理解反饋與熱量傳送對北極放大效應的貢獻

Abstract

北極放大效應指北極表面溫度變化幅度相較於全球其他區域更為顯著的現象，且在二氧化碳濃度降低和增加的情境下皆會出現，意指在二氧化碳減少造成全球冷卻的情境中，北極冷卻將比全球平均冷卻更顯著。本研究探討反饋交互作用對北極放大效應的影響，我們分析受到一系列廣泛的二氧化碳濃度（從工業化前的1/8倍到8倍）強迫的氣候模型模擬，結果表明北極放大效應確實在二氧化碳濃度下降的情況下發生，且冷卻北極放大效應的強度比二氧化碳濃度增加情境中的強度更強。反饋分析顯示，普朗克、失效率和反照率反饋是產生二氧化碳增加和減少情境中北極放大效應的主要因素，但與二氧化碳濃度減少最相關的是失效率反饋，其在冷卻情境中的強度比暖化情境中更強，不對稱的作用使得冷卻情境中有更強的北極放大效應。我們也藉由逐一關閉濕能量平衡模型中各項反饋，分析各反饋機制、大氣熱傳輸及其交互作用的貢獻。我們發現，在二氧化碳減少的冷卻模擬中，反饋交互作用對北極放大效應的貢獻比二氧化碳增加的模擬中的貢獻更強。特別的是，當二氧化碳濃度增加時，溫度垂直遞減率反饋的交互作用會導致負的北極溫度變化；而在二氧化碳濃度降低的情境下，則會產生正的溫度變化。這表明，溫度垂直遞減率反饋與其他反饋及大氣熱傳輸的交互作用是導致氣候冷卻情境下的北極放大效應比暖化情境更強的重要過程。反饋與大氣熱傳輸的交互作用通常會抵銷反饋間的交互作用。我們的結果突顯了非線性過程在產生北極放大效應對冷卻與暖化氣候不對稱反應中的重要性。

The Arctic amplification (AA), the phenomenon of amplified surface temperature response in the Arctic compared with the response elsewhere, can emerge under both reduced and increased carbon dioxide (CO2) forcings. In this study, we investigate the roles of feedback interactions contributing to AA. We analyze climate model simulations forced by a wide range of CO2 concentrations (from 1/8 to 8 times preindustrial level). Our results show that AA occurs not only under increasing CO2 but also under decreasing CO2, with the Arctic exhibiting an even stronger cooling-induced AA than the warming-induced counterpart. Moreover, the Planck, lapse-rate, and surface albedo feedbacks are identified as the primary contributors to AA in both scenarios. Among these, the lapse-rate feedback, in particular, demonstrates a stronger influence under CO2 reduction, thus reinforcing the asymmetric nature of AA in cooling versus warming climates. We also use a moist energy balance model (MEBM) to emulate the contributions of each feedback, atmospheric heat transport (AHT), and their interactions by locking the effect of each of them. We find that the contribution of feedback interactions to polar amplification is overall stronger in the CO2 reduction runs than in the CO2 increase runs. In particular, the lapse-rate feedback interaction in the CO2 increase runs leads to negative Arctic temperature change, whereas in the CO2 decrease runs leads to positive temperature change. This result indicates that the interaction of lapse-rate feedback and other feedbacks and AHT is a crucial process that gives rise to stronger AA in a cold climate state than that in a warm one. The feedback interaction with AHT generally counteracts the effect of feedback-feedback interactions. Our results highlight the importance of the nonlinear processes in producing AA asymmetric response to cooling and warming forcing agents.