How global warming impacts synoptic scale surface temperature gradients and warm-season precipitation in the United States Midwest.

Abstract:

Precipitation in the Midwestern United States is the primary source of water for 70 million people across the Mississippi Basin and the Laurentian Great Lakes, and it supplies water for $80 billion worth of agricultural products annually. The majority of precipitation in the US Midwest falls during the spring and summer (March-September), and as such, changes in warm-season precipitation have major social, environmental, and economic implications. In this talk, a unified hypothesis for anomalous warm season hydroclimatic circulation in the Midwestern US is introduced that considers contributions from geostrophic mass flux, ageostrophic mass flux, and atmospheric moisture supply. Dozens of potential predictive indices of warm season precipitation, derived from the experimental literature and based primarily on earth surface observations, are evaluated for their relative importance in nowcasting bimonthly precipitation anomalies from March until September. Predictive indices associated with ageostrophic mass flux anoamlies are important in early summer, and predictive indices associated with Atlantic-sourced integrated vapor transport are important in late summer. At a two month lead, precipitation indices associated with continental-scale thermodynamic processes which drive meso-scale circulation phenomenta are more important relative to PIs associated more with local convective phenomena, which dominate in real-time. Precipitation indices representing geostrophic mass flux anomalies are highly important throughout the warm season, but particularly in during transitional months (late spring/early fall). Several new precipitation indices established for consistency with the hypothesized mechanisms driving geostrophic mass flux, that characterize land-sea thermal contrasts in the Eastern and Western Atlantic Basin and the polar mid-latitude thermal gradient, emerge as highly important, with strong implications for explaining current, and projecting future, regional hydroclimatic shifts under climate change.