Monthly variations of water masses in the East China Seas

A simple spiciness index parameter (β) is proposed which accounts for the combined contribution of temperature (T) and salinity (S) to water density. Historical monitoring data in the East China Seas (ECSs) have been processed using a formula to determine the spatial and temporal distributions of β. Based on the values of β, three key boundaries between water masses were identified, including the diluted water boundary (DWB, β = - 2.7 ) along the coast, the equilibrium line (EL, β = 0 ) of water masses and the Kuroshio Margin (KM, β = 1.9 ) on the continental shelf. Two northward points (NP1 and NP2) of the warm water mass head, two southward points (SP1 and SP2) of the diluted water mass head along the equilibrium line and one northward point (NP3) along the boundary of the warm current water are used as indicators of the extent and variability of the current system, in view of its sensitivity to the annual variation of the intensity of the warm current and the cooler coastal current that are driven, respectively, by the Kuroshio and the Eastern Asian Winter Monsoon. It was found that the evolution of the water masses in the ECSs can be divided into three phases per year, as indicated by its monthly variations. In the winter phase from October through April, the distribution of water masses is strongly influenced by the coastal current under the East Asian Winter Monsoon. In the transitional phase from May to June, the warm current intensifies whereas the coastal current weakened. In the summer phase from July through September, the water mass system is unstable in the area. The results demonstrate that the Yellow Sea coastal current (YSCC) plays an important role in the evolution of the ECSs water masses. Specifically it causes the intensification of the Min-Zhe coastal current (MZCC) and the Yellow Sea warm current (YSWC) water, and blocks the northeastward dispersion of the Taiwan warm current (TWWC) water. The result shows that the water mass of the East China Seas can be divided into four types. The first is the diluted cold water mass containing the Bohai Sea, the Yellow Sea coastal current, the Min-Zhe coastal current and the Korea coastal current (KCC) water inside the diluted water boundary. Second is the warm and salty water mass containing the Kuroshio and the Taiwan Strait warm current (TWSWC) water beyond the Kuroshio margin. Third is the mixed coastal water containing the outspread part of the YSCC and the YSWC between the diluted water boundary and equilibrium line. Fourth is the mixed warm water containing the Taiwan warm current, the Tsushima Warm Current (TSWC) and the Yellow Sea warm current between the equilibrium line and Kuroshio margin.