Program

When the Kuroshio flows through the East China Sea (ECS), it exchanges extensively with the continental shelf water. Part of cold and high-salinity subsurface water of the Kuroshio is uplifted and intrudes onto the ECS shelf. Meanwhile, part of the less saline shelf water expands to the shelf edge and penetrates into the Kuroshio subsurface layer. The subsurface exchange of Kuroshio and ECS shelf water may be important to the salt and nutrient budget on the ECS shelf, and of significance to ecological safety in the coastal and offshore ocean of China. We try to give a further description of subsurface oceanography in the ECS and focus on: where, when and how the Kuroshio subsurface high-salinity water intrudes onto the ECS shallow shelf; where, when and how the subsurface intrusion of the less saline shelf water into the Kuroshio region takes place; and what about the seasonal and interannual variability of this two-way intrusions. In this paper, we use monthly mean gridded data from the most recent released reanalysis SODA (Simple Ocean Data Assimilation) v2.1.6, over the period 1958-2008. We use a practical method called neutral density for the subsurface study. This method is based on a neutral density variable, which is not only a function of the three state variables: salinity, temperature, and pressure, but also of longitude and latitude. Unlike discretely referenced potential density surfaces, neutral density surfaces are continuous and of the most appropriate surfaces. We use empirical orthogonal function (EOF) to extract a small number of principle components to explain most of the dominant variations of a dataset. Applying the neutral density variable and EOF analysis, dominant spatial features and corresponding temporal variability are revealed on neutral surfaces 23.0-24.5. The results indicate that annual cycles dominate the two leading EOF modes. The first EOF shows the inner-shelf oscillation features, characterized by negative anomaly of temperature/salinity over the shallow shelf less than 100m depth from winter to early spring, which is related to strong vertical mixing due to winter monsoon;  while the second represents the outer-shelf oscillation, by positive anomaly of temperature/salinity over the outer shelf and shelf break between 100m and 200m in late winter and spring, reflecting seasonal intrusion of the Kuroshio onto the ECS shelf. However, the second EOF shows strong interannual variability, anomalously low temperature/salinity water dominating during 98-02 in particular. We remove the annual signal from the data set and calculate the interannual EOFs. The first interannual EOF represents the interannual variability existing in the outer-shelf oscillation. The most remarkable characteristic of variability on interannual time scale is the anomalous strengthening of subsurface intrusion of less saline shelf water into the shelf edge and the Kuroshio region during strong La Niña years. Possible causes inducing the strengthening are found to be associated with extraordinarily large discharge of the Changjiang in the following years of the strongest 1997/98 El Niño as well as the variability of the Yellow Sea bottom cold water modulated by La Niña events.