Direct weakening of tropical circulations from masked CO2 radiative forcing
Direct weakening of tropical circulations from masked CO2 radiative forcing
复制标题
掩盖的二氧化碳辐射强迫直接削弱热带环流
DOI：
10.1073/pnas.1508268112
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
T. Merlis
中科院分区：
文献类型：
--
作者：
T. Merlis
Significance The response of the atmospheric circulation to anthropogenic carbon dioxide changes has consequences for socially important aspects of climate, including rainfall and hurricane development. A substantial body of literature relates circulation weakening to temperature changes; however, it has been recently discovered that there is also a direct weakening of tropical overturning circulations from increased carbon dioxide concentration, which are independent of temperature changes. However, the cause of this direct slowing of the circulation has not been revealed. Here, I introduce a physical mechanism for the direct circulation weakening that accounts for its ubiquity across climate simulations. The previously unidentified mechanism is demonstrated across a hierarchy of atmospheric simulations including those used in the United Nations Intergovernmental Panel on Climate Change. Climate models robustly simulate weakened mean circulations of the tropical atmosphere in direct response to increased carbon dioxide (CO2). The direct response to CO2, defined by the response to radiative forcing in the absence of changes in sea surface temperature, affects tropical precipitation and tropical cyclone genesis, and these changes have been tied to the weakening of the mean tropical circulation. The mechanism underlying this direct CO2-forced circulation change has not been elucidated. Here, I demonstrate that this circulation weakening results from spatial structure in CO2’s radiative forcing. In regions of ascending circulation, such as the intertropical convergence zone, the CO2 radiative forcing is reduced, or “masked,” by deep-convective clouds and high humidity; in subsiding regions, such as the subtropics, the CO2 radiative forcing is larger because the atmosphere is drier and deep-convective clouds are infrequent. The spatial structure of the radiative forcing reduces the need for the atmosphere to transport energy. This, in turn, weakens the mass overturning of the tropical circulation. The previously unidentified mechanism is demonstrated in a hierarchy of atmospheric general circulation model simulations with altered radiative transfer to suppress the cloud masking of the radiative forcing. The mechanism depends on the climatological distribution of clouds and humidity, rather than uncertain changes in these quantities. Masked radiative forcing thereby offers an explanation for the robustness of the direct circulation weakening under increased CO2.
