The Hydrologic Cycle and Atmospheric Rivers in CESM2 Simulations of the Last Glacial Maximum

Proxy reconstructions and model simulations of precipitation during Earth's glacial periods suggest that the locations and mechanisms of atmospheric moisture transport have changed considerably during Earth's past. We investigate the hydroclimate of the Last Glacial Maximum (LGM) using simulations with the Community Earth System Model, with a focus on the extratropics and the influence of atmospheric rivers (ARs), a key driver of modern-day moisture transport globally. Mean and extreme precipitation increase significantly over southwestern Patagonia, Iberia, and southwestern North America-mid-latitude regions affected by ARs in the modern climate-despite overall decreases elsewhere. In each, the associated moisture transport changes are different, with increased transport and AR activity mainly occurring in the North Atlantic. The overall LGM response is dominated by the response to ice sheets, with other forcings causing additional cooling and drying over the extratropics and a strong decrease of moisture transport over the subpolar North Atlantic. During the last ice age, when glaciers reached their maximum extent, atmospheric CO2 was lower and Earth's orbit was different. All of these influences caused dramatic changes in precipitation patterns around the planet. Using climate model simulations, we find that the majority of the changes to atmospheric moisture transport and precipitation patterns result from the presence of the ice sheets alone, with lower CO2 and insolation differences enhancing the ice sheet-induced patterns. These changes mainly comprise decreases in moisture transport and lower precipitation, with three notable exceptions in Patagonia, Iberia, and southwestern North America. Of these, precipitation increases over Iberia result from increased transport by plumes of moisture, known as atmospheric rivers , across the North Atlantic. Elsewhere in the world, AR activity decreases. Average and extreme precipitation declined globally during the Last Glacial Maximum with three notable mid-latitude exceptions over landMoisture transport changes mainly caused by ice sheets, with lower CO2 and altered insolation causing enhanced drying regionallyAtmospheric river activity decreased globally except over the North Atlantic, where it enhanced precipitation over Iberia