Future aerosol reductions and widening of the northern tropical belt
Future aerosol reductions and widening of the northern tropical belt
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DOI：
10.1002/2016jd024803
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发表时间：
2016-06-27
影响因子：
4.4
通讯作者：
Ajoku, Osinachi
中科院分区：
文献类型：
--
作者：
Allen, Robert J.;Ajoku, Osinachi
观测结果表明，热带在过去几十年中扩大了，这一现象与亚热带干旱区向极地迁移和大规模大气环流有关。虽然这一信号的一部分与自然气候变异有关，但研究发现，主要与温室气体和平流层臭氧损失有关的外部强迫因素。在这里，我们表明，在二十世纪世纪气溶胶的增加导致收缩的北方热带，从而抵消了部分扩大与温室气体的增加。然而，在世纪，当气溶胶排放预计将减少时，气溶胶和温室气体的影响相互加强，两者都有助于北方热带带的扩大。模型有较大的气溶胶强迫，包括气溶胶间接影响云的寿命和寿命，产生显着更大的北方半球（NH）的热带扩大比模型直接气溶胶的影响。更有针对性的模拟表明，未来减少气溶胶可以驱动北半球热带扩大温室气体一样大，和理想化的模拟显示北半球中纬度气溶胶强迫的重要性。从机制上讲，21世纪世纪气溶胶的减少在北纬40度附近达到峰值，这导致地面太阳辐射的相应最大增加，北半球中纬度对流层变暖放大，以及最大斜压性纬度的向极移动，这意味着大气环流的相应变化。如果气溶胶间接影响的模型更好地代表了真实的世界，那么未来的气溶胶变化很可能是一个重要的，如果不是主导的北半球热带带扩大的驱动力。
Observations show that the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. Although part of this signal is related to natural climate variability, studies have identified an externally forced contribution primarily associated with greenhouse gases (GHGs) and stratospheric ozone loss. Here we show that the increase in aerosols over the twentieth century has led to contraction of the northern tropical belt, thereby offsetting part of the widening associated with the increase in GHGs. Over the 21st century, however, when aerosol emissions are projected to decrease, the effects of aerosols and GHGs reinforce one another, both contributing to widening of the northern tropical belt. Models that have larger aerosol forcing, by including aerosol indirect effects on cloud albedo and lifetime, yield significantly larger Northern Hemisphere (NH) tropical widening than models with direct aerosol effects only. More targeted simulations show that future reductions in aerosols can drive NH tropical widening as large as greenhouse gases, and idealized simulations show the importance of NH midlatitude aerosol forcing. Mechanistically, the 21st century reduction in aerosols peaks near 40 degrees N, which results in a corresponding maximum increase in surface solar radiation, NH midlatitude tropospheric warming amplification, and a poleward shift in the latitude of maximum baroclinicity, implying a corresponding shift in atmospheric circulation. If models with aerosol indirect effects better represent the real world, then future aerosol changes are likely to be an important-if not dominant-driver of NH tropical belt widening.