NORTH ATLANTIC FRESHWATER FORCING DURING HEINRICH EVENTS DRIVES WETTER CLIMATE IN WESTERN NORTH AMERICA

Although much work has focused on understanding the Last Glacial Maximum (LGM; ca. 21,000 yrs BP) hydroclimate of southwestern North America from a proxy and climate modeling standpoint, substantial geochronologic evidence demonstrates that the presumably wettest conditions in this region postdate the LGM. In particular, many pluvial lakes in the Great Basin reach highstands during some portion of Heinrich Stadial 1 (HS1: 18,000 to 14,700 yrs BP). Recent climate modeling suggests enhanced subtropical moisture transport into western North America during HS1. However, coarse model resolution and idealized representation of Heinrich Events has precluded a comprehensive understanding of the dynamics of HS1 hydroclimate.

To understand the driving mechanisms behind HS1 hydroclimate in southwestern North America and to investigate the roles of moisture source changes, we perform a series of climate model experiments that track changes in precipitation, moisture source, water isotopes, and storm frequency. We use the Community Earth System Model version 1.3 at 0.9 x 1.25 degree horizontal resolution to conduct simulations of the LGM climate as well as several experiments of HS1 climate centered at 16 ka to examine the sensitivity of HS1 climate to freshwater forcing in the North Atlantic, ice sheet topography, and albedo. We find that, relative to the LGM, winter precipitation increases along the west coast of the United States in all 16 ka experiments. However, freshwater forcing yields the most pronounced precipitation anomalies due to a strong southward shift of the North Pacific jet stream and storm track, driven by an anomalous stationary wave train originating in the midlatitudes close to the dateline. As observed through moisture tagging, the equatorward shifted storm track brings moisture sourced from low latitudes toward the south and east and into California and the Great Basin. Thus, our model experiments support an influx of subtropical moisture to southwestern North America during HS1 and provide a mechanistic link between North Atlantic freshwater forcing and the occurrence of Great Basin pluvial lakes highstands, as well as wet conditions indicated by regional speleothem records, following the LGM.