DEVELOPMENT OF A MESOSCALE MODEL FOR PALEOCLIMATE STUDIES OF THE TROPICS

A regional climate model (RCM) based on the PSU/NCAR mesoscale (MM5) model is modified for application to paleoclimate investigations of Northern Africa and South America. Physical parameterizations, lateral boundaries, and surface conditions are optimized for the regions of interest, and the horizontal resolution is chosen so that topography and strong surface temperature gradients in the region of interest are well resolved. The RCM provides excellent basic-state climatologies for present-day “control” simulations, and improved confidence in the paleoclimate simulations as a result. For example, the model is able to capture small-scale features associated with East African and South American topography, coastal upwelling, and the tight meridional temperature gradients of the present-day Sahel. The higher resolution of the mesoscale also allows for more direct comparison of modeling results with the geologic record.

A series of RCM integrations is used to study the paleoclimates of the African Humid Period (AHP), and South America at the LGM. In contrast to atmospheric general circulation model (AGCM) simulations of the AHP which produce lower precipitation rates than implied by the geologic record, the RCM produces excessive precipitation over present-day Sahel and Sahara regions when the solar insolation, surface vegetation, and atmospheric CO2 levels are set at their 6 ky BP values. The simulation improves, and matches the geological record well when cold SSTAs are introduced into the eastern Atlantic. Multiple seasonal climate-mode simulations of the LGM South American climatology yield insight into the response of the atmosphere to potential radiative and surface differences between the LGM and the present.