聖嬰與沿熱帶風暴路徑之擾動能量診斷

Abstract

摘要 本研究利用三維擾動能量診斷及高解析度ECHAM4.6 T106數值模式研究聖嬰、西北太平洋大尺度環流場和熱帶風暴間的交互作用。研究結果發現，當七至九月Niño3.4區域的海溫增暖時（暖聖嬰年），伴隨季風槽與低層西風之增強和向東南延伸，西北太平洋熱帶風暴之生成區域亦向東南發展。低層之擾動正壓能量轉換和高層之擾動斜壓能量轉換均增強，此兩能量轉換之正距平自130°-150°E菲律賓海向換日線延伸。而西北太平洋地區正壓和斜壓能量轉換的增強，主要與熱帶中東太平洋海溫正距平引發之氣旋式距平、向東延伸的西風噴流和上升運動有關。大尺度環流場的變異、擾動正壓和斜壓能量轉換的增強共同有利於熱帶風暴生成區向東南的發展。 暖年時，西北太平洋之副熱帶和中緯度地區為反氣旋式距平及下沈運動盤據，此大尺度環流之配置並不利於擾動正壓能量轉換。當西北太平洋地區之熱帶風暴移行至20°N以北時，支持熱帶風暴持續發展和增強之能量來源為較強的擾動斜壓能量轉換。而斜壓能量轉換將擾動可用位能轉至擾動動能後，損失的擾動可用位能可經由擾動本身之非絕熱加熱效應產生，或經由擾動熱能垂直傳輸，將平均可用位能轉換至擾動可用位能。而後者之能量轉換機制在二維之擾動可用位能診斷中無法顯現。綜合擾動能量診斷之結果可知，暖年時，大尺度環流場及對應之擾動正壓能量轉換有利於熱帶風暴生成區向東南發展，而熱帶瞬變擾動（包含熱帶風暴）可經由擾動本身非絕熱加熱及斜壓過程，得以自我發展並增強。 為進一步釐清與聖嬰有關的海溫變異、大尺度環流場和擾動能量過程三者間之主動和被動關係，本研究採用高解析度ECHAM4.6 T106進行三組數值實驗。控制實驗以全球觀測海溫驅動ECHAM4，此模式成功模擬140°E 以東之大尺度環流場以及熱帶風暴生成、頻率的年際變化。數值實驗的結果顯示，赤道中東太平洋海溫的增暖於大尺度環流場及熱帶風暴年際變化上扮演一主動角色，其於西北太平洋上產生氣旋式環流和強西風距平，有利擾動正壓及斜壓能量轉換增強，支持瞬變擾動（包含熱帶風暴）生成及發展。 然而，赤道西太平洋之冷卻效應則會使得由中東太平洋暖海溫引發之低層氣旋式環流和西風噴流之強度減弱。 ECHAM4 T106對熱帶風暴強度的模擬明顯低估，此誤差可能來自其解析度的不足，而限制了模式中瞬變擾動（包含熱帶風暴）的自我成長過程，並進而降低模式裡瞬變擾動與大尺度環流場之間的交互作用。提高模式的解析度和/或採用海氣耦合模式可能有助改進模式內擾動場之非絕熱釋放過程，以及擾動非絕熱加熱對熱帶風暴強度、路徑和大尺度環流場影響之模擬。

Abstract The interaction among ENSO, large-scale circulation and tropical storm (TS) activity over the western North Pacific is investigated through three-dimensional eddy energetics and ECHAM4.6 T106 high resolution numerical experiments. The results show that the TS formation regions shift southeastward associated with the southeastward extension and intensification of monsoon trough and low-level westerly when the July to September Niño3.4 SST is increased (warm ENSO years). Both the eddy barotropic energy conversion at low-levels and eddy baroclinic energy conversion at high-levels enhance and extend eastward from the Philippine Sea (130°-150°E) to the date line. The enhanced barotropic and baroclinic energy conversion is mainly related to the cyclonic anomaly, eastward extension of westerly jet and anomalous ascending motion associated with warm SST anomaly over the tropical central-eastern Pacific. These anomalous large-scale environments and the barotropic energy conversion as well as the baroclinic energy conversion are favorable for the southward extension of TS formation region. The anomalous cold SST, anticyclonic anomaly and anomalous descending motion appear over the subtropical and mid-latitude western North Pacific during warm years. These large-scale circulations are not beneficial for the barotropic energy conversion in this region. The enhanced baroclinic energy conversion plays an important role in maintaining and strengthening the subsequent development of tropical storms as they propagate into north of 20°N over the western North Pacific. The loss of EAPE to EKE due to the baroclinic energy conversion is supplemented by both the EAPE generation through eddy diabatic heating and MAPE to EAPE conversion associated with the vertical heat transport of eddies, which is excluded in two-dimensional EAPE budget equation. These eddy energetic results suggested that the large-scale circulations and corresponding eddy barotropic energy conversion mainly contribute to the southeastward shift of TS genesis region. The tropical transient eddies including tropical storms may be self-development and intensify through their diabatic heating and baroclinic processes. To clarify the active and passive roles between ENSO-related SST, large-scale circulation and eddy energy process, three numerical experiments using the ECHAM4.6 T106 high resolution model are performed. The ECHAM4 control experiment, force by observed SST globally, reproduces the interannual variations of large-scale circulation, TS formation and TS frequency east of 140°E as in observation. The results of numerical experiments support that warm SST forcing over the equatorial central-eastern Pacific plays an active role in setting the favorable large-scale environments for the formation and early-development of transient eddy including TS over the western North Pacific. The equatorial central-eastern Pacific heating generates significant cyclonic anomaly and strong westerly that enhanced the eddy barotropic and baroclinic energy conversion for the growth of transient eddies including TSs. However, the cooling effect over the equatorial western Pacific would weaken the amplitudes of low-level cyclonic anomaly and westerly jet induced by warm SST anomaly over the equatorial central-eastern Pacific. The ECHAM4 T106 significantly underestimates the intensity of TS which might be resulted from the insufficient resolution. This discrepancy would restrain the strengthening of transient eddy including TS through self development process and in turn reduce the interaction between transient eddies and large-scale circulation in model. A sufficient high resolution and/or an air-sea coupled model might improve the simulations of eddy diabatic heating and its impact on the TS intensity, track and large-scale environments.