東亞-西太平洋夏季季風之年際變化

Interannual variability of the East Asia-West Pacific summer monsoon

林育鮪(Yu-Wei Lin)

指導教授 ： 林和

國立臺灣大學/理學院/大氣科學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.02966

摘要

東亞-西太平洋夏季季風(East Asia-West Pacfiic suumer monsoon, EA-WPSM)年際變化受到該區域複雜的時、空間結構，具有強烈區域敏感性。本文以EA-WPSM兩大年際變化型態為基礎，特別著重於西太平洋季風槽未曾提及的重要熱帶物理機制，並澄清溫帶海氣交互作用對於副熱帶高壓年際變化的貢獻。
EA-WPSM最主要年際變化(EOF1占有24.7%)反映著西北太平洋季風槽(monsoon trough)變化，故稱為MT mode，並直接受到當時的聖嬰/南方震盪(El Nino/Souther Oscillation，ENSO)狀態作用(相關係數r=0.87)。MT mode以130°E以東的熱帶西太平洋降水跟渦度在赤道南北兩側之帶狀變異為主體。赤道以北自菲律賓海向赤道中太平洋傾斜的正渦度帶，反應著西北太平洋季風槽；赤道以南負渦度帶被新幾內亞島嶼所分離，顯示低層兩支跨赤道氣流分別源自班達海跟索羅門海進入季風槽所造成的效果。由於季風槽主要水汽來源是由新幾內亞跨赤道氣流挾帶的水汽平流主導。應用大氣邊界層模式解析跨赤道氣流的影響因子，發現海洋大陸一帶的邊界層氣壓梯度扮演著主導的角色。此邊界層氣壓梯度的形成是當時ENSO冷卻海洋大陸海溫並透過Lindzen Nigam 機制所致。
另一EA-WPSM年際變化呈現Nitta 1987年所提的太平洋日本遙相關(Pacific Japan pattern)環流結構(EOF2占13.9%)，主要反映在太平洋跟日本南北向帶狀渦度震盪，故稱為PJ mode。PJ mode最主要環流特徵為西太平洋副熱帶高壓脊處于強烈沉降因而增強，雖ENSO領先PJ mode半年，但兩者關係(r=0.53)還有解釋空間。本文比對El Nino JJA(1)跟PJ mode在EA-WPSM的結構差異，發現黑潮延伸區在PJ mode年顯著冷卻，強化25°N到40°N間的經向海溫梯度。從經向垂直剖面亦可見西風噴流南移，高層瞬變渦流輻合亦提供副高脊高壓的增強作用。增強的副高脊亦可提供暖濕西南氣流強化其北側梅雨鋒面。高層大氣駐波活動通量(WAF)在35°N至45°N由北向南傳送能量，熱帶低層大氣向北的WAF可達30°N，反映PJ mode之溫帶/熱帶交互作用。由於夏季黑潮延伸區海洋混合層快速變薄，此季節性變化利於梅雨鋒面遮蔽短波輻射造成該區海溫冷卻之效應，形成中緯度海氣交互作用的正回饋。換言之，更險峻的海表面溫度梯度需要更有效的斜壓調節，東亞季風內的鋒面/海溫梯度/副高脊透過正回饋具備了自我維持的能力。至於PJ mode中印度洋熱容效應(IO capacity)在春季支持副高脊之預先條件(pre-condition)，反而在夏季的效用為次要。
有趣的是本文所提出的兩個物理機制都跟海溫經向梯度有關。MT mode以海洋大陸冷海溫強化經向海溫梯度，透過Lindzen Nigam 機制影響跨赤道氣流對季風槽的調整。而PJ mode則是在夏季北太平洋冷海溫出現時，強化了大氣斜壓性，透過斜壓調整過程，強化梅雨鋒面跟副熱帶高壓的耦合結構。

關鍵字

年代際變化 ； 季風槽 ； 太平洋-日本遙相關 ； 聖嬰/南方震盪 ； 副熱帶高壓 ； 梅雨鋒面

並列摘要

The interannual variability of East Asia-West Pacific summer monsoon (EA-WPSM) is a very complicated phenomenon. Because eddies with various spatial and temporal scales interact with each other, that causes results by statistical analysis to be sensitive in domain and variable. The study shows two dominant modes of the EA-WPSM which passed statistical tests to gain most comprehensive information.
The first EOF mode, called the monsoon trough (MT) mode, is directly forced by current El Nino/Southern Oscillation (ENSO) state. The MT mode is characterized by zonally elongated structure stretching from 130°E to date line. The major bands as a dipole-like pattern, lies on two sides of the equator. The band north of the equator, tilted from the Philippine Sea toward the equatorial eastern Pacific, can be identified with the monsoon trough. The band on the Southern Hemisphere is weakened by New Guinea Island, which relates to the low-level cross-equatorial flow that originates from the Banda Sea and Solomon Sea to the monsoon trough, which causes two anomalous vorticity poles. The major moisture source of the monsoon trough is the low-level mositure advection laterally driven by the New Guinea cross-equatorial flow. By decomposing contributions to the cross-equatorial flow based on the linear mixed layer model, the boundary layer pressure gradient in the Maritime Continent plays a major role. The pressure gradient is further found to be related to the densely-packed sea surface temperature (SST) gradient near the equator around New Guinea, which is well correlated with concurrent ENSO state.
The second laeding mode (EOF2 13.9%) shows the famous Pacific Japan pattern discovered by Nitta in 1987, called the PJ mode. It also shows a band structure elongated in the east-west direction and more closes the coast of East Asia, especially around the Philippines to Japan. The major feature of PJ mode is the enhanced western Pacific subtropical high ridge (WPSH) under stronger subsidence. Although this mode reveals a positive correlation with the ENSO phase in the previous winter, their correlation (r=0.53) reflects that some influences are still not found. According to the comparsion the spatial structure between El Nino JJA(1) and PJ mode summer, the significant cold SST anomalies appear over the Kuroshio Extension in the PJ mode. It deepens the meridional SST gradient between 25°N and 40°N. The sharp SST gradient needs more active baroclinic adjustment. Over the North Pacific, the southward displacement of the westerly jet and extratropical storm track are borth connected to the enhanced meiyu rainband, which changes the shortwave radiation at surface and cools SST with shallow oceanic mixed layer depth in summer. The enhanced WPSH also supplies the northeastward warm and moist low-level flow to meiyu. In addition, the wave activity flux shows significant upper southward transport around 30°N~45°N and reveals a northward transport around WPSH (15°N~30°N), which implies the interaction between the meiyu front and subtropical ridge. Therefore, the meiyu front, SST gradient and WPSH ridge among the East Asia summer monsoon via the positive feedback to promote the self-maintaining mechanism. On the other hand, the Indian Ocean capacity effect reveals a pre-condition of enhanced WPSH in spring, but it becomes a second forcing of PJ mode in summer. As a result, both MT and PJ mode have their major original forcing related to the SST gradient, through the mechanisms are completely different. The MT mode reveals the influence of near-equatorial Lindzen Nigam mechanism, and the PJ mode is associated with the enhanced baroclinic adjustment.

並列關鍵字

Inetrannual variabilty ； monsoon trough ； Pacific Japan pattern, El Nino/Southern Oscillation, subtropical high ； Meiyu

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