This study is based on climatological records of cold outbreak (friagem) events that occurred in the southern Amazon region during the austral winter months (May to September). The friagem events were identified with an objective method that used climatological data from the surface station of the Department of Airspace Control (DCEA) in Vilhena city, including the minimum temperature (T min ), the pressure reduced to mean sea level (P msl ) and the decrease in T min (DT min ). The purpose of this study was to investigate the association between atmospheric circulation and friagem events using Climate Forecast System Reanalysis (CFSR) and a composite method. One hundred and forty-four friagem events were identified with the proposed methodology. These events produced large changes in weather conditions in Amazonia, including an abrupt decrease in air temperature, an increase in the amount of dry air, and an increase in surface pressure. The main atmospheric features associated with friagem events are an intensification of high post-frontal surface pressure that is caused by the intensification of a ridge at middle and high levels when air approaches the Andes and strong southerly winds at the surface that carry cold, dry air to tropical latitudes.

RESUMOEste artigo discute um modelo de previsão combinada para a realização de prognósticos climáticos na escala sazonal. Nele, previsões pontuais de modelos estocásticos são agregadas para obter as melhores projeções no tempo. Utilizam-se modelos estocásticos autoregressivos integrados a médias móveis, de suavização exponencial e previsões por análise de correlações canônicas. O controle de qualidade das previsões é feito através da análise dos resíduos e da avaliação do percentual de redução da variância não-explicada da modelagem combinada em relação às previsões dos modelos individuais. Exemplos da aplicação desses conceitos em modelos desenvolvidos no Instituto Nacional de Meteorologia (INMET) mostram bons resultados e ilustram que as previsões do modelo combinado, superam na maior parte dos casos a de cada modelo componente, quando comparadas aos dados observados. Palavras-chave: Previsões sazonais, Modelos estocásticos; Correlações canônicas, Modelo agregado. ABSTRACT A COMBINED STOCHASTIC MODEL FOR SEASONAL PREDICTION OF PRECIPITATION IN BRAZIL.This article discusses a combined model to perform climate forecast in a seasonal scale. In it, forecasts of specific stochastic models are aggregated to obtain the best forecasts in time. Stochastic models are used in the auto regressive integrated moving average, exponential smoothing and the analysis of forecasts by canonical correlation. The quality control of the forecast is based on the residual analysis and the evaluation of the percentage of reduction of the unexplained variance of the combined model with respect to the individual ones. Examples of application of those concepts to models developed at the Brazilian National Institute of Meteorology (INMET) show good results and illustrate that the forecast of the combined model exceeds in most cases each component model, when compared to observed data.

[1] Interannual variations of high potential vorticity intrusions at 10°S in the upper troposphere during NDJFM are shown to be negatively correlated with rainfall over northern northeast Brazil (NEB). That is, higher intrusions associated with stronger equatorial westerlies may provoke droughts. Higher intrusions lead to the formation of an anomalous cyclonic vortex over NEB, which causes convergence in the upper troposphere and sinking motion at lower levels. The number of intrusions in NDJFM has highest correlation, significant at 99% confidence level, with the rainfall of the principal rainy season in FMAM. Citation: Rao, V. B., S. H. Franchito, and T. F. Barbosa (2007), Impact of high potential vorticity intrusions into the tropical upper troposphere in South Atlantic on precipitation over northeast Brazil, Geophys. Res. Lett., 34, L06704,

In this study, we examined intrusions of the high potential vorticity (HIGH-PV) at the 350 K isentropic level over the South Atlantic during the period December-January-February (