背景場在中高緯熱力強迫所致大氣環流反應所扮演角色

Role of Mean State on Atmospheric Circulation Responses to Extratropical Thermal Forcing

陳永真(Yung-Jen Chen)

指導教授 ： 黃彥婷

國立臺灣大學/理學院/大氣科學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202104040

摘要

本研究探討中高緯熱力強迫下大氣之反應與不同背景場對其反應之影響。本研究使用大氣模式耦合單層海洋，在外熱帶放入不隨時間改變之強迫，並使用兩種控制模擬：其一為以赤道經向對稱之氣候背景場，另一為類北半球冬季之氣候背景場。大氣反應的時間尺度和型態受到三種因子的影響：中緯度熱力結構反應、哈德里胞型態與氣候間熱帶輻合區(ITCZ)的位置。
首先，低層加熱強迫造成的中緯度熱力結構反應會減弱經向溫度梯度與垂直溫度梯度，兩者對斜壓性(baroclinicity)的效果彼此抗衡。在不同的氣候背景場下，兩種因子之異常對斜壓性的改變程度有所差異。在渦流熱力通量異常主導減弱斜壓性的情況下(如夏季與春秋分)，上層波動減少並使得副熱帶環流強度減弱。而如果垂直穩定度異常和渦流熱力通量異常貢獻相當(如冬季)，使得上層的波動異常微弱以致於無法調整環流強度。前者通過渦流動量通量快速地調整副熱帶環流，較後者發展早數個月。
第二，哈德里胞型態解釋了在夏半球副熱帶環流的快速反應。在夏季，哈德里胞處於渦流驅動型(eddy-driven regime)，即哈德里胞易受因斜壓性改變而造成之渦流動量通量異常調整。而在冬季，哈德里胞處於角動量守恆型(angular-momentum regime)，即副熱帶環流不易被渦流通量影響，僅緩慢地被低層傳播增溫調整。此二因子影響大氣反應的時間尺度。
最後，當增熱異常傳播至熱帶，氣候間熱帶輻合區阻斷了低層熱異常的擴散，並造成跨間熱帶輻合區之海表溫度梯度。同時，哈德里環流異常之最大值發生在氣候間熱帶輻合區附近，且跨間熱帶輻合區之海表溫度梯度與跨間熱帶輻合區之哈德里環流強度具顯著正相關性。亦即，間熱帶輻合區之阻斷效應顯著地限制了環流反應，並決定了低層熱異常是否能傳播至深熱帶與另一半球。

關鍵字

大氣動力學 ； 遙相關 ； 熱帶-溫帶交互作用 ； 間熱帶輻合區 ； 斜壓渦流

並列摘要

The study investigates the atmospheric responses to the extratropical forcing and the dependence on different background states. We conduct a set of experiments by applying time-invariant extratropical forcing in an atmospheric model coupled to an aquaplanet mixed layered ocean with two idealized control climates: an equinox-like climate and a boreal-winter-like climate. Our study suggests that the time scale and the pattern of atmospheric responses are affected by three factors: responses of midaltitude thermal structure, climatological Hadley cell regime, and the position of climatological Intertropical Convergence Zone (ITCZ).
First, the response of the midlatitude thermal structure to lower-level extratropical warming leads to decreases in meridional temperature gradient and in static stability, which have opposing effects on baroclinicity. Our results show that the thermal structure response is dependent on the background climate, arising a different degree of contribution from eddy heat flux and static stability to the baroclinicity change. If the eddy heat flux predominantly decreases the baroclinicity (i.e. summer or equinox), the upper-level wave activities are suppressed and weaken the subtropical circulation. On the other hand, if the decreased static stability equivalently contributes to the baroclinicity changes (i.e. winter), the upper-level wave activity responses would be too weak to modify local circulation. The former process results in rapid development of subtropical circulation modification through robust eddy momentum flux response, faster than the latter for about several months.
Secondly, the Hadley cell regime explains the fastest response of subtropical circulation in the summer cell, which is in the “eddy-driven regime” and substantially affected by the anomalous eddy momentum flux caused by the decreased baroclinicity. In the winter cell, which is in the “mean-momentum regime,” the subtropical circulation slowly responds to the eddy momentum flux responses and the lower-level propagating warming. The first and second factors affect the time scales for the atmospheric response.
Lastly, while the anomalous heat propagates into the tropics, the climatological ITCZ blocks the anomalous lower-level warming from spreading further and enhances the meridional sea surface temperature (SST) gradient. Meanwhile, a salient anomalous Hadley cell arises near the climatological ITCZ, with positive correlations between the anomalous cross-ITCZ SST gradient and the anomalous cross-ITCZ cell in all experiments. Namely, the ITCZ blocking effect constrains the position of robust anomalous circulation and determines whether the lower-level thermal response can propagate into the deep tropics and another hemisphere.

並列關鍵字

atmospheric dynamics ； teleconnection ； tropical-extratropical interaction ； ITCZ ； baroclinic eddy

參考文獻

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