气溶胶散射性相对占比增强对南亚北部恒河平原夏季风降水减少的贡献
李婧
气溶胶是气候变化的重要人为驱动因子之一，其也是辐射强迫不确定性最大的来源。气溶胶的辐射效应既来自于其总量变化，也来自于散射性与吸收性的相对占比，即成分比例变化。在传统的气溶胶强迫模拟试验中，二者的贡献均隐藏在气溶胶总的辐射效应当中。
图1 观测与模拟1980年代以来的南亚夏季风降水变化
Figure 1 Observed and simulated South Asia summer monsoon precipitation trends since the early 1980s
本研究聚焦于南亚北部恒河平原的夏季风降水变化，利用气候模式CESM1.2.2，系统分解了由气溶胶光学厚度AOD变化表征的气溶胶总量与由单次散射反照率SSA变化表征的散射吸收相对占比对南亚夏季风降水减少的贡献，发现SSA变化能够显著冷却南亚北部对流层中上层，抑制季风环流的抬升运动，从而导致局地降水减少。这说明散射性与吸收性的相对占比变化能够显著扰动温度廓线与局地的水循环，这一效应甚至强于气溶胶总量变化的影响。
对于CMIP6模式historical试验的进一步分析表明，绝大多数气候模式未能抓住南亚季风降水的变化趋势。虽然大多数模式可以较好地模拟出南亚北部的AOD上升趋势，但鲜有模式能够同时抓住南亚北部的SSA上升信号。对SSA变化的表征相对较好的模式所模拟的降水变化趋势也与观测更加接近，这说明CMIP6模式对于散射性与吸收性气溶胶相对占比的错误表征有可能是其降水模拟偏差的来源之一。这一结论强调了气候模式准确表征气溶胶成分的重要性，同时也说明了在更大范围内持续观测SSA的必要性。
该研究以“Increased aerosol scattering contributes to the recent monsoon rainfall decrease over the Gangetic Plain”为题发表在2023年8月的Science Bulletin上。
参考文献：
Ying, T., Li, J.*, Jiang, Z., Liu, G., Zhang, Z., Zhang, L., Dong, Y. and Zhao, C.*, 2023: Increased aerosol scattering contributes to the recent monsoon rainfall decrease over the Gangetic Plain. Science Bulletin, 68(21), 2629-2638. https://doi.org/10.1016/j.scib.2023.08.052.
Increased aerosol scattering contributes to the recent monsoon rainfall decrease over the Gangetic Plain
Aerosol radiative forcing has long been recognized to play important roles in driving local and global climate anomalies. Aerosols are also the most uncertain factor in anthropogenic forcing of climate change. The forcing of aerosols arises from the changes of both their total loading and compositions. While these two factors are implicitly embedded in climate change simulations, out study explicitly investigates the role of change relative fraction of scattering and absorbing aerosols. Using CESM1.2.2, we separate the relative importance of aerosol total loading change represented by aerosol optical depth (AOD) and aerosol compositional change represented by single scattering albedo (SSA), and find that SSA change is more effective cooling the upper troposphere in northern South Asia and suppress upward motion. This indicates that aerosol compositional change has a key role in perturbing temperature structure and hydrological cycle, which is even stronger than changing aerosol total loading alone.
Further analysis of CMIP6 historical simulations show that most of the models don’t capture the precipitation decrease in northeastern South Asia. Although these models show good consistency simulating the increasing trends of AOD, few of them successfully capture the increasing trends of SSA. Models with better representation of SSA trend tend to simulate a better precipitation trend comparing to observations. This result emphasizes the importance of accurate representation of aerosol composition and its optical properties my climate models in additional to aerosol total loading, and also shows the urgent need for SSA observations in a larger range.
This study has been published at Science Bulletin named ‘Increased aerosol scattering contributes to the recent monsoon rainfall decrease over the Gangetic Plain’.
