探討中高緯的強迫影響熱帶環流之時間尺度與機制

Abstract

間熱帶輻合區(ITCZ)會受到中高緯緯度南北不對稱的能量收支變化而南北移動。為了瞭解中緯度強迫影響熱帶的時間尺度與機制，本研究使用與不同海洋混合層深度耦合的大氣模式，並在中緯度海洋混合層中放入不隨時間改變的熱力強迫，分析系統的平衡態與時序的變化。 在平衡態的變化中，渦流扮演在中緯度調節傳遞能量的重要角色。另一方面，平均經向環流(MMC)則主宰了在熱帶能量傳遞的改變，其透過哈德里環流(Hadley Circulation)的上層環流將乾淨能(DSE)流向較冷的半球，以及下層環流把水氣、降雨帶移往較暖的半球。而在副熱帶上層的大氣，距平的渦流動量通量的輻合輻散場(S)則會和異常地球自轉渦度的經向風平流(f·v)平衡。這說明渦流和哈德里環流變化是密切相關。 雖然平衡態的改變在不同海洋混合層深度(MLD)的實驗結果是相似的，但是中緯度熱力強迫影響到熱帶環流的時間是會隨著海洋混合層深度越深而越晚發生。在中緯度強迫逐漸影響熱帶過程中，本研究認為有時間上兩階段的變化：一、當放入中緯度熱源/冷源的初期，中緯度強迫會快速地使中緯度的溫度變暖/變冷和溫度梯度明顯的減弱/增強，這導致了中緯度的渦流強度和副熱帶上層西風的減弱/增強。二、至少要六個月時間熱帶海表溫度才會被中緯度強迫影響，這時副熱帶上層渦流和哈德里環流強度會開始互相調整，間熱帶輻合區會開始移往較暖的半球。在時序變化的分析也顯示了，大氣的渦流主導將異常的能量和動量從中緯度傳到副熱帶邊緣。雖然在熱帶環流被影響之前，渦流動量通量及渦流能量通量有部分改變，可是主要的改變還是發生在熱帶環流被調整之後。而被風-蒸發-海表面溫度(WES)回饋機制所主宰的海表過程，才是扮演把異常能量從副熱帶傳到熱帶的關鍵角色。因為熱帶環流是被海表面溫度所限制，所以中緯度的強迫影響到熱帶的時間尺度會被海表過程所決定，在五十公尺深的海洋混合層下大概花八個月，而在四百公尺深的海洋混合層下則要花約五十個月。

The intertropical convergence zone (ITCZ) shifts meridionally in response to hemispheric asymmetry extratropical thermal forcings. To understand the mechanisms and the time scales of extratropical forcings influencing the tropics, this study investigates the equilibrium and the transient responses of a time invariant extratropical thermal forcing in an atmospheric model coupled to an aquaplanet slab mixed layer ocean. In equilibrium responses, the midlatitude eddy plays a key role in transporting anomalous heating/cooling equatorward in midlatitudes. The anomalous mean meridional circulation (MMC) accomplishes the changes in energy transports in the tropics, fluxing dry static energy (DSE) toward the cooler hemisphere via the upper branch of the anomalous Hadley Circulation and pushing moisture and precipitation toward the warmer hemisphere. In subtropical upper atmosphere, the anomalous eddy momentum flux divergence (S) is balanced by the anomalous meridional advection of planetary vorticity (f·v) associated with Hadley Circulation. Although the equilibrium responses are similar in the cases with various mixed layer depth (MLD), the time scales of the tropical circulation responding to the imposed extratropical forcings increase in cases with deeper MLD. We report two distinct stages of the responses to the extratropical forcings: (1) When imposing extratropical heating/cooling, the midlatitude warms/cools and decreases/increases temperature gradient rapidly, resulting in weakening/strengthening midlatitude eddies and upper-level subtropical westerly. (2) Once the tropical SST is influenced by the imposed forcings, which takes at least 6 months, eddies in the subtropical upper-level and Hadley cell strength adjust and ITCZ shifts to the warmer hemisphere. Our transient analysis suggests that atmospheric eddies play the main role of transporting anomalous energy and momentum from the extratropics to the edge of the subtropics. Although there are small changes in eddy momentum and eddy heat flux in the subtropical regions occurring before the tropical circulation adjustments, the changes are largely enhanced after the adjustments take place. Surface processes, dominated by wind-evaporation-SST (WES) feedback, appear to play a key role for transporting anomalous warming/cooling from the subtropics into the deep tropics. Since tropical circulation is constrained by SST, the time scales of extratropical forcings influencing the tropics is determined by surface processes, ranging from 8 months for the cases with 50m MLD to about 50 months for the cases with 400m MLD.