Abstract

During the last ice age, the western United States was covered by large lakes, sustained partly by higher levels of precipitation. Increased rainfall was driven by the atmospheric circulation associated with the presence of large North American ice sheets, yet Pleistocene lakes generally reached their highstands not at glacial maximum but during deglaciation. Prior modeling studies, however, showed nearly monotonic drying since the last glacial maximum. Here I show that iTraCE, a transient climate simulation of the last deglaciation, run at higher resolution and with updated boundary conditions, reproduces a robust peak in winter rainfall over the Great Basin near 16 ka. I further demonstrate that the simulated peak is driven by a transient southward shift of the midlatitude jet. The causes for the southward shift of the jet are multifactorial, with meltwater forcing, changing orbital conditions, and rising atmospheric CO2 all playing a role.