Extreme cold events in South America analyzed from a GFDL model perspective: comparison between CMIP3 and CMIP5 climate scenarios

Abstract

This paper deals with the future change projections of extreme cold events in relation to historical climate simulations, based on the comparison between two versions of the GFDL model, CMIP3 and CMIP5, in three regions of southeastern South America where these events are frequent. To this end, the compositions of up to five extreme events below 0 °C at 850 hPa were considered. They were derived from daily data collected from May to September of the NCEP/NCAR reanalysis and GFDL Coupled Atmospheric and Oceanic Global Circulation Model. The study periods run from 1961 to 1990 and from 2081 to 2100 for the most critical scenarios: A2 (GFDL-CM2) and RCP8.5 (GFDL-CM3). Sea level pressure, temperature, and wind were analyzed at 850 hPa and wind also at 250 hPa, for each of the areas under study, representing tropical (Area 1) and subtropical (Area 2 and Area 3) latitudes. The ability of the model to simulate the historical climate, represented by the reanalysis, increased as latitude decreased, CM3 version being better for subtropical latitudes than CM2. Depending on GFDL model version, temperature was higher or lower than those shown by the reanalysis in the tropical latitudes. In contrast, at subtropical latitudes, both versions of the model present lower values. An increase in mean temperature is expected in the future in the three areas, mainly at tropical latitudes (Area 1) according to both versions of the model, which is in line with the results reported by other global models. In Area 2, a further increase in temperature is projected in CM3 version, while, for Area 3, the greatest projection is in CM2 version. Changes are expected for the future climate, primarily explained by the configuration of the circulation fields, such as in the case of the subtropical areas in CM2, whose pattern is more related to that typically associated with radiative cooling, rather than the advective cooling shown in the historical climate. This result is in contrast with the results of the CM3 version, which displays a pattern associated with cold air advection from the south for both the historical and future climate. As regards the tropical area, both versions of the model reveal a considerable reduction in the number of extreme cold events.