西北太平洋夏季季風環流與海洋次表層溫度之年際變化

Abstract

本研究利用EOF與SVD等統計方法，分析海洋同化資料與大氣再分析資料，探討6~9月西北太平洋海氣耦合模的年際變化特性。EOF與SVD都顯示主導海溫變化的主要結構為位於菲律賓海0~20°N東西向，極大值位於10°N的海溫與熱容量（0~300公尺）的整體變化，搭配的大氣低層環流則為一橫跨菲律賓海的緯向風。熱容量與低層的風場關係為高（低）熱容量搭配東（西）風距平。進一步分析發現，此一西北太平洋耦合模與太平洋的meridional mode與ENSO有關。在西北太平洋耦合模發生之前的春季，有一氣旋環流距平存在於北半球熱帶與副熱帶中太平洋，因此在中太平洋的主要環流距平為0~15°N的西南風距平，導致北半球赤道洋流減弱，熱帶中與東太平洋熱容量上升。此一結構與meridional mode極其相似。從春季至夏季氣旋環流距平逐漸西移至菲律賓海，此時0~15°N的菲律賓海為西風距平主宰，且熱容量為負距平，10°S~10°N太平洋則為西風距平，赤道東太平洋呈現El Niño的特徵。從夏季至冬季，此一海氣耦合結構持續加強，呈現顯著的El Niño海氣耦合關係。在此一發展過程中，赤道次表層暖海水由換日線，沿著躍溫層逐漸往東且往海表面移動，至冬季時最強。研究結果顯示，此一夏季西北太平洋海氣耦合模，為meridional mode發展成El Niño過程中主要結構之一。

This study applied EOF and SVD analysis to the assimilated ocean data and atmospheric reanalysis to explore the interannual variability of the leading coupled ocean-atmosphere mode during the period from June to September. The leading mode is characterized by the warm (cool) sea surface temperature and heat content anomaly in the Philippine Sea between the equator and 20°N and the coupled easterly (westerly) wind anomaly in the lower troposphere. The pattern was found to be closely associated with the Pacific meridional mode and ENSO. In the case of warm phase, the coupled mode was led by a cyclonic circulation in the subtropical central Pacific, which is characterized by the southwesterly near the dateline between 0~15°N. The westerly anomaly in the cyclonic circulation led to the weakening of the North Equatorial Current and therefore the further cooling in the tropical western North Pacific and the warming in the eastern Pacific. By the boreal summer, the cyclonic circulation shifts farther westward to the Philippine Sea, and the westerly prevails in almost the whole tropical Pacific. This is consistent with the reduced convection in the Philippine Sea, enhanced convection in the eastern Pacific, and a weakened Walker circulation. The Bjerknes mechanism would lead to the positive feedback between the atmosphere and ocean, and results in the occurrence of El Ni o. During the evolution, a pool of subsurface warm water moves from the dateline to the eastern Pacific from spring to winter. This is also a major feature in the ENSO. The summertime coupled mode in the tropical Western North Pacific identified in this study is apparently an important linkage between the Pacific meridional mode that occurs in the boreal spring and the El Niño that peaks in the boreal winter.