ETA characterization of the 1997-1998 warm season Chaco jet cases

The pattern of frequency of occurrence of low-level jets just east of the Andes (SALLJ) cases for the austral summer, obtained both from analyses and reanalyses data archives displays a major maximum near Santa Cruz de la Sierra (Bolivia) and a secondary maximum penetrating to higher latitudes (30ºS). One of the prominent roles of warm and moist low-level jets is to efficiently transport moisture from low to high latitudes and the other is to act as a deep convection forcing mechanism in downwind extended areas where associated moisture flux convergence is favored. The mentioned secondary SALLJ frequency maximum may be related to enhanced precipitation episodes over northern and central Argentina, Uruguay and southern Brazil. This study focuses in a particular set of SALLJ cases for the 1997-1998 warm season (part of an El Niño event) characterized by a more extended penetration to the south (denoted as Chaco jets). The objective is to study the differences between the composite Chaco jet patterns of low-level circulation, vertically integrated moisture flux convergence, equivalent potential temperatures, sea level pressure, accumulated precipitation fields and the corresponding mean patterns for the whole warm season. The ETA regional model operative forecasts produced at CPTEC (Brazil) for the period September 1997-February 1998 over a domain covering South America provides the data set for this study. Main differences between the mean fields and the Chaco jet composite fields show for the latter a stronger meridional wind speed maximum and water vapor transport at the four synoptic times and a longer occurrence of the Chaco jet daily duration. Besides, these differences fields reveal a deeper Chaco-Northwestern Argentina thermal low co-located with a low-level higher equivalent potential temperatures. They also show a stronger vertically integrated moisture flux convergence mostly over Paraguay, northeastern Argentina, southern Brazil and Uruguay and a similar region with heavier precipitation.