Upper ocean responses to category 5 typhoon Megi in the western north Pacific
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Upper ocean responses to category 5 typhoon Megi in the western north Pacific CHEN Xiaoyan1,2 , PAN Delu1,2∗ , HE Xianqiang1,2 , BAI Yan2 , WANG Difeng2 1 2
Department of Earth Sciences, Zhejiang University, Hangzhou 310027, China State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China
Received 27 May 2011; accepted 8 August 2011 © The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2012
Abstract Category 5 typhoon Megi was the most intense typhoon in 2010 of the world. It lingered in the South China Sea (SCS) for 5 d and caused a significant phytoplankton bloom detected by the satellite image. In this study, the authors investigated the ocean biological and physical responses to typhoon Megi by using chlorophyll-a (chl-a) concentration, sea surface temperature (SST), sea surface height anomaly (SSHA), sea surface wind measurements derived from different satellites and in situ data. The chl-a concentration (>3 mg/m3 ) increased thirty times in the SCS after the typhoon passage in comparison with the mean level of October averaged from 2002 to 2009. With the relationship of wind stress curl and upwelling, the authors found that the speed of upwelling was over ten times during typhoon than pre-typhoon period. Moreover, the mixed layer deepened about 20 m. These reveal that the enhancement of chl-a concentration was triggered by strong vertical mixing and upwelling. Along the track of typhoon, the maximum sea surface cooling (6–8◦ C) took place in the SCS where the moving speed of typhoon was only 1.4–2.8 m/s and the mixed layer depth was about 20 m in pre-typhoon period. However, the SST drop at the east of the Philippines is only 1–2◦ C where the translation speed of typhoon was 5.5–6.9 m/s and the mixed layer depth was about 40 m in pre-typhoon period. So the extent of the SST drop was probably due to the moving speed of typhoon and the depth of the mixed layer. In addition, the region with the largest decline of the sea surface height anomaly can indicate the location where the maximum cooling occurs. Key words: sea surface temperature, chlorophyll-a concentration, sea surface height anomaly, upwelling, typhoon Megi, ocean remote sensing
is abundant light for photosynthesis but often lack of nutrients. Therefore it will contribute to the growth of phytoplankton and take a significant impact on the increase of ocean primary production (Lin et al., 2003; Siswanto et al., 2008; Siswanto et al., 2009; Zhao et al., 2009). Lin et al. (2003) suggested that the longneglected contribution of typhoons to the South China Sea’s annual new production may be as much as 20%– 30%. Therefore the amounts of carbon fixation by typhoons can not be ignored. At the same time, sea surface cooling is another major feature after typhoon passage (Price, 1981; Stramma et al., 1986; Sakaida et al., 1998; Cione and Uhlhom, 2003; Tsai et al., 2008). From the previous studies, the SST drop is due to a unique combination of typhoon’
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