The role of soil moisture on summer atmospheric circulation climatology in the Northern Hemisphere

Soil moisture–atmosphere interactions intensify extremes like heat waves and droughts. Atmospheric circulation and soil moisture are key drivers for both local and remote extreme events via dynamical and thermodynamical mechanisms. Understanding the interaction between soil moisture and atmosphere dynamics, including potential feedback loops, is crucial for both climate attribution studies and sub-seasonal to seasonal forecasts. Here, we study the effect of soil moisture on large-scale atmospheric circulation using large ensemble simulations from the fully coupled climate model EC-Earth 3 from 2009 to 2016. The atmosphere evolves freely in all the experiments. Four sets of experiments are carried out with one control run in which the interaction between the atmosphere and the land is fully interactive. In contrast, in the other three experiments soil moisture is prescribed. The main finding of this study is that soil moisture impacts the climatological mean state of the atmospheric circulation in the Northern Hemisphere during the summer season (June to August) and especially in July. Specifically, we observe poleward shifts of subtropical jets and a stronger polar front jet in the experiment with the prescribed soil moisture climatology of the control experiment with interactive soil moisture. Additionally, by allowing the two-way interaction between land and atmosphere (control experiment), the wave amplitudes over land are strengthened by approximately 24% (compared to soil moisture prescribed), which implies the key role of land-atmosphere coupling in modulating atmospheric wave dynamics. The Stationary wave patterns over North America are triggered in the experiment with prescribed climatological soil moisture of the ERA land reanalysis. In addition, we find that interactive soil moisture leads to higher mean summer surface temperatures for most land areas up to + 1.5 k and even higher (+ 3 k) for temperature extremes (90th near-surface daily temperature). We conclude that soil moisture impacts the atmospheric circulation. Therefore, we expect circulation changes triggered by drying soils in large areas of the summer continents in projections of our future climate.