Falling Trend of Western Disturbances in Future Climate Simulations
Falling Trend of Western Disturbances in Future Climate Simulations
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DOI：
10.1175/jcli-d-18-0601.1
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发表时间：
2019-08-01
影响因子：
4.9
通讯作者：
Shaffrey, Len C.
中科院分区：
文献类型：
--
作者：
Hunt, Kieran M. R.;Turner, Andrew G.;Shaffrey, Len C.
西部扰动（WDs）是由副热带西风急流平流到巴基斯坦和印度北部的天气尺度气旋天气系统。在那里，它们负责大部分冬季降水，这对rabi作物的农业至关重要，也会导致更极端的降水事件，这可能导致当地的洪水和雪崩。尽管它们很重要，但目前还没有尝试客观地确定WDs在未来气候gcm中的命运。本文采用跟踪算法建立了CMIP5历史和未来代表性浓度路径（RCP）实验中WDs的目录。结果表明，在常规商业模式（RCP8.5）的未来气候模拟中，到21世纪末，WD频率将下降约15%，最大的相对变化出现在季风前和季风后的月份。同时，平均WD强度将降低，中心涡度预计将在同一时期减少12%左右的气旋性。冬季副热带急流的预估扩宽和减弱以及经向风切变和对流层中斜压涡度趋势的减弱也解释了WD频率的变化。探讨了这些变化对区域降水的影响。WD频率和强度的下降将导致巴基斯坦和印度北部冬季平均降雨量减少约为平均值的15%，这取决于模式代表相关过程的能力。然而，对极端降水事件的影响尚不清楚。
Western disturbances (WDs) are synoptic-scale cyclonic weather systems advected over Pakistan and northern India by the subtropical westerly jet stream. There, they are responsible for most of the winter precipitation, which is crucial for agriculture of the rabi crop as well for as more extreme precipitation events, which can lead to local flooding and avalanches. Despite their importance, there has not yet been an attempt to objectively determine the fate of WDs in future climate GCMs. Here, a tracking algorithm is used to build up a catalog of WDs in both CMIP5 historical and representative concentration pathway (RCP) experiments of the future. It is shown that in business-as-usual (RCP8.5) future climate simulations, WD frequency falls by around 15% by the end of the twenty-first century, with the largest relative changes coming in pre- and postmonsoon months. Meanwhile, mean WD intensity will decrease, with central vorticity expected to become less cyclonic by about 12% over the same period. Changes in WD frequency are attributed to the projected widening and weakening of the winter subtropical jet as well as decreasing meridional wind shear and midtropospheric baroclinic vorticity tendency, which also explain the changes in intensity. The impact of these changes on regional precipitation is explored. The decline in WD frequency and intensity will cause a decrease in mean winter rainfall over Pakistan and northern India amounting to about 15% of the mean-subject to the ability of the models to represent the responsible processes. The effect on extreme precipitation events, however, remains unclear.