在海氣耦合模式下探討熱帶太平洋氣候對中高緯度強迫之快慢反應

Abstract

古氣候證據和模式研究指出：中高緯地區的氣候變化對熱帶氣候有所影響。為瞭解中高緯的輻射改變影響熱帶太平洋氣候的機制，以及海洋動力在其中的角色，本研究使用海氣耦合的全球氣候模式，分別在南、北半球中高緯地區放入擾動太陽輻射，進而製造半球間熱力不對稱的情境，以分析氣候系統隨時間的反應。 實驗的頭十年，海表溫度呈現了明顯的半球間不對稱結構。首先輻射改變引起的中高緯海表溫度變化，會藉由風-蒸發-海表溫度反饋機制(wind-evaporation-SST feedback)，沿著副熱帶太平洋的盛行風向往赤道延伸，進而改變副熱帶東西向海表溫度梯度。 與之相比，第二十至三十年，海表溫度仍能觀察到半球間的不對稱，但不同的是，在赤道冷舌區相比前期有明顯變冷或變暖，本研究認為其生成涉及大氣和海洋的動力機制如下：以北半球中高緯加冷實驗為例，在跨赤道哈德里環流的結構下，伴隨的底層風場往較暖的南半球吹，在南太平洋形成西北風距平，擾動赤道南邊的斜溫層，改變海水熱含量。風場擾動斜溫層有兩種可能的機制：1) 熱帶東風減弱將使斜溫層的東西梯度減緩，使南太平洋西側的斜溫層變淺。2) 西北風距平藉由艾克曼抽吸(Ekman suction)造成向上運動，抬升斜溫層。由此，在原斜溫層附近的海水會變冷，接著藉由平均海流往赤道移動，最終赤道冷舌區的海表溫度變冷，同時也激發熱帶的正反饋加強海溫反應。

Paleoclimatic evidence and modeling studies suggest that extratropical climate changes have an influence on the tropical climate. This study aims to investigate the responses of the tropical Pacific climate to extratropical radiative forcings, with a particular focus on the role of ocean dynamics. We perturb the insolation over either the NH or SH extratropics to create an interhemispheric thermal contrast in a global coupled general circulation model. In the first decade, the sea surface temperature (SST) response shows a significant interhemispheric contrast. Responding to the change of interhemispheric temperature gradient, the extratropical SST anomalies propagate to the subtropical Pacific along the Eastern Pacific which can be explained by the wind-evaporation-SST (WES) footprinting mechanism, resulting in an east-west SST contrast in the subtropics. In contrast, the slow SST response shows that the interhemispheric contrast still exists, but the enhanced equatorial response (EER), more significant warming or cooling over the equatorial cold tongue region, develops in the 2nd to the 3rd decade after imposing extratropical radiative forcings. We proposed a coupled atmospheric and oceanic dynamical mechanism for the EER formation. Take the NH subpolar radiative cooling case as an example. In the southern tropical Pacific, the anomalous northerly surface wind associated with the cross-equatorial Hadley circulation in the fast response perturbs the depth of the thermocline, resulting in the anomalous cooling near the thermocline in the Western Central Pacific. Here are two possible ways to perturb the thermocline. 1) Weakened tropical easterly winds in the Southern Pacific flattens the thermocline with shallower in the Western Central Pacific. 2) The anomalous surface winds induce the Ekman suction, which raises the local oceanic thermocline. In turn, the anomalous cooling propagates north and east toward equatorial Eastern Pacific by the mean advection process. Then, after the EER response forms, there are some tropical feedbacks to enhance the slow EER response.