北太平洋副熱帶高壓冬季及冬春轉換的控制因子

Abstract

本研究探討北太平洋副熱帶高壓在北半球冬季與冬季轉春季時期的控制因子。我們的分析顯示北太平洋洋盆的西風噴流對於控制副熱帶高壓的強度及位置具有關鍵的作用。其中，噴流強度在緯向上的差異使冬季沉降場有兩個不同的來源：西、中太平洋受平均翻轉環流 (time-mean overturning circulation) 控制與東太平洋受反氣旋式羅士培波碎裂 (anticyclonic Rossby wave breaking) 影響。準確來說，在西、中太平洋較強的噴流抑制了該區的羅士培波碎裂，這使此處有較強風切渦度的帶狀副熱帶高壓被熱帶對流透過熱力驅動區域哈德里胞 (thermally-driven local Hadley cell) 以及噴流出區的地轉調整環流所主導。另一方面，相較於西邊的強西風噴流，東太平洋較弱的西風以及其伴隨的位渦 (potential vorticity) 梯度使此處較容易發生羅士培波碎裂。透過羅士培波碎裂的合成分析，我們也發現波動碎裂可以加強東太平洋局部的沉降進而使該區有更強的曲率渦度與封閉反氣旋環流。針對副熱帶高壓的季節轉換，我們發現東太平洋有較強曲率渦度的封閉高壓在冬季轉春季時於中太平洋明顯的快速向西擴張，而此西擴可歸因於中太平洋的噴流於此時期減小，並與南半球熱帶加熱的削弱有關。此季節轉換的分析也進一步支持我們將噴流視為副熱帶高壓控制因子的猜想。而這樣由噴流參與其中的概念不只揭示了冬季北太平洋副熱帶高壓的控制因子，也為研究外熱帶-熱帶交互作用的過程提供了可能的框架。

This study investigates the controlling factors of the North Pacific subtropical high, focusing on boreal winter and the rapid transition in spring. We show that the jet in the Pacific basin plays a critical role in controlling the strength and the location of the North Pacific subtropical high. Specifically, the zonal differences in the jet strength lead to two distinct sources of subsidence in winter: the time-mean overturning in the west and the anticyclonic Rossby wave breaking in the east. The strong jet in the western and central North Pacific inhibits wave breaking. The subtropical high belt with strong shear vorticity in this region is then controlled by the tropical convection through the thermally-driven local Hadley cell and the time-mean geostrophic adjustment at the jet exit region. On the other hand, the weak jet and associated PV gradient in the eastern Pacific result in higher wave breaking frequency. The composite analysis of Rossby wave breaking days further suggests the strengthening effect of wave breaking on the local subsidence and the closed anticyclone with relatively strong curvature vorticity. From winter to spring, the closed anticyclone in the eastern Pacific rapidly expands westward, which can be attributed to the weakening of the jet and the upstream Southern Hemispheric tropical heating. The seasonal transition supports the hypothesis of the jet in controlling the North Pacific subtropical high in winter. The proposed concept of the jet-mediated controlling factor of the subtropical high also provides a framework to investigate the mechanisms of the extratropical-tropical interaction.