Simulation of the Tropical Climate of an Ice Age

Numerical time integrations of a general circulation model of the atmosphere are performed with both modern and ice age boundary conditions. It is shown that the climate of continental portions of the tropics in the ice age simulation is much drier than that of the modern climate simulation. According to comparisons of results from the two experiments, tropical continental aridity of the ice age results from stronger surface outflow from (or weaker surface inflow into) continents. The intensification of outflow from (or weakening of inflow into) tropical continental regions results from the fact that in response to ice age boundary conditions, atmospheric temperature is reduced more over continents than over oceans. With the exception of high latitudes, boundary condition differences between the two experiments consist mainly of changes of the prescribed values of sea surface temperature and continental albedo. In order to evaluate the relative contributions of these changes in producing continental tropical aridity in the ice age simulation, a third numerical experiment is time-integrated in which a hybrid combination of ice age sea surface temperatures and modern continental albedo values is prescribed. From intercomparisons between results from this and the previous two experiments it is shown that the effect of increased continental albedo is mainly responsible for the weak Asian monsoon in the ice age simulation.