菲律賓海反氣旋之維持機制

Abstract

聖嬰發展年北半球冬季，菲律賓外海會出現一低層反氣旋變異：這個反氣旋變異於十月生成後，迅速地在十二月發展到巔峰，並持續存在至隔年春夏，盤踞菲律賓海長達半年之久。菲律賓海反氣旋不僅對東亞季風區年際變化有顯著影響，亦被視為全球大氣對聖嬰反應之主要現象之一。 本文研究維持菲律賓海反氣旋之機制，結果顯示以下兩種影響途徑： (一) 遠端強迫過程 聖嬰發展時，全球熱帶對流層大氣會因赤道東太平洋變異而增暖，形成圍繞熱帶的環狀結構，同時在副熱帶呈現冷卻狀態。因之在聖嬰發展年秋末，南亞上空形成強烈經向溫度梯度，引發北緯10~20度附近高空副熱帶噴流。此噴流條出區位於華南外海，透過次環流之地轉調整，橫跨出區的非地轉風於其右側高空造成輻合區，提供菲律賓外海沉降的動力強迫作用，並經由Sverdrup平衡產生低層高壓環流。 (二) 局地風-蒸發-暖海回饋過程 當反氣旋存在於冬季西北太平洋地區的東北季風背景場時，風-蒸發效應將顯現：反氣旋東南側總風場的風速增強，加強海表蒸發，導致海面溫度冷卻；而西北側則反之。然而只有在海溫冷卻區域重合氣候場暖海深對流條件時，風-蒸發造成的些微海溫降低才能有效地引起其上空對流劇烈減弱，引發非絕熱冷卻改變環流場。伴有暖海條件的海溫冷區位於菲律賓海反氣旋的東南角，提供理想的強迫位置令羅士培反應正確地落在菲律賓海，使之回饋到原反氣旋，此過程並透過模式模擬充分驗証。這個局地海氣交互作用形成菲律賓海反氣旋自我維持之正回饋系統。 這兩種機制過程同時維持聖嬰發展年十月起至隔年四月間的菲律賓海反氣旋的存在，並且皆支持反氣旋在十月生成後急遽成長及隨後鞏固的現象。

During Nino development a low-level anticyclone often appears over the Philippine Sea in boreal winter, which, after its formation in October, grows rapidly and reaches peak around December. It will last until the next spring/summer for a sustained period more than half year. This Philippine Sea anticyclone (PSA) exerts major impacts on interannual variability of the East Asian’s climate, is also regarded one of the most dominant responses of global circulation from ENSO. This study is focused on the mechanism to maintain the PSA, our results show two vital paths : (1) Remote forcing: During the developing stage of Nino, the whole tropical belt will be warmed up to form an annular structure. In subtropical area it exhibits compensating cooling. The local manifestation in the South Asia shows a sharp upper temperature gradient in winter, that leads to the intensification of subtropical jet around 10~20N. This jet streak has its exit near the South China shore which induces sub-circulation via geostrophic adjustment. The result is a convergent zone associated with the cross-exit ageostrophic winds on the left side of exit, an area coincident with the upper troposphere over the PSA. Hence the strong subsidence, a result of dynamical forcing, produces this low-level anomalous high according to the Sverdrup balance. (2) Local wind-evaporation-warm ocean feedback: Once the PSA is formed it is located within the background of northeastern winds during the northern winter, the effect of wind-evaporation takes over: that is, on the southeastern half of the PSA the stronger total wind speed increases evaporation rate that cools the sea surface temperature and an opposite effect happens on the northwestern half of the PSA. Nevertheless, in terms of feedback of SST to atmosphere, this wind-evaporation effect must be added an extra condition: only over warm ocean when deep convection presents the anomaly of surface can be carried to free atmosphere that effectively modifies the atmospheric circulation. The coincidence between the wind-evaporation area and climatological warm ocean locates on the southeastern corner of the PSA, the ideal spot to put forth the Rossby response that has the correct positive feedback as the wind-evaporation cooling to circulation. This self-maintained in situ air-sea interaction can be fully verified by model simulation. Based on above two processes the PSA is expected to stay from October of Nino development year to the next April. It also gives clues to how the PSA rapid grows after October and establishes for later months.