中文摘要：

基于2005年至2009年、2011年和2013年各年九月份南海开放航次获取的南海东北部120°E断面的水文观测资料,运用了地转流诊断和模态分解两种方法,研究了该断面流场结构和体积输运的年际变化特征。2005年、2006年、2007年和2013年流场呈显著斜压特征,断面上、下层流速方向相反;而2008、2009年和2011流场垂向变化不明显,呈现准正压结构。断面体积输运沿深度分布呈现三种方式：一致向西（2005年、2007年和2011年）,上西下东（2008年和2013年）和上东下西（2006年和2009年）。断面净体积输运亦有显著年际变化,在2005年出现西向最大–11.2Sv,在2013年出现东向最大9.1Sv,而在2009年仅为西向-1.2Sv。模态分解表明,准正压结构的年份,流场主要被正压模态控制,但第一斜压模态亦不可忽略;而斜压结构的年份,流场由正压模态和第一斜压模态共同主导。

英文摘要：

Using hydrographic data with geostrophic calculation and normal mode analysis along the 120°E transect in September from 2005 to 2009, 2011, and 2013, we revealed the vertical characteristics and interannual variation of the velocity fields in the northeastern South China Sea （SCS）. Results show that, the direction of velocity fields in 2005, 2006, 2007, and 2013 changed with depth and presented a baroclinic structure, and were dominated by barotropic and the first baroclinic modes as revealed in normal mode analysis. Differently, the velocity fields were uniform in vertical structures in 2008, 2009, and 2011, which was resulted mainly from the barotropic mode contributions. The volume transport along the section in depth can be classified into three types： westward transport in all depths （i.e., 2005, 2007, and 2011）, westward transport in the upper layer and eastward transport in deeper layers （2008 and 2013）, and eastward transport in upper layer and westward in lower layer （2006 and 2009）. The net volume transport presented an obvious interannual variability. The maximal westward transport was -ll.2Sv in 2005, whereas the maximal eastward transport was 9.1Sv in 2013. The minimal transport occurs in 2009 for only -1.2Sv. In addition, the quasi barotropic volume transport is dominated by the barotropic mode and the first baroclinic mode, in which the barotropic mode is bigger than the the first baroclinic mode. For the baroclinic volume transport, the barotropic mode and the first two baroclinic modes are the leading modes.