The Argentinian Chaco Province is located in an area with a strong zonal gradient of annual precipitation, wetter in the east and drier in the west. Precipitation variations affect farming in the east and it may make the access to water sources difficult in the dry western lands known as the “Impenetrable”. Low-frequency precipitation variability is studied in the period 1955-2010. The annual precipitation cycle can be divided into two seasons: a wet season (from October to April) and a dry season (from May to September). A spatial Principal Component Analysis classification yields four subregions: subregion I to the east, subregion II to the west, subregion III to north-center and subregion IV to south-center. Only two of these were studied in the present paper, the one most economically important and the one with more water access problems, these are the two extremes in the precipitation gradient, the subregion I the wettest and the subregion II the drier. For these subregions a representative precipitation series was performed and studied. The exploratory study of potentials forcings reveals that seasonal accumulated precipitation is modulated by ENSO and regional anomalous circulations that increase anomalous northerly warm and wet air mass transport.