海洋大陸於聖嬰期間的海陸空間降水不對稱

Abstract

海洋大陸處在東南亞赤道地區,位於印度洋與西太平洋之間,它是由大大小 小的島嶼組成,受周圍溫暖的海洋 (赤道暖池) 包圍。此區的降水豐沛,而其降 水和區域海溫變化息息相關且有相當明顯的年際震盪變化。在聖嬰發生期間,由 於沃克環流強度的改變,進而對海洋大陸地區的降水產生影響:聖嬰發生時,沃 克環流上升中心向中太平洋移動,海洋大陸的對流活動被異常的沉降作用抑制, 因此降水變少。在反聖嬰發生時,則有利海洋大陸對流活動的產生。對於海洋大 陸而言,除了大尺度環流的變化,當地局部環流的日夜循環(像是海陸風、山谷 風及積雲聚合過程) 對此地而言是很重要的降水機制。 在本研究中,前半部主要分析衛星以及測站的觀測降水資料,指出聖嬰事件 對海洋大陸降水的遙相關影響僅限於海洋上的降水,陸地降水受到聖嬰事件的反 應並不明顯。而後半部的研究採取數值工具模擬,考慮到解析度與模擬降水日循 環的表現,因此我們使用天氣研究與預報模式 (Advanced Research Weather Research and Forecasting Model, WRF-ARW) 作為模式模擬工具,進行多組海洋 大陸降水模擬敏感性測試:在控制組實驗中有模擬出在聖嬰發生時海陸的降水不 對稱反應。另一組實驗藉由移除婆羅洲陸地,模擬去掉陸海溫度差異所帶來的局 部日夜環流變化。結果顯示,在移除婆羅洲後,該地的降水反而會受聖嬰事件影 響。由此可知,由於海洋大陸的特殊海陸分佈,在聖嬰現象發生時,局部日夜環 流變化可能有利於調節海洋大陸的降水,因此降水變化在海陸呈現不對稱的空間 分布。

The Maritime Continent (MC) is located in the Southeast Asia and is composed of many islands and peninsulas, surrounding by warm ocean. It has one of the largest regional rainfall rates in the world, and through strong convective heating, the MC plays an important role in the Earth’s energy balance and hydrological cycle. The rainfall over the MC is also correlated with local SST and interannual variability. During El Nino, large scale downdraft anomaly imposes on the MC accompanying with decreased precipitation; in contrast, more active convection and precipitation occur in this region during La Nina. In addition to the interannual variability, the local diurnal variations in the land-sea breeze, mountain-valley winds, and cumulus merger process are the major contributions to the precipitation over the MC islands. In this study, we analyze several global precipitation data sets, demonstrating that precipitation shows an asymmetric response over the land and nearby oceans in the MC during different phases of ENSO (El Nino- Southern Oscillation). This study aims to reveal the role of the diurnal cycle in the asymmetric precipitation pattern over the MC during the ENSO events. Concerning the coarse resolution and its inability to simulate land-sea breeze in a global model, we adopt WRF-ARW (Advanced Research Weather Research and Forecasting Model) to setup several sensitivity experiments for a better understanding in precipitation changes in the MC region. Two simulations are conducted: one is control run and the other is water run (in order to remove diurnal-cycle changes by changing Borneo from land to ocean). The diurnal-cycle and the precipitation asymmetry between Borneo and Celebes Sea are well captured in the control run. In the water run, the diurnal-cycle disappeared and the precipitation over Borneo becomes more sensitive to large scale circulation han that in the control run. The results imply that the supplement of precipitation from diurnal cycle over Borneo is one of the major reasons that the precipitation slightly fluctuated during the ENSO events on the island.