Low-frequency variability of the shallow meridional overturning circulation in the South China Sea
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Low-frequency variability of the shallow meridional overturning circulation in the South China Sea YANG Zhitong1, 2, LUO Yiyong2* 1 College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao 266100, China 2 Physical Oceanography Laboratory, Ocean University of China, Qingdao 266100, China
Received 26 July 2015; accepted 23 October 2015 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2016
Abstract
The low-frequency variability of the shallow meridional overturning circulation (MOC) in the South China Sea (SCS) is investigated using a Simple Ocean Data Assimilation (SODA) product for the period of 1900–2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells: a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport (LST) that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current (NEC) bifurcation and thus a stronger transport of the Kuroshio east of Luzon. Key words: South China Sea, meridional overturning circulation, Luzon Strait transport, North Equatorial Current bifurcation Citation: Yang Zhitong, Luo Yiyong. 2016. Low-frequency variability of the shallow meridional overturning circulation in the South China Sea. Acta Oceanologica Sinica, 35(3): 10–20, doi: 10.1007/s13131-016-0826-9
1 Introduction The South China Sea (SCS), extending from the equator to 23°N and 99°E to 121°E in the western tropical Pacific Ocean, is the largest semi-enclosed marginal ocean basin with a total area of 3.5 million km2 (Fig. 1). The maximum water depth is slightly more than 5 000 m with and the average water depth is ~2 000 m. It is surrounded by the Chinese mainland to the north, the Philippine Islands to the east, the Indonesia to the south, and the IndoChina Peninsula to the west, with connections to the surrounding oceans through straits. The upper layer circulation in the SCS is driven by the seasonally reversed monsoon system (e.g., Wyrtki, 1961; Xie et al., 2003; Liu et al., 2008; Yang and Wu, 2012; Lan et al., 2013; Shu et al., 2014). The winter monsoon over the SCS is strongly northeasterly, whereas the winds in summer reverse direction to southwesterly (Liu and Xie, 1999; Xie et al., 2003). Previous studies have found that the meridional ove
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