亞洲季風春夏轉換期的菲律賓海準雙週震盪與南海5-6月熱帶氣旋活躍度關係探討
Authors: 卓盈旻 卓盈旻 , Yin-Min Cho 盧孟明 , Mong-Ming Lu 吳靜軒 
Published: November 2024

<p>南海與菲律賓海區域對流與環流活動的季節、季內、年際、年代際等多種時間尺度變化是亞澳季風區季節轉變時的重要現象。分析1991-2023年3-6月春夏轉換期外逸長波輻射資料，發現這段期間的季內尺度深對流活動以菲律賓海(125&deg;E-135&deg;E, 10&deg;N-20&deg;N)的10-30天準雙週震盪(QBWO)最顯著。運用帶通濾波法挑選出震幅較大且週期明顯的71個菲律賓海QBWO個案進行合成分析，看到伴隨QBWO交替出現的對流發展和抑制相位，分別搭配橫跨南海與菲律賓海的850hPa低層大氣風場偏西南和東北方向的距平，前者加強南海上的正渦漩擾動有利熱帶氣旋(TC)生成，後者雖不利南海生成TC但有利西北太平洋TC向西移入南海。另外，分析發現5-6月南海TC和菲律賓海QBWO的活躍程度與特徵在1996-2011年和2012-2022年兩段時期差異極大，前期5-6月南海TC活躍，5月TC多數在南海生成，但6月TC多數在西北太平洋生成，5-6月菲律賓海QBWO相當活躍。後期5月南海完全沒有TC活動，菲律賓海QBWO也難以辨識，但6月南海TC和菲律賓海QBWO都轉為活躍，且TC多數在南海生成。造成這兩個時期差異的可能原因是1996-2011年期間印太季風區深對流活動提早進入夏季型態，孟加拉灣與菲律賓海之間的西風偏強；而2012-2022年印度洋海溫異常偏高使菲律賓海與東印度洋之間的海溫梯度加大致始東風距平增強，即西風異常偏弱。印度洋深對流活動對南海的影響在2023年的5-6月特別突出，這一年赤道印度洋和菲律賓海的對流均異常活躍，但南海這兩個月全無颱風活動。本文提出的春夏轉換期南海5-6月熱帶氣旋活動與菲律賓海的次月尺度深對流活動的關係，為研究亞澳季風區的區域氣候變異及改善區域展期預測提供了可結合區域氣候監測分析的思路。</p>
<p>&nbsp;</p><p>The multiple-scale atmospheric convective activities over the regions of South China Sea and Philippine Sea are important to understanding the regional climate characteristic during the boreal spring-to-summer transition period of the broad Asian-Australian monsoon system. In this study, we analyzed the Outgoing Longwave Radiation (OLR) data during March-June from 1991-2023 and found that the strongest sub-seasonal variability is with the periods of 10-30 day (QBWO) over the Philippine Sea(125&deg;E-135&deg;E, 10&deg;N-20&deg;N). After applying the band-pass filter to OLR data, we selected 71 strong QBWO cases and performed composite analysis to reveal the circulation and precipitation patterns associated with the QBWO. The results show clear alternation of the enhanced and suppressed phases of the deep convection over the Philippine Sea and the associated anomalous southwesterly and northeasterly winds across the South China Sea (SCS) and the Philippine Sea. The southwesterly anomalies form a favorable condition for the tropical cyclone (TC) genesis to occur over the SCS, while the northeasterly anomalies create a favorable condition for the western North Pacific TCs to propagate westward into the SCS. In addition, we identified two contrast periods of 1996-2011 and 2012-2022 based on the QBWO days. During the former period, the TC activity over the SCS during May and June was quite active and so was the Philippine Sea QBWO. The majority of the TCs were born over the SCS during May, while the majority of the TCs were born over the western Pacific during June. In contrast to 1996-2011, the entire period of 2012-2022 did not see any TC activity during May over the SCS. The TC activity resumed in June, but the majority are the SCS-born TCs. The contrast is attributed to the strong phase reversal of the low-level zonal wind from the Bay of Bengal to the Philippine Sea. During the period of 1996-2011, the seasonal transition over Indian Ocean-western Pacific region is earlier than normal which induced strong westerly anomalies across the SCS during May, whereas during the period of 2012-2022 the low-level zonal wind anomalies over the SCS turned to strong easterly anomalies associated with abnormally warm sea surface temperature (SST) over the Indian Ocean. The strong influence of Indian Ocean SST on the SCS climate during May is demonstrated using the 2023 case, which is a year with extremely warm eastern Indian Ocean SST near the equator, most active QBWO over the Philippine Sea but no TC activity over the SCS. In conclusion, we found that the tropical cyclone activity over the SCS and the QBWO activity over the Philippine Sea are usrful indicators showing the characteristics of spring to summer seasonal transition over the Asian-Australian monsoon region.</p>
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