Equilibration and variability in a Last Glacial Maximum climate simulation with CCSM3

Abstract

We present results from a 1862 year simulation of the Last Glacial Maximum (LGM) with the Community Climate System Model version 3 (CCSM3). A quasi steady state is reached after approximately 100 years of integration when the initial cooling trend in the annual global mean atmospheric surface temperature (T s ) levels off and even reverses. After another 150 years of integration the climate continues to cool and reaches a new equilibrium after a total of 800 years of integration. The cause of the continued adjustment of the climate to LGM forcing and boundary conditions is found in the abyssal ocean which is cooling at a rate decreasing from 0.15°C per century until the new equilibrium is reached. The new equilibrium differs substantially from the first quasi steady state with 1.1°C colder global mean T s and regional differences of 5–15°C in the North Atlantic region and a 30% reduction of the strength of the Atlantic meridional overturning circulation (AMOC). Further, the variability in the global mean T s is significantly larger in the new equilibrium. This variability is associated with coupled ocean–atmosphere–sea ice variations in the North Atlantic region.