Future Drying Subtropical East Asia in Winter: Mechanism and Observational Constraint

Coupled climate models project robust wintertime wetting trend over midlatitude East Asia under global warming scenarios, but the projected change in precipitation shows large intermodel uncertainty over subtropical East Asia from southern China to southwestern Japan. Based on an ensemble of climate models participating in CMIP6, this study shows that the weakened southern branch westerly jet (SWJ) on the southern side of Tibetan Plateau (TP) plays a key role in suppressing subtropical East Asian precipitation. The SWJ is deflected into southwesterly wind on the south-eastern side of TP, bringing ascent and precipitation to subtropical East Asia primarily through isentropic gliding. As a re-sult of the poleward and upward shift of the planetary-scale westerly jet under global warming, the SWJ becomes weaker and it acts to suppress subtropical East Asian precipitation by weakening the southwesterly wind and ascent. The SWJ- precipitation linkage also exists on interannual time scales, but the sensitivity of precipitation to interannual SWJ variabil-ity is systematically underestimated by the models compared with observation. The combined effects of the change in SWJ strength and the sensitivity of precipitation to SWJ explain about 40% of the intermodel spread of the projected precipita-tion changes. Observational constraint on the SWJ-precipitation relationship amplifies the projected drying trend and nar-rows the intermodel spread. It shows that the regional-averaged precipitation over subtropical East Asia decreases by 3.3% per degree of warming, and the amplitude of precipitation reduction over subtropical East Asia (southern China) is about 1.4 (3.4) times the raw projection.

Automated summary

This study shows that the weakened southern branch westerly jet (SWJ) on the southern side of Tibetan Plateau plays a key role in suppressing subtropical East Asian precipitation. Global warming weakens the SWJ, which further acts to suppress precipitation by weakening the southwesterly wind and ascent. The observational constraint on the SWJ-precipitation relationship amplifies the projected drying trend and narrows the intermodel spread.