The Influence of a Single Northern Hemisphere Continent on Tropical Precipitation and Climate in Idealized GCM Experiments

Abstract:

Many recent studies have shown the utility of constraining the mean location of tropical precipitation with the zonal energetic budget. Here, we use that framework to understand how the addition of a single continent in one hemisphere changes tropical precipitation in general circulation model (GCM) simulations. We use two GCMs from the GFDL model hierarchy. The first is a gray radiation model (GRaM) that lacks a greenhouse effect and cloud feedbacks, but contains full dynamics. The second is the AM2 model that includes radiative feedbacks and complex physical parameterizations. There is one continent placed in the tropics of the northern hemisphere (NH) in each model; elsewhere, there is a slab ocean model. Two albedos are tested for the continent: one for a "dark" continent with an albedo similiar to the ocean and one for a "light" continent with a higher albedo. In the GRaM model, advection of hot, dry air off the continent into the deep tropics displaces tropical precipitation into the southern hemisphere (SH). As a result, the location of zonal precipitation is in the SH, despite the NH as a whole being warmer than the SH because of the addition of the continent. The effect is much larger in the simulation with the light continent, which can be explained energetically through the extra reflection of insolation in the NH. In the AM2 simulations, the location of tropical precipitation is also constrained by the energetic budget; however, in addition to the southward deviation of tropical precipitation near the continent, there is a partially compensating northward precipitation displacement that largely depends on the hemispheric imbalance of radiation caused by the higher albedo. This study further highlights the use of energetic arguments to explain changes in the tropical climate.