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Ten-year seasonal climate reforecasts over South America are obtained using the Eta Regional Climate Model at 40 km resolution, driven by the large-scale forcing from the global atmospheric model of the Center for Weather Forecasts and Climate Studies. The objective of this work is to evaluate these regional reforecasts. The dataset is comprised of four-month seasonal forecasts performed on a monthly basis between 2001 and 2010. An ensemble of fi ve members is constructed from fi ve slightly different initial conditions to partially reduce the uncertainty in the seasonal forecasts. The seasonal mean precipitation and 2-meter temperature forecasts are compared with the observations. The comparison shows that, in general, forecasted precipitation is underestimated in the central part of the continent in the austral summer, whereas the forecasted 2 meter temperature is underestimated in most parts of the continent and throughout the year. Skill scores show higher skill in the northern part of the continent and lower skill in the southern part of the continent, but mixed skill signs are seen in the central part of the continent. During the El Niño and La Niña seasons, the forecast skill scores clearly increase. The downscaling of the Eta model seasonal forecasts provides added value over the driver global model forecasts, especially during rainy periods.

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Modelos numéricos têm importante papel na previsão de tempo e clima, amparando assim as tomadas de decisões em diversos setores. O aprimoramento destes modelos pode ser feito a partir de parametrizações de processos de superfície, como o balanço de umidade do solo. Esse balanço é obtido a partir de parâmetros hidrológicos definidos para cada tipo de solo, e pela umidade inicial inserida no modelo. O objetivo desse trabalho é estudar a influência das condições iniciais do solo e do tipo de solo na previsão sazonal do modelo Eta. São realizados dois experimentos (ETA1 e ETA2) a partir de integrações do modelo Eta, com 4 meses e meio de integração e utilizando reanálises como condições inicial e de contorno. O período analisado são as estações secas e chuvosas no Brasil, entre 1979 e 2012. ETA1 foi realizado como uma simulação controle, com umidade inicial do solo climatológica e nove tipos de solo. Em ETA2, utilizou-se umidade inicial do solo calculada e 26 tipos de solo. Para analisar a climatologia simulada, dados de reanálises permitiram a avaliação dos erros de algumas variáveis do modelo. A partir da análise dos experimentos, conclui-se que o modelo Eta manifesta sensibilidade a alterações no solo quando diferentes condições de solo são utilizadas na integração do modelo. ETA2 simula temperaturas sistematicamente mais baixas e precipitação mais acurada em algumas regiões do Brasil.

This work aimed to evaluate changes in water balance components (precipitation, evapotranspiration, and water availability) and precipitation extremes projected under global warming levels (GWLs) of 1.5 °C and 2 °C, in Brazil. An ensemble of eight twenty-first-century projections with the Eta Regional Climate Model and their driving Global Climate Models (CanESM2, HadGEM2-ES, MIROC5, and BESM) were used. Projections of two Representative Concentration Pathway scenarios, RCP4.5 and RCP8.5, considered intermediate and high concentration, respectively, were used. The results indicate that the RCP8.5 scenario under 2 °C GWL is likely to have a higher impact on the water balance components, amplifying trends in drier conditions and increasing the number of consecutive dry days in some regions of Brazil, particularly in the North and Northeast regions. On the other hand, the projections indicate the opposite sign for the South region, with trends toward wetter conditions and significant increases in extreme rainfall. The 0.5 °C difference between the GWLs contributes to intensifying reductions (increases) from 4 to 7% in water availability, mainly in the North-Northeast (South) regions. The projected changes could have serious consequences, such as increases in the number of drought events in hydrographic regions of the Northeast region of Brazil and increases in flood events in the South of the country. The results here presented can contribute to the formulation of adaptive planning strategies aimed at ensuring Brazil’s water security towards climate change. Supplementary Information The online version contains supplementary material available at 10.1007/s10113-023-02042-1.