Tropical Cooling at the Last Glacial Maximum: An Atmosphere–Mixed Layer Ocean Model Simulation

Abstract

The sensitivity of tropical temperature to glacial forcing is examined by using an atmosphere–mixed layer ocean (A–MLO) model to simulate the climate of the last glacial maximum (LGM) following specifications established by the Paleoclimate Modeling Intercomparison Project. Changes in continental ice, orbital parameters, atmospheric CO2, and sea level constitute a global mean radiative forcing of −4.20 W m−2, with the vast majority of this forcing coming, in nearly equal portions, from the changes in continental ice and CO2. In response to this forcing, the global mean surface air temperature decreases by 4.0 K, with the largest cooling in the extratropical Northern Hemisphere. In the Tropics, a more modest cooling of 2.0 K (averaged from 30°N to 30°S) is simulated, but with considerable spatial variability resulting from the interhemispheric asymmetry in radiative forcing, contrast between oceanic and continental response, advective effects, and changes in soil moisture. Analysis of the tropical energy balance reveals that the decrease in top-of-atmosphere longwave emission associated with the tropical cooling is balanced primarily by the combination of increased reflection of shortwave radiation by clouds and increased atmospheric heat transport to the extratropics. Comparisons with a variety of paleodata indicate that the overall tropical cooling is comparable to paleoceanographic reconstructions based on alkenones and species abundances of planktonic microorganisms, but smaller than the cooling inferred from noble gases in aquifers, pollen, snow line depression, and the isotopic composition of corals. The differences in the magnitude of tropical cooling reconstructed from the different proxies preclude a definitive evaluation of the realism of the tropical sensitivity of the model. Nonetheless, the comparisons with paleodata suggest that it is unlikely that the A–MLO model exaggerates the actual climate sensitivity. The similarity between the sensitivity coefficients (i.e., the ratio of the change in global mean surface air temperature to the change in global mean radiative forcing) for the LGM simulation and a simulation of CO2 doubling suggests that similar climate feedbacks are involved in the responses to these two perturbations. More comprehensive simulation of the tropical temperature sensitivity to glacial forcing will require the use of coupled models, for which a number of technical obstacles remain.