The North American Cordillera – an Impediment to Growing the Continent-wide Laurentide Ice Sheet

Abstract:

Much effort has been devoted to understanding the climate of the Last Glacial Maximum (LGM) when the continent-wide Laurentide Ice Sheet resided in North America. It has been established, however, that the massive LGM ice sheet was a relatively short lasting (5–15 kyrs) "glacial extreme", and the longer build-up phase (around 75–85 kyrs in length) was dominated by an east-heavy ice configuration and largely ice-free conditions in the interior of the continent.

The present study aims to investigate whether first-order (large scale) interactions between the atmosphere and ice sheets can explain the asymmetric ice evolution and the limited glaciation of the continental interior in the larger part of the last glacial cycle. Simulations are conducted using a comprehensive atmospheric circulation model asynchronously coupled to an ice-sheet model. The atmospheric state is updated for every 2×10⁶ km³ increase in ice volume and the coupled model is integrated to steady state. We resort to a simplified model setup, using a triangular representation of North America, with and without the influence of the Cordilleran region, in order to isolate the influence of the mountain range on the ice sheet evolution.

In the first experiment we use a flat continent with no background topography. The ice sheet evolves fairly zonally symmetric and the equilibrium ice sheet is continent wide and has the highest point in the center of the continent. In the second experiment we include a simplified representation of the Cordilleran region. The ice sheet's equilibrium state is here structurally different from the flat continent case; the center of mass is strongly shifted to the east and the interior of the continent remains ice-free – an outline resembling the geologically determined pre-LGM state. The ice sheet induces both mechanically and thermally forced stationary waves that result in a reduced cloudiness and hence an increased downwelling of radiation at the surface. This yields warmer summer temperatures that prevent the westward ice expansion. The climate is also more arid in the lee of the Cordillera, which further helps limit the expansion of the ice sheet. Hence, the present study suggests that the Cordillera played a key role for the spatial evolution of the pre-LGM ice sheet in North America.