南中國海(South China Sea, SCS)為四周環繞陸地與島嶼的半封閉海域，其海平面高度變化對於周圍環境影響甚大，且此處位於低緯度地區，應考慮聖嬰現象 (El Niño–Southern Oscillation, ENSO)的影響。因此本研究分析1993年至2014年測高衛星所測量的海平面高度變化及2003年至2014年GRACE(Gravity Recovery and Climate Experiment)重力資料變化，同時將南中國海與其他海域連通的海峽納入考量，分析1993年至2014年海峽地轉流流速變化情形。透過經驗正交方程(Empirical Orthogonal Function, EOF)分析，結果發現南中國海海平面高度移除季節性訊號後，其Mode-1和多變量聖嬰指數(Multivariate ENSO Index, MEI)相關係數達到0.3；於GRACE重力資料變化能夠得知進水量的變化，移除季節性訊號後EOF分析Mode-1和MEI的相關性有0.26：顯示反聖嬰年時海平面高度明顯上升，於聖嬰年反之。海峽地轉流流速變化，以呂宋海峽為例，與MEI也有0.28的高相關性，於聖嬰年時進入南中國海的流速增強，代表海峽進出量有調節南中國海水量的功能，並非影響整體南海水量的主要因素。綜合測高資料及GRACE重力資料分析結果，南中國海水量於聖嬰年時水量較少，反聖嬰年時水量較多。

The South China Sea (SCS) is a semi-enclosed marginal sea surrounded by continents and islands, thus, water can only exchange through the straits. In this study, we analyze the water budget variations of SCS during 1993-2014, using (i) the AVISO-released surface geostrophic current (SGC) daily data computed by combining the satellite altimeter data in reference to the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) geoid; (ii) the Gravity Recovery and Climate Experiment (GRACE) monthly time-variable gravity signifying the water mass variation; and (iii) daily sea level variation (SLV) from AVISO satellite altimeter data. We find from (i) that SGC water intrusion into SCS through the Luzon Strait, normally stronger in winter and weaker in summer, is correlated at the coefficient of 0.28 with the Multivariate El Niño Southern Oscillation (ENSO) Index (MEI) apparently under the influence of the variation of monsoon and the strength of Kuroshio in response to the ENSO. As the water mass budget will also be reflected in the gravity and SLV, we use the GRACE data (ii) to find the water mass redistributions and the satellite altimeter data (iii) for the SLV in SCS, both showing similar annual variation patterns. We also conduct the Empirical Orthogonal Function (EOF) analysis on the non-seasonal datasets. We find both leading modes showing unison water rise-and-fall behavior in the whole SCS. Their time series have correlation coefficients up to 0.44 and 0.30, respectively, with the MI. This means SCS water budget is under ENSO influence, not only in the intrusion but also reflected in the mass and sea level height. The SCS water budget variation is larger in normal winters and smaller during El Niño years.

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