The meridional sea level oscillation of the China seas and the Gulf of Thailand

Analyses of sea level anomalies(SLA) from satellite altimetry,ranging from 1992 to 2012,indicate that,as a semi-enclosed narrow water body,the China seas (consisted of the Bohai Sea,the Huanghai Sea,the East China Sea,and the South China Sea) and the Gulf of Thailand are subject to significant meridional sea level oscillation. The north-south SLA seesaw is strongly seasonal,with sea level being high in the south but low in the north during winter seasons,and vise versa in summer. The sea level difference between the Gulf of Bohai and the Gulf of Thailand is taken as a measure of the oscillation,which has a range of over 80 cm and a climatological annual mean of 63 cm. The analysis shows that,in the seasonal time scale,the meridional oscillation is almost totally dominated by variations of the East Asian monsoon,which indicates that monsoon is the main cause of the seasonal seesaw. Further analysis reveals that,in addition to seasonal variation,the meridional sea level oscillation is also subjected to considerable interannual variation. Unlike the oscillation in the seasonal time scale,its interannual variation appears correlated no only with the fluctuations of the East Asian Monsoon but also with the basin scale variation of the Pacific. The influences of remote forcings are comparable with that of the East Asian monsoon,and the seesaw generally lags two months behind the atmospheric forcings. The results from a multi-input,linear partial coherience model shows that,among the mumberous input factors other than the East Asia Monsoon Index,the interannual variation of the sea level oscillation index is correlated with SOI and WP. It is thus suggested that,in addition to the East Asian monsoon,remote forcings are from the tropics as well as the mid-latitude. The east-west sea level seesaw in the tropical Pacific associated with the southern oscillation,and the possible effects of interannual migration of the mid-latitude East Asian jet stream on the sea level,are both possible to cause interannual variation of the north-south sea level seesaw in the area. Further investigation is needed to determine their mechanism.