Components and Mechanisms of the North American hydrologic cycle since the Last Glacial Maximum

Abstract:

Proxy reconstructions of the evolving hydroclimate of North America since the Last Glacial Maximum (LGM; 21 ka) are relatively abundant, and many general circulation models have been used to simulate the climate of time periods between the LGM and the present. However, detailed analyses of the components of the moisture budget and mechanisms responsible for changing precipitation and evaporation patterns have been sparse, as have investigations of the underlying synoptic processes and their response to climate forcings. We present the evolution of the moisture budget over North America over this time period as discerned from multiple climate models, and corresponding model–data comparisons, with a view toward better understanding mechanisms of changes. Simulations tend to suggest that much of North America was wetter than the preindustrial period through much of the last deglaciation, largely due to increased moisture convergence by the mean flow over the west of the continent and by transient eddies over the east. The agreement with hydroclimate proxy records is much stronger in the west, and appears correlated with the seasonality of precipitation, and to a lesser extent to regions where thermodynamic changes are more important. During the Holocene, much of North America was drier than the preindustrial, with enhanced seasonality and concomitant meridional shifts of the midlatitude circulation. Finally, atmospheric rivers, which can straddle the decomposition into mean flow and transient eddies, emerge as an important synoptic process affecting midlatitude hydroclimate, whose behavior is strongly affected by external forcings.