The atmospheric conditions preceding the occurrence of a strong low level jets east of the Andes during January 1998

The existence of a strong northerly low level current inmediately east of the Andes has been documented in previous works. Some of them have been carried out using analyzed data sets, like those provided by NCEP (National Centers for Environmental Prediction) or ECMWF(European Centre for Medium Range Weather Forecasts), a few ones have used observed wind data, which are extremely scarce over central South America. With this same objective, Eta regional operational model outputs, provided by CPTEC(Centro de Previsão de Tempo e Estudos Climáticos - Brazil-), have been analyzed during 1997-1998 warm season. This previous work provided a high-resolution database suitable to detect Low level jet (llj) occurrence (following Bonner´s criterion) during that period. Particularly strong events, characterized by a pronounced southern penetration of the llj, and consequently of tropical air, have been isolated during this warm season. In this work, a subset of these strong events, named Chaco jets (see companion paper for further details), is analyzed using both, Eta operational model outputs, provided by CPTEC and NCEP-reanalyzes which are freely available at the Climate Diagnostic Center web site. During January 1998, 11 Chaco jet events have been isolated. An ensamble of these days, constructed from 2 days before the strong event to one day after is being analyzed in this work. The main objective is the characterization of the atmospheric circulation over South America preceding the occurrence of the Chaco jets, to provide some hints of how this pattern evolves and which are the phenomena favoring its formation. Also the conditions associated to its decay are briefly described. This analysis has been carried out through the inspection of synoptic scale patterns that could be responsible for low level wind acceleration. The associated diurnal cycle is also studied. The analysis of mean sea level pressure and equivalent potential temperature shows the strong interaction between the northern Argentina thermal low (NAL) and the llj intensification and subsequent southern penetration. It also depicts the strong correlation between weak SACZ associated circulation, described in previous works, and Chaco jet occurrence, at least over the period under study.