Climatic changes between 20th century and pre-industrial times over South America in regional model simulations

Abstract

Two simulations with a regional climate model are analyzed for climaticchanges between the late 20th century and a pre-industrial periodover central and southern South America. The model simulations havebeen forced with large-scale boundary data from the global simulationperformed with a coupled atmosphere-ocean general circulation model.The regional simulations have been carried out on a 0.44� � 0.44�grid (approx. 50 km � 50 km horizontal resolution). The differencesin the external forcings are related to a changed greenhouse gascontent of the atmosphere, being higher in the present-day simulation.For validation purposes the climate model is analyzed using a fiveyear long simulation between 1993 and 1997 forced with re-analysisdata. The climate model reproduces the main climatic features reasonablywell, especially when comparing model output co-located with observationalstation data. However, the comparison between observed and simulatedclimate is hampered by the sparse meteorological station networkin South America.The present-day simulation is compared with the pre-industrial simulationfor atmospheric fields of near-surface temperatures, precipitation,sea level pressure and zonal wind. Higher temperatures in the present-daysimulation are evident over entire South America, mostly pronouncedover the southern region of the Andes Mountains and the Parana basin.During southern winter the higher temperatures prevail over the entirecontinent, with largest differences over the central Andes Mountainsand the Amazonian basin.Precipitation differences show a more heterogeneous pattern, especiallyover tropical regions. This might be explained by changes in convectiveprocesses acting on small scales. During southern summer wetter conditionsare evident over the Amazonian and Parana basin in the present-daysimulation. Precipitation increases are evident over Patagonia togetherwith decreases to the north along the western slope of the AndesMountains. During southern winter also a dipole pattern along theAndes Mountains with wetter conditions over the southern parts anddrier conditions over the central parts is evident. An interestingfeature relates to precipitation changes with changing sign withina few 10th of kilometers along the southern parts of the Andes mountainchain. This pattern can be explained by changes in large-scale circulationrelated to latitudinal changes of the extratropical southern hemisphericwesterlies.