The oceanic responses to Typhoon Rananim on the East China Sea
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The oceanic responses to Typhoon Rananim on the East China Sea Dan Xu1, Zhiyuan Li2, Zhanhong Wan3*, Zongfu Ren3, Zhongshui Zou3*, Xiuyang Lv3, Shizhu Luo3 1 College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, China 2 Collegeof Hydraulic and Environmental Engineering, Zhejiang University of Water Resources and Electric Power,
Hangzhou 310018, China 3 Ocean College, Zhejiang University, Zhoushan 316021, China
Received 31 March 2019; accepted 27 September 2019 © Chinese Society for Oceanography and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract
Many typhoons pass through the East China Sea (ECS) and the oceanic responses to typhoons on the ECS shelf are very energetic. However, these responses are not well studied because of the complicated background oceanic environment. The sea surface temperature (SST) response to a severe Typhoon Rananim in August 2004 on the ECS shelf was observed by the merged cloud-penetrating microwave and infrared SST data. The observed SST response shows an extensive SST cooling with a maximum cooling of 3°C on the ECS shelf and the SST cooling lags the typhoon by about one day. A numerical model is designed to simulate the oceanic responses to Rananim. The numerical model reasonably simulates the observed SST response and thereby provides a more comprehensive investigation on the oceanic temperature and current responses. The simulation shows that Rananim deepens the ocean mix layer by more than 10 m on the ECS shelf and causes a cooling in the whole mixed layer. Both upwelling and entrainment are responsible for the cooling. Rananim significantly deforms the background Taiwan Warm Current on the ECS shelf and generates strong Ekman current at the surface. After the typhoon disappears, the surface current rotates clockwise and vertically, the current is featured by near inertial oscillation with upward propagating phase. Key words: typhoon, sea surface temperature, numerical model, mix layer, near inertial oscillation Citation: Xu Dan, Li Zhiyuan, Wan Zhanhong, Ren Zongfu, Zou Zhongshui, Lv Xiuyang, Luo Shizhu. 2020. The oceanic responses to Typhoon Rananim on the East China Sea. Acta Oceanologica Sinica, 39(7): 69–78, doi: 10.1007/s13131-020-1573-5
1 Introduction Typhoons are very transient and violent atmospheric events in the tropical and subtropical oceans and are widely studied. When a typhoon passes, the strong wind stress generates intensive thermal and physical oceanic responses (Price, 1981; Brink, 1989; Shay, 2010). A very prominent thermal response is the sea surface temperature (SST) cooling (Price, 1981; Wentz et al., 2000). The maximum SST drop caused by a typhoon is about 10.8°C (Chiang et al., 2011). Usually, the SST response is asymmetric. In the northern hemisphere, the SST cooling is much stronger to the right side of the typhoon track (Wright, 1969; Price, 1981). In addition to the SST cooling, a typhoon also deepens the mix layer (ML) and causes the whole ML cooling. The strong vertical mixing and Ekman pumping
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