全球暖化下熱帶區域環流改變之能量限制

Abstract

在全球暖化下，大氣環流的變化是未來氣候預測中主要的不確定來源之一。過去的研究成功利用能量限制來解釋熱帶環流的整體減弱以及間熱帶輻合區 (ITCZ) 的移動。在本篇論文中，我進一步探討了全球暖化下大氣能量收支與熱帶環流在區域尺度上的關聯性。根據CMIP6氣候模式的abrubt-4xCO2 模擬 (將二氧化碳的濃度突然提高至四倍)，本篇論文提出兩項關鍵發現。第一項發現是，大氣能量收支的改變在陸地與海洋之間的演化具有明顯差異，這種海陸能量差異解釋了模式平均的沃克環流 (Walker Circulation) 變化在初始暖化階段與準平衡階段之間的反轉。第二項發現指出，北半球夏季時，歐亞大陸中高緯度的雲量減少 (導致太陽輻射增加)，以及南大洋的熱吸收，促使熱帶的深對流向北移動，進而增強南亞季風降水。

第一項關鍵發現是基於我對全球暖化下大氣能量源 (energy source) 以及能量匯 (energy sink) 變化的診斷。在最初的二十年間，能量改變的空間分布主要和海陸差異有關，大氣的能量源主要集中在陸地上。隨著時間推移，熱帶東太平洋區域的能量源逐漸發展，而陸地上的能量源則逐漸減弱。沃克環流的變化可以通過以上能量源的相對強度解釋：當陸地的能量源較強時，沃克環流傾向於增強；而當東太平洋的能量來源較強時，沃克環流則趨於減弱。從能量角度來看，初期沃克環流的增強有助於將過多的能量從陸地重新分配到海洋；相對的，在長時間尺度下，赤道東太平洋區域的逐漸升溫增強了該處的能量源，驅動沃克環流的減弱。

第二項關鍵發現則是基於對季節尺度下大氣能量收支改變的進一步分析。在暖化下，大氣能量收支的改變具有顯著的季節性。在北半球夏季期間，南亞地區的南北向能量梯度顯著增強，驅動了異常的向南能量傳送；此能量傳送的改變則使深對流向北移動至南亞地區。進一步分析顯示，南亞區域能量梯度的增強主要由兩個因素驅動：歐亞中高緯度地區的雲回饋 (形成異常的能量源)，以及南大洋的海洋吸熱(形成異常的能量匯)。進一步的氣候模式實驗驗證了這一假說：在抑制這兩個區域過程後，南亞季風降水的增加被削弱了68%。大氣水氣收支的診斷顯示，深對流的北移在增強南亞季風降水方面扮演了關鍵角色；這與傳統的解釋 (即南亞季風降水的增加主要是由於大氣濕度上升) 有所不同。

本論文的研究結果顯示，從能量觀點出發，有助於理解熱帶區域環流的變化，並指出大氣能量收支的變化可用來解釋全球暖化下區域環流的轉變。

Changes in atmospheric circulation under global warming are major sources of uncertainty in future climate projections. Previous studies have successfully accounted for the overall weakening of tropical circulation and the shift of the zonal-mean ITCZ by applying energetic constraints. In this dissertation, I take a further step by investigating the connection between energy and tropical circulation on regional scales under global warming. The dissertation presents two key findings. First, a robust land–sea contrast in the evolution of the anomalous atmospheric energy budget, as revealed by CMIP6 abrupt-4xCO₂ simulations, accounts for the reversal of model-mean Walker circulation changes between the initial warming stage and the quasi-equilibrium stage. Second, the combination of reduced solar reflection due to decreased cloud cover and enhanced heat uptake over the Southern Ocean, particularly during boreal summer, leads to a robust northward shift of the deep convections that enhances the South Asian monsoon rainfall.

The first key finding is built on my analysis and interpretation of the spatial distribution of anomalous energy sources and sinks evolution under global warming. In the first two decades, a “land-sea contrasting” pattern emerges, with anomalous energy sources mainly over land. Over time, an energy source develops and intensifies over the tropical eastern Pacific, while the energy source over lands diminishes. The response of Walker circulation depends on the relative strength of these energy sources: it tends to strengthen when the Afro-Eurasian energy source dominates and weakens when the eastern Pacific energy source prevails. From the energetic perspective, the Walker circulation strengthening acts to redistribute the excessive energy from land to ocean and the ocean processes contribute to the inter-model spread. On the other hand, the long-term weakening of Walker circulation is driven by the gradually developing energy source in the equatorial eastern Pacific as the region warms by the slowdown of subtropical cell and warm water ventilation.

The second key finding arise as the analysis goes beyond annual-mean changes. There are significant seasonal variations in the atmospheric energy budget. The meridional energy gradient over the South Asia is found to be largely enhanced during the boreal summer, driving anomalous southward energy transport which demands the northward shift of deep convections toward the South Asia. Further analysis shows that the enhanced meridional energy gradient over the South Asia during boreal summer is driven by two key factors: a positive cloud feedback over extra-tropical Eurasia (creating an energy source) and ocean heat uptake over the Southern Ocean (creating an energy sink). Mechanism-denial experiments confirm this hypothesis: suppressing these two regional processes reduces the excess SAM rainfall by 68%. This shifting of deep convections is found to play a critical role in enhancing the South Asian monsoon rainfall; this mechanism contrasts with the conventional explanation that links increased South Asian monsoon rainfall primarily to higher atmospheric moisture.

The results in this dissertation demonstrate the implications of energetic perspective on the changes in tropical regional circulations, highlighting that the responses of atmospheric energy budget can be used to understand and constrain the changes in regional circulation under global warming.