CHANGES IN SUMMER PRECIPITACION IN SOUTHEASTERN SOUTH AMERICA BY THE WCRP/CMIP5 MODELS

DÍAZ LEANDRO; VERA CAROLINA

Buenos Aires

Taller; Adaptación al cambio climático en sectores clave en América del Sur; 2012

Universidad de Buenos Aires

Several previous works have shown evidences of positive trends in both mean and extremes of the austral summer precipitation over southeastern South America (SESA) during the 20th century. Moreover, most of the WCRP/CMIP3 models project over the same region, positive austral summer precipitation trends during the 21st century. Recently, the projected precipitation trend has been related to changes in the activity of the leading pattern of interannual variability of summer precipitation. Nevertheless, despite of the fact that the tendency can be identified in both observations and numerical climate simulations, there is almost no previous works that have made progresses in describing the causes that explain such tendencies observed during the 20th century. It is then our goal to provide answers to that question, and in this work the results of the preliminary analysis performed using the recently available WCRP/CMIP5 climate simulations are presented. In particular, an assessment of the skill of the WCRP/CMIP5 climate simulations in representing the mean and variability of the austral summer precipitation is discussed. Also, the results of a first exploration of the possible causes explaining the summer precipitation trend observed in SESA during the 20th century and described by previous works is included. Historical simulations of 33 WCRP/CMIP5 coupled general circulation models (CGCM) are considered in order to make a preliminary evaluation of the representation of the austral summer (December-January-February, DJF) rainfall mean and variability in South America. The simulated climatological mean rainfall computed over the last part of the 20th century shows, when compared to observations, for most of the WCRP/CMIP5 models, similar biases than those shown by the previous version of the models (WCRP/CMIP3). Model errors are still largest over the northern coast of South America and in less extent over SESA. Nevertheless, results show an improvement of the representation of the interannual variability first leading pattern of precipitation. The pattern, identified from an Empirical Orthogonal Function analysis, is characterized by a dipole-like structure with centers of action of opposite sign located over SESA and the South Atlantic Convergence Zone (SACZ), which is simulated by all models considered. The DJF rainfall trends computed over the 1861-2005 period are also studied. Results reveal that most of the models show a significant positive trend over SESA that seems to be associated with the frequency increase of the seasonal dipole positive events, which are characterized by precipitation increase (decrease) in SESA (SACZ). Moreover, it is found that the precipitation trend is significantly different from the model dispersion over central-eastern Argentina. However, the uncertainties related to internal variability (represented by the each model members) are large. Simulations known as Historical_Nat and Historical_GHG are also analyzed. The first simulation set has been obtained considering the natural forcing (volcanism, solar radiation variability, etc) only while leaving constant the Greenhouse gases (GHG) concentrations. On the other hand, the second set of simulations has been obtained considering the variations of the GHG forcing while leaving constant the natural forcing. Preliminary studies reveal that the anthropogenic forcing (associated to GHG concentration increase) seems to at least partially explain the positive rainfall trend.