Amplified subtropical stationary waves in boreal summer and their implications for regional water extremes
The linkage between climate change and increased frequency/magnitude of weather extremes remains an open question in the scientific field.Here we investigate such a dynamical linkage by focusing on an amplification trend of the northern subtropical stationarywaves found in recent decades. Specifically, we show that inmultiplemodern reanalysis products, a robust positive trend exists in awave amplitude index defined through the summer-mean tropospheric stream function field. Pronounced changes in the subtropical atmospheric circulation accompany this wave amplification, including an intensified SouthAsianmonsoon and strengthened subtropical highs over theNorth Pacific andNorthAtlantic oceans. Throughmodifying the characteristics of large-scale moisture transport, these circulation changes are coupled to changes in the regional precipitation amount and the occurrence of water extremes including both droughts and heavy rainfall events. Given this connection, amplified stationarywaves have likely contributed to the elevated occurrence probabilities of droughts in the central United States,Mexico, Japan, and northernChina, as well as those of heavy rainfall events in SouthAsia, southeasternChina, and the easternUnited States. These results suggest that as climatewarming continues, the amplification of subtropical stationarywaves will increase the risk of water extremes over the above-mentioned regions.