Components of the ice age circulation

Four general climate model (GCM) simulations are compared to investigate the influence of ice age boundary conditions on atmospheric dynamics and regional climate patterns. Starting with a simulation of the current climate, the ice age distributions of (1) sea surface temperatures; (2) 10‐m‐thick land ice in locations of ice age ice sheets; and (3) ice sheets elevated to their proper altitude were added sequentially. The results show that these different boundary conditions often impart conflicting influences, with the full ice age simulation representing a compromise between different tendencies inherent in the different boundary components. In particular, the ice age sea surface temperatures stablize the atmosphere over the oceans, increase the frequency of storms tracking through central North America, and amplify transient eddy energy without increasing baroclinic generation. Low‐elevation ice generates low pressure over eastern North America and southern Europe in winter, while increasing cloud cover and cooling the land in summer. Elevation of the ice sheets cools the land in winter, further intensifies storms off northeastern North America, induces subsidence warming downstream of the European ice sheets in summer, and increases both transient and stationary eddy energy through increased baroclinicity. In all of the experiments the atmosphere is transporting a similar amount of energy poleward, consistent with the similarity in sea surface temperature gradients, but the eddy characteristics employed in the process vary with the longitudinal distribution of the boundary conditions. Other results show that the Broad raised ice sheets have both a topographic and a thermal effect (due to the reduced optical thickness above) which occur simultaneously and lead to a greater stationary eddy forcing of the zonal mean flow. The stationary wave change in the northern hemisphere is accompanied by a similar change in the southern hemisphere, implying a dynamical interhemispheric connection. The Walker circulation and the July Hadley circulation weaken as a result of the altered sea surface temperature patterns, but the poleward extent of the Hadley cell and the zonally averaged jet stream show little difference from current climate values. The results are compared with two‐dimensional model expectations, paleoclimate evidence, and previous ice age simulations.