LGM Summer Climate on the Southern Margin of the Laurentide Ice Sheet: Wet or Dry?

High-resolution regional climate simulations are conducted using Polar MM5 with a domain centered over North America to explore the summer climate of the Last Glacial Maximum (LGM) roughly 21,000 calendar years ago. The simulations use appropriate LGM boundary conditions: orbital forcing, continental ice sheets, reduced trace gas concentrations, lowered sea level, and paleovegetation. A tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate provides the initial and lateral boundary conditions for Polar MM5 as well as prescribed sea surface temperatures and sea ice extent.

The LGM summer climate features a pronounced low level thermal gradient along the southern margin of the Laurentide Ice Sheet, which anchors the mid-tropospheric jet stream and facilitates the development of synoptic cyclones that track over the ice sheet; some of these produce heavy rain along the southern margin. Precipitation on the southern margin is orographically enhanced as moist southerly low-level flow from the Gulf of Mexico (resembling a contemporary Great Plains low level jet configuration) is drawn up the ice sheet slope in advance of the cyclones. Composites of wet and dry periods on the Laurentide Ice Sheet southern margin illustrate two distinctly different atmospheric flow regimes. Given the episodic nature of the rain events and the interannual variability simulated in Polar MM5, the modeled wet conditions in the Great Plains during the LGM summer are reconcilable with general aridity across the region inferred from geological proxy evidence.