Subsurface oceanography in southern East China Sea: spatiotemporal characteristics of salinity and temperature on isopycnal surfaces

On the southern East China Sea (ECS) shelf, intrusions of the subsurface Kuroshio water are mainly through the shelf edge off northeast Taiwan. Fifty-one year (1958-2008) monthly reanalysis data of temperature, salinity and pressure over the southern ECS shelf were analyzed on neutral surfaces using time-domain Empirical Orthogonal Function (EOF). We retained first the annual signal in data preprocessing. The results indicated that annual cycles dominate the two leading EOF modes. The first EOF shows an inner-shelf oscillation characterized by less saline and colder water, while the second EOF represents an outer-shelf oscillation reflecting seasonal intrusions of the Kuroshio subsurface water onto the shelf. The second EOF also shows strong interannual characteristics. We removed the annual signal from the data set and recalculated interannual EOF. The first interannual EOF represents significant interannual variability existing in the outer-shelf oscillation. Its most remarkable characteristic is that continuously negative anomalies keep dominating during 1998-2002, suggesting a strengthening of penetration of low-salinity water over the southern ECS shelf toward the Kuroshio subsurface around the shelf edge. We concluded that the onshore intrusion of the high-salinity Kuroshio water and the seaward penetration of low-salinity shelf water are a pair of contradictions in the two-way intrusions, which are modulated by strong interannual signals.