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Increased atmospheric moisture content and changing land-ocean thermal contrasts are usually implicated in the East Asian summer rainfall response to global warming. We highlight an additional influence whereby increased meridional stationary eddy circulation downstream of the Tibetan Plateau drives a stronger moisture flux convergence over southeastern China, intensifying the early summer rainband. This effect occurs robustly across the Community Earth System Model large ensemble Representative Control Pathway 8.5 simulations, and only for its pre-Meiyu stage (mid-May through late June). The westerly jet impinging on the Plateau is also enhanced, suggesting an orographic influence on the westerlies as the root cause. The increased westerlies result from warmer tropical tropospheric temperatures that enhance the meridional temperature gradient at the latitudes of the Plateau. Our results highlight the potential role of the westerlies in altering the East Asian early summer monsoon rainfall in the late 21st century.

Plain Language Summary Early summer rainfall over East Asia is projected to increase in the future as consequence of two related physical changes resulting from global warming: more moisture in the air with warmer temperatures, and stronger monsoonal circulations. A set of state-of-the-art simulations of the future climate that we analyzed suggests that the rainfall increase will be especially pronounced in the "pre-Meiyu" stage (mid-May to late June) when the East Asian rainband extends from southeastern China to southern Japan; at the end of the century, rainfall over southeastern China is projected to increase up to 40%. The cause of this intensification is different: it occurs because the westerlies "linger" at the latitudes of the Tibetan Plateau during this time instead of migrating northward, and the resulting atmospheric flow downstream brings about stronger convergent flows that concentrates and delivers more moisture to the pre-Meiyu rainband. More work, and using other models, will be needed to demonstrate that this future pre-Meiyu intensification is real. However, the significance of our result from a climate science viewpoint is in demonstrating that the change in the westerlies impinging on the Tibetan Plateau can have a first-order effect on future changes to the East Asian summer rainfall climate.