Observed and Model-Simulated Dramatic Bottom Temperature Variations During a Weakened Typhoon in the Northern Yellow Sea

Abstract

Observations and a high-resolution ocean model were utilized to investigate the significant bottom temperature response in the north Yellow Sea (NYS) during the weakening of Typhoon Lekima in August 2019. The cyclonic typhoon winds generated prominent cyclonic currents, with eastward (westward) currents along the southern (northern) coast of the NYS. Concurrently, high sea levels occurred along the northern and southern coasts, whereas low sea levels were present in the central NYS. This sea level pattern persisted for a week after the typhoon's dissipation, maintaining the eastward (westward) currents along the southern (northern) coast. The sustained eastward alongshore current transported warm bottom water along the northern coast of the Shandong Peninsula, resulting in a significant bottom temperature increase over 10°C during 4 days. Simultaneously, the westward alongshore current carried cold water from the Yellow Sea Cold Water Mass (YSCWM) along the southern coast of the Liaodong Peninsula, leading to a prominent bottom temperature decrease of 13°C over 6 days. Near-inertial oscillations disturbed the thermocline and created favorable conditions for the bottom warm water advection. Temperature budget analysis further confirmed the dominant role of horizontal temperature advection in these dramatic bottom temperature anomalies. The relatively weaker wind disturbance could still drive strong bottom temperature change in the YSCWM boundary with sharp temperature gradient, which underscores the impact of weaker synoptic weather systems on significant bottom temperature variations in coastal regions with sharp temperature front, which are often ignored compared to much stronger weather events such as typhoons.