Atmospheric blocking events over Southeast Pacific and Southwest Atlantic oceans in CMIP6 present-day climate

Episodes of atmospheric blocking over the Southeastern Pacific and Southweastern Atlantic oceans significantly affect the weather over South America. These blocking events have been associated with heatwaves, droughts, and abnormally wet periods. Given the significant impacts that atmospheric blockings can have on regional climate, it is important to investigate their simulation in general circulation models (GCMs) and document any inherent biases within these models. If models are unable to accurately capture blocking under present-day conditions, it undermines the confidence in their future projections of blocking frequency.

However, few studies have examined the representation of Southern Hemisphere blockings in GCMs, especially for the Southeastern Pacific and Southweastern Atlantic regions. Therefore, in this study, we investigate the representation of blocking events in a set of 13 GCMs from the Coupled Model Intercomparison Project Phase 6 (CMIP6) within the domain of the SE Pacific, with boundaries from latitudes 40°S to 65°S and longitudes from 140°W to 82.5°W, and the SW Atlantic, with boundaries from latitudes 40°S to 65°S and longitudes 80°W to 10°W. These areas are of primary interest due to their proximity to the South American continent.

Historical runs are employed to analyze blocking conditions in the recent-past climate, focusing on the period from 1985 to 2014. Geopotential height at 500 hPa (Z500) data from the Fifth Generation of the European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5) is utilized to represent the observed state of the atmosphere within the specified domain covering the Southeastern Pacific and Southern Atlantic Oceans.

Overall, the analyzed GCMs generally underestimate blocking events in the Pacific sector, although the MPI-ESM1-2-HR and MPI-ESM1-2-LR models closely reproduce observed numbers, with around -10% and -9% underestimation, respectively. For the Atlantic sector, the performance of the CMIP6 models is more varied, and the models seem to have more difficulty in capturing the observed blocking events, with differences exceeding +/-25% compared to ERA5. Nevertheless, most CMIP6 models accurately capture the seasonal distribution of blocking events, with the highest occurrence in winter and the lowest in summer. Additionally, they reproduce the center position, overall area, and Z500 anomalies of the blockings.

This research results from the R&D Project from ENGIE Brasil Energia and Companhia Energética Estreito (R&D-00403-0054/2022), and regulated by the Brazilian National Electric Energy Agency (ANEEL).