Over the last four decades (1980–2020), the summer westerlies that prevail in South Asia along the monsoon trough region have weakened by about 25% based on multiple reanalysis datasets. Trends in a range of climate model simulations suggest that the weakening is driven by multiple anthropogenic forcings. Over the period, sea-level pressure has increased by 0.6–1.0 hPa over South Asia’s northwestern regions, induced by cooling due to aerosol emission and changes in land use and land cover, and has decreased over the Arabian Peninsula mainly due to warming by greenhouse gases. These changes in temperature and pressure act to weaken the regional pressure gradient, deflecting the subtropical westerlies from South Asia toward the Arabian Peninsula and weakening the winds in the monsoon trough and its adjacent region. The slowing down of winds appears to result in an anomalous moisture loading and increase in rainfall over the semi-arid northwestern South Asia. This weakening and its associated changes in regional climate are highly relevant to policymaking across South Asia.

根据多个再分析数据集，在过去四十年（1980-2020 年）中，南亚季风槽地区盛行的夏季西风带减弱了约 25%。一系列气候模式模拟的趋势表明，这种减弱是由多种人为强迫驱动的。在此期间，由于气溶胶排放和土地利用和土地覆盖变化导致的冷却，南亚西北部地区的海平面压力增加了 0.6-1.0 hPa，而阿拉伯半岛的海平面压力则有所下降，这主要是由于温室气体变暖。这些温度和压力的变化削弱了区域气压梯度，使亚热带西风带从南亚偏转到阿拉伯半岛，并减弱了季风槽及其邻近地区的风。风速减慢似乎导致南亚西北部半干旱地区的异常水分负荷和降雨量增加。这种减弱及其相关的区域气候变化与整个南亚的政策制定高度相关。