| The South China Sea is the largest semi-enclosed marginal sea in the western Pacific.it exchanges water with the East China Sea,the Western Pacific,the Indonesian Seas and the Indian Ocean through multiple straits.Influenced by local mixing and coastal runoff,the water masses in the South China Sea degenerate significantly,thus,the definition and division method of water mass for the open ocean are not fully applicable to the South China Sea.In this study,the shallow water mass is defined as: the great water body with similar physical and chemical properties,regular seasonal variability,and both homogeneity and anisotropy,which is consistent with the principle of the K-means clustering algorithm.Based on the WOD13 temperature and salinity observations from 1966 to 2013,the K-means algorithm is used to classifiy the water masses in the South China Sea.In combination with the system clustering and T-S scatter diagram,we analyze the seasonal variability of the South China Sea water masses,including its distribution,temperature and salinity properties,and the degeneration processes.In the process of computation,the functions of in-cluster-distance and density values are used to determine the numbers and initial center of the water masses.The results show that there are ten water masses in the South China Sea,including the Coastal Diluted Water(F),the nearshore Mixed Water(M),the South China Sea Surface Water(S),the Thai Bay-Sunda Strait Shelf Water(TS)(only occurs in summer),the Kuroshio Surface Water(KS),the South China Sea Subsurface Water(U),the Kuroshio Subsurface Water(KU),the South China Sea Subsurface – Intermediate Mixed Water(UI),the South China Sea Intermediate Water(I),the South China Sea Deep Water(D).The temperatures of water masses are higher in summer and lower in winter,with the amplitudes of variation are gradually reduced from the surface to the bottom.The seasonal variation of salinity of the surface water masses(F,M,S,TS)is high in winter and low in summer,which mainly depends on the amount of river discharge and precipitation.The undersurface water masses(U,UI,I,D)are strongly affected by the Pacific Ocean water in the according layers,thus the seasonal variability of these water masses are not obvious,especially for the Intermediate Water Mass,the low salinity characteristics is prominent.The KS and KU are formed through the surface and subsurface Kuroshio water invasion into the South China Sea(stronger in winter than in summer,stronger in subsurface than in the surface),and the seasonal variation of both temperature and salinity properties are not obvious.The properties and seasonal variabilities of the water masses derived from the K-means algorithm are in agreement with the existing conclusions,suggesting that the improved K-means algorithm is efficient and accurate in the shallow water mass division. |
