Amplified subtropical stationary waves in boreal summer and their implications for regional water extremes

Abstract:

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 stationary waves found in recent decades. Specifically, we show that in multiple modern reanalysis products, a robust positive trend exists in a wave amplitude index defined through summer-mean tropospheric stream function field. Pronounced changes in the subtropical atmospheric circulation accompany this wave amplification, including intensified South Asian monsoon and strengthened subtropical highs over the North Pacific and North Atlantic oceans. Through modifying characteristics of large-scale moisture transport, these circulation changes are in turn coupled with changes in regional precipitation amount and the occurrence of water extremes including both droughts and heavy rainfall events. Given this connection, the amplified stationary waves have likely contributed to the elevated occurrence probabilities of droughts in the central United States, Mexico, Japan and northern China as well as those of heavy rainfall events in South Asia, southeastern China and eastern United States. Since the amplifying stationary waves are a robust feature in models’ projection of future climate, our results suggest an increased risk of water extremes over the above-mentioned regions in the near future.