熱帶太平洋對外熱帶強迫力的反應：人為氣候變遷與自然變異的啟示

Abstract

為了理解人為氣候變化下的熱帶氣候變化，本論文利用一完全耦合的氣候模型—地球系統社群模式1.2.0版，研究熱帶太平洋對於外熱帶輻射強迫的瞬態氣候響應。透過在任一半球的外熱帶區域施加突變的太陽輻射改變，本論文檢視海洋表面與次表層溫度的不同反應，並探討驅動這些變化的基本機制。 在海洋表面，異常海表溫度的演變可分為兩個階段：(1) 在最初的三年內，赤道區海表溫反應在北半球外熱帶呈現與輻射強迫相反的符號，而在南半球外熱帶則與輻射強迫符號一致。在此階段，加熱北半球外熱帶對冷卻赤道太平洋的作用比冷卻南半球外熱帶更有效，因北半球外熱帶的異常暖化被氣候場的輻合雨帶阻擋，只能從西側進入赤道太平洋，從而更有效地觸發了皮耶克尼斯反饋機制。(2) 在十年後，所有實驗均顯示出增強的赤道反應，且其符號與輻射強迫一致，這歸因於受迫半球的海洋經向翻轉環流的減緩。南半球受迫情況的赤道海表溫反應更強，表明南半球外熱帶對於十年尺度上熱帶太平洋具有顯著的控制作用。 在次表層，觀察到兩半球亞熱帶太平洋區域內帶狀的溫度異常，這些異常在受迫半球呈現與外熱帶強迫相反的符號，而在另一半球則與強迫符號一致。次表層溫度響應在2-3年內出現，其可能主要由風應力驅動的海洋動力學（如斯維爾德魯普傳送和艾克曼抽吸）所引起。其作用時間尺度快於通常預期的海洋動力機制（數十年或更長），突顯了耦合的大氣-海洋交互作用在形成兩半球亞熱帶次表層溫度反應中的關鍵作用。 本論文的研究結果強調了耦合大氣-海洋動力機制在塑造熱帶氣候反應的時間與空間演變中的關鍵角色。由於北半球在全球變暖下經歷了顯著的地表暖化，這些結果對於理解人為氣候變化與熱帶氣候變率具有重要意義，並有助於解釋觀測到的熱帶太平洋海表溫趨勢、氣候模型中模擬的歷史溫度變化以及極端事件的變化（如熱帶氣旋活動與聖嬰事件）。此外，透過基於機制的診斷方法，這些結果為調和模型與觀測的差異提供了潛在途徑，並促使進一步透過更具現實性的實驗研究相關的時間尺度。

With the goal of understanding the tropical climate changes under anthropogenic climate changes, this dissertation investigates the transient climate responses of the tropical Pacific to extratropical radiative forcings, using a fully coupled climate model, the Community Earth System Model (CESM), version 1.2.0. By imposing abrupt incoming solar radiation changes in extratropical regions of either hemisphere, this dissertation examines the distinct responses of both surface and subsurface ocean temperature and explores the underlying mechanisms driving these changes. At the surface, a two-stage evolution of anomalous sea surface temperature (SST) is recognized: (1) in the initial three years, the equatorial SST responses exhibit an opposite sign to the forcings in the northern extratropics but align with those in the southern extratropics. At this stage, heating the northern extratropics is more effective at cooling the equatorial Pacific than cooling the southern extratropics. This occurs because the anomalous warming in the northern extratropics is blocked by the rainband and can only enter the equatorial Pacific from the west, triggering Bjerknes feedback more effectively. (2) Over a decade, all experiments show enhanced equatorial responses aligning with the signs of the forcings, attributable to the slowdown of the oceanic meridional overturning circulation in the forced hemisphere. The south-perturbed cases experience stronger equatorial SST responses, suggesting the significant control of the southern extratropics on tropical Pacific on decadal timescales. As for the subsurface, band-like temperature anomalies in the subtropical Pacific regions in both hemispheres are observed, which show the opposite sign against the extratropical forcing in the forced hemisphere and the same sign with the forging in the other hemisphere. The subsurface temperature responses develop within 2-3 years, which could be majorly driven by wind stress-induced ocean dynamics, such as Sverdrup transport and Ekman pumping. The operating timescale is faster than those expected by the oceanic dynamics, which are decadal or longer, highlighting the role of coupled atmosphere-ocean interactions in forming the subsurface temperature responses in both hemisphere subtropics. The findings in this dissertation underscore the critical role of coupled atmosphere-ocean dynamics in shaping the temporal and spatial evolution of tropical climate response to hemispherically asymmetric forcings. Since the Northern Hemisphere has experienced significant surface warming under global warming, these results have important implications for understanding the anthropogenic climate changes and tropical climate variability and help interpret the observed tropical Pacific SST trend, the simulated historical temperature changes in climate models, and shifts in extreme events including tropical cyclone activity and ENSO dynamics. Additionally, these results offer potential avenues for reconciling model-observation discrepancies through a mechanism-based diagnostic approach, urging further investigation of associated timescales through realistic experiments.