1975/76年氣候突變對西北太平洋熱帶氣旋生成的影響

The Impact of 1975/76 Climate Shift on Tropical Cyclone Formation in Western North Pacific

林和(Ho Lin) ； 吳宜昭(Yi-Chao Wu)

《大氣科學》 29卷3期 (2001/09) Pp. 257-272

摘要

本研究之重點在探討1975/76年氣候突變(Climate Shift)前後西北太平洋區熱帶氣旋生成的改變。在適當區分出亞洲夏季季風的次季節後，發現熱帶氣旋生成有顯著的超十年週期變化，此變化在盛夏（七月中至九月上旬）尤其明顯。超十年週期變化的空間分佈可以歸納為一西北／東南走向的羅士培波，其節點落於(5~12.5°N，150~165°E)及(17.5~25°N，125~140°E)區域。據此，我們定義了一熱帶氣旋羅士培指數，並區隔出前後兩紀元相位之明顯相異。根據變化顯著的年份進行合成分析，結果顯示這是超十年週期變化、ENSO及南北半球季風系統複雜的交互作用而導致的結果。假設一類似ENSO暖態的超十年週期基本態存在，東太平洋赤道的高海溫距平引起中西太平洋的西風距平。當ENSO處於暖態且澳洲冬季季風到達鼎盛之際，此基本態將觸動區域（南半球）哈德雷環流的爆發。此區域哈德雷環流上升帶位於10°N、140°~170°E，在澳洲東南方的地面冷高壓處下沉。在上升帶，對流被跨赤道流及熱帶西風距平北側之氣旋式風切加强，孕育了許多熱帶氣旋。此區域（南半球）哈德雷環流顯然自成一緊密且可自我加強的循環結購。在另一方面，位於西北太平洋東南象限的增強對流激發出一向西北方傳遞的羅士培波列，它有效地減弱了亞洲夏季季風。季節擺動改變了羅士培波的傳播方向，導致在第二紀元鄰近台灣的區域，熱帶氣旋生成在夏季減少，卻在秋季增多。

關鍵字

熱帶氣旋 ； 氣候突變 ； 超十年變化 ； 羅士培波 ； 澳洲冬季季風

並列摘要

This study is aimed to investigate changes of tropical cyclone (TC) formation of the western North Pacific before and after the 1975/76 climate shift. Based on properly divided sub-seasons of Asian summer monsoon, changes of TC formation in interdecadal time scale can be found loud and clear particularly during late summer (mid-July to tenth of September). A NW-SE aligned Rossby wave pattern, which foci fall into boxes (5~12.5°N, 150~165°E) and (17.5~25°N, 125~140°E), dictates spatial pattern. Thus, a TC Rossby index was defined and two epochs, each represents different phase of interdecadal variation, were identified. Composite study performed on those outstanding years revealed complicated interplay between interdecadal change, ENSO and monsoon systems of BOTH hemispheres. The interdecadal change is assumed an ENSO-like pattern, an enlarged anomalous SST pattern that induces westerly winds in central and western Pacific during the second epoch. When Australian winter monsoon reaches its peak this basic state tends to trigger an outbreak of local (southern) Hadley cell during warm phase of ENSO, with subsiding leg settles on the surface cold high near the southeastern comer of Australia and rising leg around10°N, 140°~170°E. The convection patch, that hatches TC, is fueled by the cross-equatorial flow and cyclonic shear sustained by the decadal-long also warm-phase ENSO tropical westerlies. The local (southern) Hadley Cell apparently drives a compact and self-intensified cycle. On the other hand this enhanced convection on SE quadrant of western North Pacific emits a Rossby wavetrain toward NW direction that, effectively weakens Asian summer monsoon. The seasonal swing of Rossby wave propagation causes someareas (near Taiwan) where TC decreases in summer and increases in autumn after climate shift.

並列關鍵字

Tropical cyclone ； Climate shift ； Interdecadal variation ； Rossby wave ； Australian winter monsoon