渦漩在南海引起的季節內變化

Abstract

本文以長期錨錠觀測之海流流速及水溫資料與由衛星高度計量測之海水位異常(Sea Level Anomaly, SLA)，探討於南海中、北部地區水文(海流流速與海水溫度)季節內變化的特性及其成因。結果顯示此季節內變化一般在海流上是以順、逆時針方向旋轉交替出現的渦漩形式出現，而海水溫度隨著海流方向的順時針/逆時針旋轉有升溫/降溫趨勢。此外，水文季節內的變化週期因地略有不同，於南海北部其變化週期約介於數天到一個月；而於南海中部其變化週期則為數天到二個月。 另一方面，SLA資料顯示，南海內存在不同尺度的渦漩，此渦漩的性質具有區域特性。於南海北部渦漩因受地形的限制，故水平尺度略小，半徑約為0.5-1度，其生成後的行進方向略沿著北南海地形以逆時針的方向行進；而於南海中部因為廣闊海盆，故渦漩的尺度較大約為半徑1-2度的水平尺度。渦漩的行進方向則較不定，然多為向西行進。 經交相比對SLA與長期觀測的海流與海溫資料，顯示兩者具有高相關，當冷/暖渦漩經過錨錠測站，則海流與海溫相對應地呈現逆時針/順時針旋轉與降溫/升溫。此現象於南海中部更為明顯，渦漩的影響深度可達約水下500公尺。此外，渦漩對部份斜溫層深度的季節內變化也有影響。綜合分析結果顯示，南海中尺度渦漩的存在為南海內海流與海溫季節內變化的重要因子。

Long-term moored current velocity, sea temperature and sea level anomaly (SLA) measured from satellite altimetry are used to study the characteristics and causes of hydrographic (current and sea temperature) intra-seasonal variation in the northern and central South China Sea (SCS). The results indicate that the current intra-seasonal variations are generally in the form of clockwise and anti-clockwise rotating eddies alternately, and the sea temperatures have tendency to increase/decrease in the clockwise/anti-clockwise rotations. Besides, the periods of intra-seasonal variations vary with areas, from several days to a month in the northern SCS and several days to two months in the central SCS. SLA data display that there are eddies with different scales, and these eddies have regional characteristics. In the northern SCS, limited by topography, the horizontal scales of eddies are less, and the radius of them are about 0.5-1 degree; in the central SCS, the scales are larger due to the broad ocean basin, and the radius are about 1-2 degree. The moving directions of eddies are not constant, but usually westward. Comparing SLA with long-term current and sea temperature data, they are in high correlation. While cold/warm eddies pass the mooring stations, current directions display anti-clockwise/clockwise rotation and temperatures decrease/increase. The phenomena are more obvious in the central SCS. The affection of eddies can reach to about 500-meter depth. Also, eddies have impact on some intra-seasonal variation in the depth of thermocline. Above all indicate that the existence of mesoscale eddies are important factors of the current and sea temperature intra-seasonal variation in SCS.