Summer aridity in the United States: Response to mid-Holocene changes in insolation and sea surface temperature

We examine the response of summer precipitation to mid-Holocene insolation forcing and insolation-induced changes in sea surface temperature. Using a high-resolution nested climate modeling system, we find that mid-Holocene insolation forcing results in drier-than-present conditions over the central continental United States ( U. S.) and northern Rocky Mountains, as well as wetter-than-present conditions over the Atlantic seaboard and northwestern Great Plains. We find that changes in summer precipitation are dominated by changes in large-scale processes, with similar patterns of change in the global and nested models. We also find that insolation-induced changes in sea surface temperature do not change the basic pattern of precipitation response, primarily because the dynamical response is very similar with and without sea surface temperature changes. Notably, drier-than-present conditions over the central U. S. are associated with enhanced anticyclonic circulation aloft over the mid-continent and reduced low-level moisture content over the Gulf of Mexico and south-central U. S., while wetter-than-present conditions over the Atlantic seaboard are associated with enhanced low-level cyclonic circulation and elevated low-level moisture content. The simulated patterns of precipitation and soil moisture agree with proxy moisture records from most regions, indicating both that insolation was the strongest determinant of mid-Holocene summer aridity in the continental U. S. and that high-resolution nested climate modeling systems are able to capture the basic response of midlatitude warm-season aridity to changes in external climate forcing.