Using in situ data, satellite observations, and model outputs, we analyzed the interannual variability in coastal upwelling intensity in the northern South China Sea. Comparing coastal upwelling observed from three cruises during the summers of 2008 and 2016, we found that coastal upwelling was stronger during 2016 compared to 2008, although the local upwelling favorable wind was stronger in 2008. The stronger near-bottom cross-shelf current and shallower thermocline in the slope resulted in stronger upwelling intensity during the summer of 2016. The topographic position index (TPI), which is defined by the sea surface temperature difference between one center cell and its neighbors, was used to quantify the interannual variability in upwelling. Stronger (weaker) upwelling intensity occurred during the summers of 2007, 2008, 2011, 2015, and 2016 (2004, 2009, 2012, and 2014) when the local wind was more favorable (less favorable) to coastal upwelling. The correlation coefficient between the area-weighted TPI and alongshore wind speed was -0.60, thereby confirming that local wind is the primary dynamical factor controlling the interannual variability in upwelling intensity. The correlation coefficient between the area-weighted TPI and the eastward boundary current transport averaged between the 75- and 100-m isobaths on the shelf was -0.42, indicating that the interannual variability in large-scale circulation in the northern South China Sea also contributes to the interannual variability in upwelling intensity. The anomalously shallow thermocline in the summer of 2016 was likely associated with the strong 2015–2016 El Ni?o event through planetary wave propagations.

局地风和海流对南海北部沿岸上升流强度年际变化的贡献

中文摘要：利用现场资料、卫星观测和模式输出，我们分析了南海北部沿岸上升流强度的年际变化。通过对比2008年和2016年夏季三个航次观测到的沿岸上升流，我们发现虽然2008年局部上升流优势风更强，但2016年沿岸上升流比2008年更强。倾斜地形上较强的近底跨陆架流和较浅的温跃层导致2016年夏季上升流的强度更强。利用地形位置指数(TPI)量化上升流的年际变化，TPI 是由一个中心单元与其相邻单元之间的海面温度差定义的。2007、2008、2011、2015和2016年(2004、2009、2012和2014年)夏季的上升流强度更强（更弱），因为局地风对沿岸上升流更有利(不太有利)。面积加权TPI与沿岸风速的相关系数为-0.60，从而证实局地风是控制上升流强度年际变化的主要动力因子。面积加权TPI与陆架上75米和100米等深线之间的东向边界流输运平均值的相关系数为-0.42，这表明南海北部大尺度环流的年际变化也对上升流强度的年际变化有贡献。2016年夏季异常浅的温跃层可能与2015-2016年通过行星波传播的强El Ni?o事件有关。