Post-Eemian Glacial Inception. Part I: The Impact of Summer Seasonal Temperature Bias

Abstract

Post-Eemian glacial inception [the transition between marine oxygen isotopic stage (MOIS) 5e and MOIS 5d] began approximately 117 000 years before present (117 kyr BP) and led to significant Northern Hemisphere glaciation within the ensuing 5000 yr. Previous sensitivity studies with atmospheric general circulation models (AGCMs) have had difficulty producing glacial nucleation in high northern latitude regions of the globe. A base simulation of this process has been conducted using the Canadian Centre for Climate Modelling and Analysis (CCCma) GCMII with mixed layer slab ocean model constrained so as to ensure that the model reproduces the set of Atmospheric Model Intercomparison Project 2 (AMIP2) modern sea surface temperatures (SSTs) under conditions of modern radiative forcing. This simulation demonstrates that entry into glacial conditions at 116 kyr BP requires only the introduction of post-Eemian orbital insolation and standard preindustrial CO2 concentrations. Two additional sensitivity experiments are also described herein in which the associated modern control climates have modified oceanic heat transports and solar radiation parameterizations. These simulations produce modern Northern Hemisphere sea surface temperatures that are either cold biased or warm biased with respect to the AMIP2 SSTs. Three modern control and three post-Eemian simulations are therefore employed to investigate the sensitivity of the onset of large-scale glaciation at high northern latitudes to the summer seasonal temperature bias in the model. Also discussed are the elements of the hydrological cycle at 116 kyr BP in order to more precisely isolate the primary causes of the onset of perennial snow cover. In particular, one novel feature is described that is characteristic of the two post-Eemian simulations that do initiate glaciation, namely, the absence of perennial snow cover in the Alaskan region, a result that is in accord with geological evidence.