| 摘要: | 在東南亞地區，季內震盪(Intraseasonal Oscillations，簡稱ISOs)是此區域季風變異度中的一個重要分量，不論在調節雨季/夏季季風的肇始、夏季季風的活躍-中斷生命週期以及強降雨事件中，都扮演著顯著的要角。然而在不同的背景流場影響下，ISOs在夏季次季節中也呈現出不同的特徵。落座於東南亞最東邊的越南中部地區，可劃分成兩個不同的氣候子區域：中部高地(Central Highlands，簡稱CH)以及越南中部海岸地區(Central Coast of Vietnam，簡稱CCV)。CH為越南貢獻了高達90%的咖啡產量以及25%的水力發電潛能。相形之下，CCV的雨季經常發生強降雨與洪水事件，也為此地區造成巨大的生命財產損失。所以，了解越南中部地區雨季期間降雨的ISOs，對於天氣及次季節氣候預報能力之提升至關重要，除有助於提高咖啡與水力發電的產能外，也能協助降低大雨與水患所造成的災害。因此，本論文的主要目標是透過使用高解析網格降雨和再分析資料，探討越南中部地區降雨次季節中ISOs的演變以及其對降雨變化的影響。
首先，在CH地區7月下旬觀測到雨季中的第二階段強降雨特徵(Second Rainy Season，簡稱SRS)，有別於在4月下旬至5月上旬肇始的傳統雨季。因此，在第一部分將探討1983-2010年期間SRS肇始日(SRS Onset Date，簡稱SRSOD)的長期氣候狀態與年際變化情形，以及IOSs在其中所扮演的角色。經統計發現28年的平均SRSOD落在7月28日，標準差為17天，且CH地區的SRSOD與每年10-20天模和20-60天模的發展相位是同步的。令人驚訝的是，SRSOD顯著的年際變化具有3種不同型態：提前(延後)肇始的SRS有著多(少)1個月的雨期並於7月上旬(直到8月中旬)發生，以及正常的SRS雨期發生在7月下旬。幾乎所有提前肇始的SRS年都發生在聖嬰事件(El Ni?o)的發展期，特別是在Ni?o 3.4海表溫度(Sea Surface Temperature，簡稱SST)從1月到12月持續增加的期間。水氣收支分析顯示，水氣通量輻合輻散的年際變化係反應了7月較暖的赤道太平洋SST距平所造成，促使CH地區降雨加劇導致SRS肇始提早。
其次，在亞洲季風中擁有最長雨季的東南亞夏季季風(Southeast Asian Summer Monsoon簡稱SEAM)，可以透過SEAM指數以及中南半島降雨指數分成初夏(5/13~7/31)及夏末(8/1~10/17)。初夏時期，西南風盛行並影響整個東南亞地區；到了夏末，此區轉為受到季風槽所控制，導致越南CH地區的雨季呈現兩個截然不同的強降雨階段。因此，CH中降雨的準雙周震盪(Quasi-Biweekly Variation，簡稱QBV)在兩個次季節中顯現出不同的特徵。與初夏相比，特別在1997-2010年期間，QBV於夏末時期增強，且在CH地區降雨年際變化的調制上扮演了重要的角色。赤道西太平洋水氣通量的輻合(輻散)距平，在強且濕(弱且乾)的夏末增加(減弱)了QBV的強度，從而造成CH地區降雨的增加(減少)。
最後，東南亞的秋季(9-11月份)透過不同的環流背景場可被分成初秋(9/1-10/17)以及晚秋(10/18-11/30)。季風槽是控制東南亞初秋的關鍵因素；相反地，東北風伴隨著西伯利亞高壓增強所帶來的氣壓驟升(Pressure Surge)，則在晚秋扮演著重要的角色。對CCV的秋季降雨而言，QBV(12-24天週期)是最顯著的ISO，其次則是準9天震盪(Quasi-9-day Variation，簡稱Q9V，7-11天週期)。因此，藉由對1983-2010年期間通帶濾波處理後的降雨及再分析資料進行8相位合成分析，以探討初秋和晚秋時熱帶-溫帶之間的交互作用所伴隨的CCV地區降雨之QBV以及Q9V。初秋時期降雨的QBV主要是由赤道西太平洋生成的西北向移動之熱帶低壓擾動(Tropical Depression Disturbances，簡稱TDDs)所引發，隨後受來自俄羅斯東北的氣壓驟升而增強。相反地，晚秋時期降雨的QBV是受到西伯利亞高壓的氣壓驟升、赤道西太平洋東風，以及地形的效應共同影響。至於Q9V在初秋時期的濕相位首先受到來自東海移動至CCV地區的氣旋式TDD，以及自菲律賓海西傳至中國南方的反氣旋式TDD相互作用所造成的，接著被來自日本海向西南延伸的高壓距平所增強。然而，在晚秋時期降雨的Q9V受制於來自赤道西太平洋向西傳遞的TDDs、西伯利亞高壓之氣壓驟升以及地形之間的相互作用所影響。
;The intraseasonal oscillations (ISOs) are an important component of the monsoon variability, which play a significant role in modulating the rainy season/summer monsoon onset date, active/break cycles of summer monsoon, and heavy rainfall event in the Southeast Asia. The ISOs exhibit different characteristics between summer subseasons due to the influence of the distinct background flows. Located in the easternmost Southeast Asia, central Vietnam could be divided into two climatic subregions including the Central Highlands (CH) and central coast of Vietnam (CCV). The CH contributes up to 90% of the country’s total coffee production and 25% of its total hydropower potential. By contrast, heavy rainfall and flood event often appear during the rainy season in the CCV, resulting in great damage on human lives and properties. Thus, understanding about the ISOs of rainfall in central Vietnam during rainy subseasons is essential to improve the capability of prediction for weather and subseasonal climate, which may enhance coffee yields and hydropower production as well as reduce the damage caused by heavy rainfall and flood events. Hence, the main goal of this dissertation is to investigate the evolution of ISOs and their influence on rainfall variability during the rainy subseasons in central Vietnam by analysing high-resolution gridded rainfall and reanalysis data.
Firstly, a second rainy season (SRS) is observed in late July in the CH, which is distinct from the conventional rainy season that occurs in late April – early May. Therefore, the first part examines the climatology of and internnual variation in the SRS onset date (SRSOD) during 1983-2010 period as well as the role of the ISOs in the SRSOD. The average SRSOD for 28 years is July 28, with a standard deviation of 17 days. The SRSOD over the CH is synchronized with the developing phases of the 10-20-day and 20-60-day modes for each year. Surprisingly, the pronounced interannual variation in the SRS onset date has led to three apparent regimes: an early (late) SRS with a 1 month longer (shorter) rainfall period occurring in early July (until mid-August) and a normal SRS starting in late July. Almost all the early SRS years occur during El Ni?o developing phases, particularly during the Ni?o3.4 sea surface temperature (SST) increase from January through December. Water vapor budget analyses reveal that the interannual variation in the divergent water vapor flux is in response to the warmer July tropical Pacific SST anomalies, resulting in rainfall enhancement over the CH and eventually inducing early SRS onset.
Secondly, Southeast Asian summer monsoon (SEAM), which owns one longest rainy season in the Asian monsoon, can be divided into early (13 May-31 July) and late summer (1 August-17 October) based on SEAM index and the Indochina Peninsula rainfall index. The southwesterly prevails over the entire Southeast Asia in early summer while the monsoon trough controls this region in late summer, leading to two distinct rainy seasons in the Central Highlands (CH) of Vietnam. Consequently, the quasi-biweekly variation (QBV) of rainfall in the CH shows different characteristics between two subseasons. Compared to early summer, the QBV in late summer strengthens and plays more important role in modulating interannual variation of rainfall mean over the CH, especially during 1997-2010. The anomalous convergence (divergence) of water vapor flux over the tropical western Pacific enhances (reduces) the QBV intensity in the strong-and-wet (weak-and-dry) late summers, thus inducing more (less) rainfall over the CH.
Finally, in Southeast Asia, the autumn (Sepember-November) could be divided into early (Sepember-17 October) and late (18 October-November) autumn with the distinct background flow. Monsoon trough is the critical factor that controls Southeast Asia in early autumn. In contrast, the northeasterly wind accompanied with pressure surge, which is attributed to the intensification of the Siberian High, plays the key role in late autumn. The QBV (12-24-day variation) is the most significant ISO of rainfall over the CCV in the autumn, followed by quasi-9day variation (Q9V, 7-11-day). Thus, the tropical-extratropical interaction associated with the QBV and Q9V of rainfall in the CCV is then investigated during both early and late autumn by performing 8-phase composite analysis to bandpass-filtered precipitation and reanalysis data during 1983-2010 period. For the QBV, in early autumn, the QBV of rainfall is primarily induced by the northwestward-moving tropical depression disturbances (TDDs) originated from the equatorial western Pacific, which is then enhanced by pressure surge from northeast Russia. On the contrary in late autumn, the QBV of rainfall is regulated by the collaborative effects of pressure surge from the Siberian High, the easterly wind from the tropical western Pacific, and topography. For the Q9V, in early autumn, the wet phase of Q9V is first caused by the interplay of a cyclonic TDD moving from the East Sea to the CCV and an anticyclonic TDD propagating from the Philippine Sea to southern China, which is then strengthened by the southwestward extension of the high pressure anomaly from the Japan Sea. However, in late autumn, the Q9V of rainfall is governed by the interaction of the westward-migrating of TDDs from the tropical western Pacific, pressure surge from the Siberian High, and topography. |
