熱帶季內振盪的辨識與對熱帶以外天氣的影響—聚焦東亞冬季降水

Abstract

本研究以奇異分解法（Singular Value Decomposition）發展一個SVD2D_DJFM熱帶季內振盪（MJO）冬季指標，來重新檢視MJO對熱帶以外區域天氣的影響。相較於廣為被應用的全季節性RMM指標（Wheeler and Hendon 2004），SVD2D_DJFM更能解析出熱帶對流處於冬季氣候背景下，主要偶極冷、熱源在經向以及緯向的重要分布特徵；同時也更能掌握伴隨MJO熱帶波動、連結熱帶以外區域的大尺度環流結構。觀測資料分析顯示，SVD2D_DJFM更能辨識出因為受到海洋大陸連結西太平洋附近（MCWP）冷源強迫力影響而導致的東亞降水。在東亞降水受MJO影響的波動周期中，MCWP不論是單極型式或是以配搭印度洋反向熱源的偶極型式呈現，都是MJO熱帶冷、熱源強迫力的重要組成。MCWP冷源對影響東亞天氣的重要性也透過線性斜壓模式（LBM）的數值模擬得到證實。與過去的研究所強調MJO印度洋熱源強迫力在南亞副熱帶引發向下游東傳的羅士培波列相比，MCWP冷源造成的經向環流變化對東亞冬季降水的影響更為直接。此外，本研究指出菲律賓附近一個熱帶次中心（Sub_PH）的存在與否亦是MCWP冷源強迫力影響東亞降水的關鍵。Sub_PH次中心伴隨整個MCWP冷源，於副熱帶區強化斜壓結構羅士培波環流的形成，為東亞帶來暖濕南來氣流，加上局地哈得里經向環流亦受MCWP對流抑制活動影響而減弱冬季風環流，都提供了東亞有利降水的環境條件。研究最後亦加入使用動力預報模式進行相關探究，發現MCWP冷源可影響模式對MJO波動的預報能力。在對流好發於印度洋的MJO事件中，具有MCWP冷源領先出現的分組類別能有更好的預報表現，研究結果顯示MCWP冷源對週尺度以上展期天氣預報的應用價值，也更加突顯MCWP冷源強迫力在MJO波動中的重要性。

The influences of the Madden–Julian oscillation (MJO) on East Asian (EA) winter rainfall is revisited in this study by introducing an MJO index SVD2D_DJFM through a singular value decomposition (SVD) approach to resolve the seasonal-dependent spatiotemporal evolution of the MJO and associated circulation. This method uses two-dimensional instead of latitudinally averaged variables in the commonly used real-time multivariate MJO (RMM) index. A comparison of the two approaches is made and the result shows the SVD composite reveals a more conspicuous and coherent variation throughout the MJO cycle. In particular, the SVD analysis identifies the convection anomalies over Maritime Continent and Western Pacific (MCWP) as a major cause of enhanced rainfall in EA at RMM phases 8 and 1 (SVD phase S1–S2). Our research shows that the MCWP cooling is as important as the IO heating in the MJO phase (RMM P2–P3/SVD S3–S4) that was regarded the major cause of EA rainfall in most previous studies. The observational analysis in our study suggests that the cooling and heating over MCWP, either in the form of monopole or dipole mode, is an important component of the MJO heating that results in stronger regional circulation along with subtropical Rossby wave responses influencing weather in East Asia. This suggestion is substantiated by linear baroclinic model (LBM) experiments that support the interpretation of the observational analysis, in that the MCWP-cooling induced anomalous meridional circulation is a more direct cause of enhanced EA rainfall than the IO-heating (or the IO-MCWP heating dipole) induced Rossby wave teleconnection. We note that the suppressed convection over east of Philippines (Sub_PH) is a critical component in the MCWP cooling that is coupled with significant baroclinic subtropical Rossby wave gyres, which bring low-level southerly warm flow to the EA region. The important role of MCWP tropical forcing in the MJO cycle is further revealed in the prediction model. Our analysis shows MJO events carrying strong convection over Indian Ocean with a precursory MCWP cooling are more predictable in CFSv2 model than those without the precursory cooling. These results highlight the important role of the MWCP tropical forcing on the extended weather forecasts.