了解區域強迫對熱帶環流之季節性影響

Abstract

本研究欲了解熱帶大氣環流對南大洋和赤道地區兩種區域熱力強迫的季節敏感度，並以CESM模式耦合無海洋環流變化但允許海氣交互作用之單層海洋為工具。在加熱南大洋的實驗中，每個月皆出現異常的跨赤道哈德里胞，該哈德里胞之上層將高位能的空氣往北半球輸送，下層將水氣往南半球輸送，而該異常哈德里胞強度呈現弱季節性，季節變動不超過其年平均異常之25%；哈德里胞的氣候平均場之季節特性，使該異常哈德里胞顯著地加強冬半球哈德里胞的角動量平流，造成冬半球的副熱帶噴流之變化比夏半球強。在加熱赤道的實驗中，每個月皆出現一對在近赤道地區異常增強的哈德里胞，其加強了兩個半球的副熱帶噴流，反應類似聖嬰現象；因赤道加熱造成的異常上升運動與氣候平均場的上升運動皆發生於夏半球，故該異常哈德里胞之季節性為夏季較強；副熱帶噴流在冬季的反應並不如南大洋加熱實驗顯著，正是因為夏季較強的異常哈德里胞抵銷了副熱帶噴流的部分季節敏感度。此外，兩實驗中異常哈德里胞之季節性皆與地表短波輻射和潛熱通量緊密連結，而短波和潛熱的季節性變化可以追溯到雲和風的季節性變化，雲和風之季節性變化則與它們的氣候平均場有所關聯。另外，藉由CESM海洋-大氣耦合模式中對於真實氣膠排放之模擬，我們發現其哈德里胞與副熱帶噴流之變化與兩者之關係相似於理想實驗的結果，說明了在人為氣候變遷下，上述理想實驗之機制之應用性。

The Community Earth System Model (CESM) coupled to a mixed layer slab ocean is used to understand the seasonal sensitivity of atmospheric tropical circulation to two idealized regional thermal forcings over the Southern Ocean and the equatorial region. In response to heating over the Southern Ocean, an anomalous cross-equatorial overturning circulation with less than 25% variation takes place year-round, transporting air with high geopotential energy toward the Northern Hemisphere through its upper branch and pushing moisture transport toward the Southern Hemisphere through its lower branch. Due to the seasonality of climatological Hadley cell (HC), the anomalous cross-equatorial cell has a stronger effect on the cross-equatorial winter HC and almost no influence on the summer cell. As a result, there are more significant changes in the advection of angular momentum flux and the strength of subtropical jets (STJs) in winter months in both hemispheres. In response to equatorial heating, a dipole of enhanced anomalous HCs develops in the inner tropics, similar to those during El Niño, strengthening both STJs. The anomalous enhanced HC is stronger in summer months, because the anomalous ascending motion caused by the equatorial warming takes place in the summer hemisphere, where the climatological ascending motion occurs. The stronger STJ response during winter months is not as significant as that in the Southern Ocean heating experiment, since the stronger anomalous HC in the summer hemisphere compensates some of the seasonal sensitivity of STJs. Moreover, in both experiments, the seasonality of anomalous HC is tightly linked with sea surface temperature (SST), which is closely related to the seasonality of surface shortwave and latent heat fluxes. The seasonal changes of shortwave and latent heat can be traced back to the seasonal changes of cloud and wind, both of which could be understood and predicted by the properties of their climatological annual cycle. The seasonality of a realistic aerosols forcing experiment in CESM fully-coupled model with the dynamical ocean is compared to that of the two idealized experiments, demonstrating that the mechanisms identified from this simplified modeling study could be useful for understanding the HC and STJ responses to anthropogenic climate change.