m. The rapid ocean mixing coincided with a tidal transition followed by several hours of sustained winds and waves with limited mixing and cooling. It is suggested that this rapid ocean destratification might reduce the intensity of typhoon Soulik before landfall.