Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models
Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models
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DOI：
10.5194/acp-15-6367-2015
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发表时间：
2014-05
影响因子：
6.3
通讯作者：
Liang Guo;A. Turner;E. Highwood
中科院分区：
文献类型：
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作者：
Liang Guo;A. Turner;E. Highwood
抽象的。比较单一强迫品种的20世纪世纪的历史实验中的一个子集的模型从第五耦合模式相互比较项目（CMIP 5）显示，南亚夏季季风降雨增加到今天的温室气体（GHG）的实验相对于工业化前的水平，而减少在人为气溶胶的实验。这些单一强迫实验与所有强迫的历史实验的比较表明，气溶胶排放主导南亚季风降雨趋势，在最近几十年，特别是在20世纪50年代至70年代。在这些实验中，南亚季风降水的变化大致遵循在同一时期的半球间温度梯度的时间演变，这表明大尺度背景状态的贡献有关的气溶胶排放的不对称分布的赤道。通过检查24个可用的所有强迫的历史实验，我们表明，包括气溶胶间接影响的模式占主导地位的负降雨趋势。实际上，仅包括气溶胶直接辐射效应的模型显示季风降雨量增加，这与这些模型中温室气体排放增加和行星变暖对季风降雨量的主导作用一致。对于南亚，在间接影响的模型中，降雨量减少与陆面蒸发量减少有关，而不是来自水平水汽通量的异常，这表明间接影响对当地气溶胶排放的影响。这一点得到了南亚地区气溶胶含量和云滴数趋势的证实。因此，虽然遥远的气溶胶和它们在赤道附近的不对称分布在确定南亚季风作为全球季风之一的半球间温度分布方面发挥了作用，但间接气溶胶效应对当地气溶胶排放的影响也在季风降雨量下降方面发挥了作用。季风降雨对气溶胶排放增加的响应之间的差异只包含直接气溶胶效应的模型和那些也包含间接效应的模型需要迫切调查，因为建议未来亚洲人为气溶胶排放固有的代表性浓度途径（RCPs）用于未来气候预测可能会变得乐观。此外，两组模式都显示中国降水量减少，也与当地气溶胶机制有关。我们假设，在CMIP5模式中，中国上空的气溶胶排放量足够大，即使在没有间接气溶胶影响的情况下，也会导致季风降雨量下降。印度的情况并非如此。
Abstract. Comparison of single-forcing varieties of 20th century historical experiments in a subset of models from the Fifth Coupled Model Intercomparison Project (CMIP5) reveals that South Asian summer monsoon rainfall increases towards the present day in Greenhouse Gas (GHG)-only experiments with respect to pre-industrial levels, while it decreases in anthropogenic aerosol-only experiments. Comparison of these single-forcing experiments with the all-forcings historical experiment suggests aerosol emissions have dominated South Asian monsoon rainfall trends in recent decades, especially during the 1950s to 1970s. The variations in South Asian monsoon rainfall in these experiments follows approximately the time evolution of inter-hemispheric temperature gradient over the same period, suggesting a contribution from the large-scale background state relating to the asymmetric distribution of aerosol emissions about the equator. By examining the 24 available all-forcings historical experiments, we show that models including aerosol indirect effects dominate the negative rainfall trend. Indeed, models including only the direct radiative effect of aerosol show an increase in monsoon rainfall, consistent with the dominance of increasing greenhouse gas emissions and planetary warming on monsoon rainfall in those models. For South Asia, reduced rainfall in the models with indirect effects is related to decreased evaporation at the land surface rather than from anomalies in horizontal moisture flux, suggesting the impact of indirect effects on local aerosol emissions. This is confirmed by examination of aerosol loading and cloud droplet number trends over the South Asia region. Thus, while remote aerosols and their asymmetric distribution about the equator play a role in setting the inter-hemispheric temperature distribution on which the South Asian monsoon, as one of the global monsoons, operates, the addition of indirect aerosol effects acting on very local aerosol emissions also plays a role in declining monsoon rainfall. The disparity between the response of monsoon rainfall to increasing aerosol emissions in models containing direct aerosol effects only and those also containing indirect effects needs to be urgently investigated since the suggested future decline in Asian anthropogenic aerosol emissions inherent to the representative concentration pathways (RCPs) used for future climate projection may turn out to be optimistic. In addition, both groups of models show declining rainfall over China, also relating to local aerosol mechanisms. We hypothesize that aerosol emissions over China are large enough, in the CMIP5 models, to cause declining monsoon rainfall even in the absence of indirect aerosol effects. The same is not true for India.