Weather regimes in subtropical South America and their impacts over Uruguay

This study addresses the interannual variability and trends in weather regimes in subtropical South America and their impact over Uruguay. Weather regimes are found by applying a clustering method to the sea level pressure field in every season. We evaluate for all seasons their frequency of occurrence and transition pathways among them, their statistical relationship with global climate variability modes and their impact on precipitation and temperature in Uruguay. Overall, we find that there are five weather regimes that represent well the atmospheric dynamics over all seasons with two additional regimes in austral winter. The interannual variability in their frequency of occurrence and preferred transitions are sensitive to the phases of El Nino-Southern Oscillation and to other modes of variability. In addition, some weather regimes show positive and others negative trends in their frequency of occurrence over the last 70 years. In particular, we find that the weather regime that represents the development of a low-pressure system over Uruguay has a negative trend (10-15% over 70 years) in all seasons except spring.