PNNL

The Science

Understanding the effects of global warming on regional precipitation is an important challenge. The midwestern United States currently receives abundant rainfall, but climate model projections of rainy-season rainfall changes show high uncertainty. This study revealed the critical role of a poleward jet shift in driving seasonally dependent precipitation change in the U.S. Midwest. While future rainy-season mean precipitation does not show a notable change relative to current precipitation, there is a substantial increase (decrease) in the late-spring (late-summer) precipitation driven by the poleward jet shift. These results deepen scientific understanding of and enhance confidence in model-projected Midwest hydroclimate changes.

The Impact

The U.S. Midwest represents one of the most agriculturally intense areas in the world. Its hydroclimatic response to global warming has significant societal and economic impacts. The change in precipitation patterns will likely make extremely wet late springs and extremely dry late summers much more common. This implies an increased risk of late-spring deluges and late-summer droughts for the Midwest.

Summary

This study investigates the response of the U.S. Midwest hydroclimate to global warming, based on both the Coupled Model Intercomparison Project Phase 6 and the Initial-condition Large-ensemble Simulations. Future hydroclimate changes in the Midwest will feature increased precipitation in April-May, but reduced precipitation in July-August. The warming-induced poleward jet shift drives these patterns as it enhances the low-level southerly winds in late spring but suppresses weather disturbances, or the storm track, in late summer. The effect of the poleward jet shift can be understood as an analog to the climatological seasonal progression. As the westerly jet shifts poleward under global warming, it becomes closer to (farther away from) the Midwest before (after) June and leads to more (less) precipitation. The extremely wet late springs and extremely dry late summers will occur much more frequently in a warmer future. A once-in-a-century event in the historical period could become a once-in-a-decade event by the end of this century under a high emission scenario. The severity of such climate impacts has been obscured in previous projections that just examined the rainy-season mean.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov

Funding

This research was supported by the Department of Energy, Office of Science, Biological and Environmental Research as part of the Regional and Global Model Analysis and Multisector Dynamics program areas.