大西洋多年代際震盪之季節不對稱性效應對台灣東北湧升流之影響

Abstract

本研究使用Advanced Very High Resolution Radiometer (AVHRR) 的Optimum Interpolation Sea Surface Temperature (OISST) 資料來分析台灣東北湧升流之長期變異，以夏季(6月至8月)和冬季(12月至隔年2月)兩個季節的湧升流為主，使用經驗正交函數(Empirical Orthogonal Functions)定義台灣東北湧升流指標，接著以整體經驗模態分解法(Ensemble Empirical Mode Decomposition)分析長期的湧升流指標，研究發現不論是夏季還是冬季的低頻訊號皆與大西洋多年代際震盪(Atlantic Multidecadal Oscillation，AMO)存在良好的相關性，且具有季節上的不對稱(seasonal-asymmetry)。AMO正相位時期，北大西洋的海面溫度異常增暖，此異常暖化會影響到太平洋的氣候，夏、冬兩季的動力機制並不相同，分別可以使用太平洋副熱帶高壓(Pacific subtropical high)以及大氣羅斯比波波列(Rossby Waves)來解釋此種跨洋盆的氣候影響。夏季時副熱帶高壓中心增強並往西延展，在北太平洋副熱帶區產生負的風應力旋度異常，導致黑潮(Kuroshio)的增強。冬季則因為羅斯比波波列傳遞的過程中，中緯度西風帶減弱以及在北太平洋副熱帶區產生正的風應力旋度異常，導致北太平洋副熱帶環流(North Pacific subtropical gyre, NPSG)以及黑潮突然的減弱，而黑潮強度與湧升流強度有關，當夏季台灣東邊的黑潮增強時，黑潮較不容易入侵到台灣東北角，使湧升流增強，最後導致湧升流的海域溫度因此下降，而冬季台灣東邊的黑潮減弱時，黑潮較容易入侵到台灣東北角，使湧升流減弱，最後導致湧升流的海域溫度因此上升。

This study analyzed long-term variabilities of the upwelling off northeast Taiwan using Optimum Interpolation Sea Surface Temperature (OISST) data. We first defined the upwelling domain by empirical orthogonal function (EOF) analyses. The Ensemble Empirical Mode Decomposition (EEMD) had then been applied to analyze the long-term upwelling indices. The result indicated that low frequency signals were closely correlated with the Atlantic Multidecadal Oscillation (AMO) while a seasonal asymmetry was identified between summer and winter.During the positive AMO phase, an anomalously warm North Atlantic will impact on the Pacific climate. Distinct mechanisms for the transbasin influence between summer and winter had been proposed. In summer, the Pacific subtropical high intensified and extended westward, resulting in negative wind stress curl anomalies in the subtropical Pacific which strengthened the Kuroshio. On the other hand, reduced mid-latitude westerlies led to positive wind stress curl anomalies in the subtropical Pacific, weakening the North Pacific subtropical gyre (NPSG) and Kuroshio during winter. The intensity of the upstream Kuroshio was highly related to the variability of upwelling off northeast Taiwan. A strengthened Kuroshio would lessen its intrusion onto the East China Sea shelf, sequentially enhancing the upwelling in summer, and weakened Kuroshio would expand its intrusion onto the East China Sea shelf, sequentially reducing the upwelling in winter.