Regional and large-scale circulation anomalies associated with wet and dry conditions in southeastern South America (SESA), as detected by rainfall and outgoing long-wave radiation over Uruguay, are studied for austral spring (OND) and summer (JFM), when rainfall associated to deep convection is more frequent. Additionally, ENSO impact on precipitation in this region is significant in these seasons, particularly during spring.

The analysis was performed at a sub-monthly timescale, considering that circulation anomalies calculated on monthly or seasonal basis could hide the essential nature of the mechanisms producing rainfall, particularly in this region where precipitation is highly episodic.

The convective cloudiness anomaly field during wet and dry periods in Uruguay is characterized by a marked dipolar structure along eastern South America from 10 S to 40 S, with the centers of the dipole located over the south Atlantic convergence zone (SACZ) and over a broad region including Uruguay, the southern states of Brazil and northeastern Argentina. Thus, when convective cloudiness is anomalously intense (weak) over this region the SACZ is weaker (stronger) than average. Sub-monthly periods of enhanced convection and rainfall over Uruguay are associated with a warm-core anticyclonic circulation anomaly in the middle and upper troposphere, centered on 34 S – 45 W, approximately. The previous feature is part of a much larger wave-like quasi-barotropic structure that includes alternating centers of negative and positive geopotential height and temperature anomalies in the southern portion of the continent, and further upstream in the southern Pacific. At the regional scale, the anticyclonic circulation anomaly in the region between the SACZ and southeastern South America is consistent with an anomalously strong subtropical jet. Furthermore, an intensified Chaco low in northwestern Argentina leads to a reinforced northwesterly flow of warm and moist air toward SESA from the Amazon basin. No major differences between spring (OND) and summer (JFM) were detected in these circulation anomalies. On the other hand, dry periods in Uruguay are characterized by regional and large-scale circulation anomalies that are broadly opposite to those described before. The regional characteristics of the dipolar structure of OLR anomaly fields during wet and dry periods remain unchanged for El Niño and non-El Niño conditions in the central Pacific, highlighting the regional nature of this phenomenon.

Results presented here suggest that intraseasonal and interannual rainfall anomalies and perhaps those occurring at longer time scales on eastern South America (10 S – 40 S) are, to a large extent, built upon recurrent shorter timescale circulation anomalies linked to the functioning of a dipole in convective cloudiness over eastern tropical and subtropical South America. This phenomenon that appears as an intrinsic mode of atmospheric variability, is one of the key factors of climate dynamics in South America during spring and summer.

Considering the extent of circulation anomalies described in this work in relation with wet and dry conditions in Uruguay, it seems plausible that they also characterize the occurrence of extremes in the rainfall regime over a broader region in southeastern South America.