Effects of Biases in the Westerly Winds on the Simulation of Tropical-Extratropical Cloudband Events Over South America by Two Global Climate Models: BAM-1.2 and HadGEM3-GC3.1

Tropical-extratropical cloudband events are typical of the subtropical South American climate, occurring mainly during the rainy season (November-March) and produce more than 60% of the rainy season precipitation. When persisting for >4 days, the cloud bands are classified as South Atlantic Convergence Zones. Similar events also occur over the South Pacific Ocean and Southern Africa. Therefore, the correct representation of cloud bands in climate models is fundamental for the accuracy of simulated subtropical precipitation. Using an objective detection algorithm, we compare the cloudband events observed over South America to simulations from two global models: the Brazilian Global Atmospheric Model version 1.2 (BAM-1.2) and the Hadley Centre Global Environment Model in the Global Coupled configuration 3.1 (HadGEM3-GC3.1) in two spatial resolution, n96 and n216. The observational cloudband baseline is identified in the satellite-derived NOAA CDR OLR dataset. Precipitation and circulation characteristics for the observed cloud bands are drawn from ERA5 reanalysis data. All three models correctly simulate the general location and frequency of the cloud bands during the rainy season and their associated circulation features. However, HadGEM3-CG3.1 models delay the onset of the cloudband season while BAM-1.2 overestimates the persistence of the events in December and January. In all models, misrepresentations of events are traced back to biases in the westerlies over midlatitudes and subtropics. Biases in the westerly wind climatology over midlatitudes change the simulated maximum Rossby wave number, altering the envelope for the synoptic-scale Rossby Wave activity necessary for the cloud band formation. In BAM-1.2, the westerly wind bias increases the meridional shear of the zonal wind, favouring the occurrence of cut-off lows, which result in more persistent cloud band events. In both HadGEM3-CG3.1 configurations, the location of the westerlies during the beginning of the rainy season inhibits the penetration of the synoptic-scale extratropical disturbances into lower latitudes, which delays the onset of the cloudband season. These results add critical insight to the interaction between mean state biases and synoptic-scale rainfall events over South America.