The Last Glacial Maximum climate over the Laurentide Ice Sheet: high-resolution simulations using Polar MM5

Regional climate simulations are conducted using a high spatial resolution (60 km) domain centered over North America during the Last Glacial Maximum (LGM), roughly 21,000 years before present (21 kBP), when much of continent was covered by the Laurentide Ice Sheet (LIS). The objective is to describe the temperature, precipitation, and flow regimes that contribute to LIS growth and ablation. The atmospheric model is a version of the National Center for Atmospheric Research/Pennsylvania State University (NCAR/PSU) fifth generation mesoscale model (MM5) that has been modified for high latitude applications by the Polar Meteorology Group at the Ohio State University Byrd Polar Research Center. The so-called PMM5 (http://www-bprc.mps.ohio-state.edu/PolarMet/pmm5.html) has been tested extensively over present-day Greenland and Antarctica and shown to have minimal bias. Since the contemporary ice sheets likely represent the range of conditions that existed over the LIS, we expect a reasonable depiction of the LGM atmosphere using PMM5.

In addition to the expansive LIS, the PMM5 LGM boundary conditions include 21 kBP orbital forcing, reduced CO2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level commensurate with the ice sheet volume. Output from a global climate model (GCM) simulation with similar LGM boundary conditions provides the initial and lateral boundary conditions for the PMM5 LGM run.

The model atmosphere responds strongly to the LGM boundary conditions. Cooling over the LIS drives a well-defined low level katabatic flow during winter months, exceeding 15-20 m s-1 along much of the sloped ice sheet terrain. Time series of surface temperatures at points along the LIS southern margin show that sufficient warming occurs during summer for ablation to occur. At upper levels from November(March, the flow is split around an anticyclone over the LIS, with a northern branch over Beringia and the Canadian Arctic islands and a southern branch impacting southwestern North America. This split flow pattern, which has a first order influence on the distribution of precipitation in the Laurentide domain, is not present in recent GCM simulations of the LGM. Possible reasons for the presence of the split flow, including ice sheet configuration, model resolution, and improved model physics, are under investigation.