REGIONAL CLIMATE CHANGE EXPERIMENT OVER SOUTHERN SOUTH AMERICA: PART I: PRESENT CLIMATE CONDITIONS (1981-1990)

SILVINA SOLMAN; MARIO N. NUÑEZ; MARIA F. CABRÉ

Foz do Iguacu, PR, Brazil

Congreso; 8th International Conference on Southern Hemisphere Meteorology and Oceanography; 2006

American Meteorological Society

This study presents the results from a regional climate simulation of the present-day climate, corresponding to the period 1981-1990, over southern South America, using the MM5 regional climate model nested within a high resolution version of the Hadley Centre global atmospheric model HadAM3. The analysis of the simulation is focused on evaluating the capability of the nested modelling system in representing spatial patterns of seasonal mean climate and its annual cycle, with two main objectives. First, to quantify the capability of the dynamical downscaling tool to represent present-day climate conditions and to assess the feasibility to produce useful estimates of regional climate change projections. This simulation is the basis to examine the climate change simulations resulting for the A2 and B2 forcing scenarios which are reported in a separate study. Second, to identify critical aspects of regional climate simulation over a barely unexplored region. The regional simulation reproduces many mesoscale climate features that are triggered by regional forcings, not well captured by the low resolution driving model. Overall, the regional model improves the representation of the mean climate upon the general circulation model in many aspects. In particular, significant improvements have been found in the regional simulation for near surface circulation features. The first feature to note is that the regional model exhibits a better performance in the representation of the low-level circulation, not well represented in the driving model, such as the topographically-induced low level cyclonic circulation during summer months over northern Argentina. Nevertheless, it fails in reproducing the correct position of the low pressure system, and, in consequence, this results in a large bias in the precipitation field. The misrepresentation of this system induces a poor representation of the low-level jet, which is critical in determining summer precipitation in subtropical South America, as it serves as conduit of moisture supply from the Amazon basin. Thus, much of the deficiency in the simulation of rainfall may be caused by the deficiency of the regional model in simulating this pattern.