Recent Tropical Expansion: Natural Variability or Forced Response?
Recent Tropical Expansion: Natural Variability or Forced Response?
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DOI：
10.1175/jcli-d-18-0444.1
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发表时间：
2019-03-01
影响因子：
4.9
通讯作者：
Staten, Paul W.
中科院分区：
文献类型：
--
作者：
Grise, Kevin M.;Davis, Sean M.;Staten, Paul W.
先前的研究已经记录了自1979年以来地球哈德利环流下沉分支的极移，但对这些观测到的变化的原因和全球气候模型捕捉它们的能力存在分歧。这篇综合论文重新审查了过去文献中一些相互矛盾的说法，并发现现代再分析所表明的热带扩张在模式对1979-2005年期间的历史模拟的范围内。早期的结论认为模式低估了观测到的趋势，这一结论依赖于使用旧的再分析的质量流函数来定义哈德利环流。最近观测到的热带扩张在北半球（NH）和南半球（SH）的年平均规模相似，但模式表明，驱动扩张的因素在两个半球之间有所不同。在北半球，温室气体（GHGs）的增加和平流层臭氧的耗竭促成了20世纪后期的热带扩张，如果温室气体继续增加，预计北半球热带边缘在21世纪将进一步向极地移动，即使平流层臭氧浓度恢复。在北半球，温室气体对热带扩张的贡献要小得多，即使到21世纪末，在大的自然变率背景下，温室气体对热带扩张的贡献仍然难以探测。为了解释两个半球最近相似的热带扩张率，必须考虑到自然变率。最近的大气-海洋耦合变率，包括太平洋年代际振荡，促进了热带扩张。然而，在以观测到的海面温度为强迫条件的模式中，由于大气内部变率，热带扩张率仍有很大差异。
Previous studies have documented a poleward shift in the subsiding branches of Earth's Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models' historical simulations for the period 1979-2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere-ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.