Revisiting Western United States Hydroclimate During the Last Deglaciation.

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 new transient climate simulation of the last deglaciation, reproduces a robust peak in winter rainfall over the Great Basin near 16 ka. The simulated peak is driven by a transient strengthening and southward shift of the midlatitude jet. While meltwater forcing is an important driver of changes to the North Pacific Jet, changing orbital conditions and rising atmospheric CO2 also shift the jet south and contribute to wetter conditions over the western US during deglaciation. Plain Language Summary: At the height of the last ice age, the western United States was covered by large lakes such as Lake Bonneville and Lake Lahontan that required more rainfall to be sustained. It is believed that the 3–4 km thick North American ice sheet over Canada acted as an obstacle to the atmospheric flow, forcing storms to be deflected southward over the southwestern US and bringing more winter rain. While wet conditions during glacial maximum are attributable to the presence of a large ice sheet, why does geological evidence indicate that most lakes attained their maximum size not when the ice sheet was at it largest, but rather when the ice sheet was already melting? Here, I use climate simulations to show that other factors, such as changes to Earth's tilt and temperature, in conjunction with changes in ocean circulation, caused a temporary further increase in rainfall that explains lake expansions during the deglaciation. Key Points: Geological evidence indicates many lakes over the western United States reached highstands during the last deglaciationiTraCE, a transient simulation of the last deglaciation, shows wetter conditions at 16 ka than at 20 ka and compares well to proxy evidenceOrbital conditions, rising atmospheric CO2, and meltwater flux all contribute to lake expansions inferred during Heinrich Stadial 1 [ABSTRACT FROM AUTHOR]