Seasonal climate and the onset of the rainy season in western‐central Brazil simulated by Global Eta Framework model

Abstract

The seasonal cycle of precipitation in tropical South America is determined by the monsoonal system. The transition from dry to wet season occurs in austral spring (September–November, SON) when intense convection from northwestern South America rapidly shifts southwards to the southern Amazon Basin and western‐central Brazil (WCB) in October and further to the southeast of Brazil in November. This study evaluates ability of the global atmospheric model, Global Eta Framework (GEF), at 25‐km horizontal resolution, to simulate the onset of the rainy season in WCB region. The simulations are based on a five‐member ensemble seasonal integrations for the years 2011 and 2013. Evaluation of mean global simulated fields, such as 200‐hPa wind, 500‐hPa geopotential height, 850‐hPa temperature and wind, and MSLP at the surface, for the SON period indicates high level of agreement with reanalyses and observations, both in spatial distribution and intensity for most of the variables. The variable of the lowest skill is precipitation, which is overestimated over some tropical oceanic regions and underestimated over tropical continental regions, including South America. The onset of the rainy season is determined using methods based on precipitation and outgoing long‐wave radiation (OLR). The threshold based on simulated precipitation is also calculated as an alternative method for defining monsoon onset. Comparison of the 5‐day averaged values (pentads) of precipitation and OLR of all members of the ensemble and the ensemble mean against the observed data shows the ability of GEF to reproduce the typical pattern of transition from dry to wet season in WCB, although most of ensemble members tend to underestimate precipitation and overestimate OLR. The onset date is delayed for few pentads in the model simulations.