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Extreme cold events can have several consequences in different sectors, including human health, infrastructure, agriculture, and the economy. The frost events that affect central and northeastern Argentina are associated with stationary Rossby waves triggered by tropical heat sources and convection, which result in intense polar air advection. However, the processes responsible for these cold events include complex interactions between these wave trains and mechanisms at different atmospheric scales, such as the Madden–Julian Oscillation (MJO), which can alter the temperature patterns. This study investigates the impacts of the MJO on air temperature and associated circulation in years with extreme occurrence frequency of cold events in central southern South America during the winter. The results showed that the MJO convection can enhance or weaken the temperature anomalies during winters with a maximum and minimum frequency of generalized frosts (GF) occurrence due to Rossby wave train propagation triggered in the tropical region. Moreover, the effect on the anomaly patterns in these events depends on the MJO phase. Our analysis shows that individual MJO phases can modulate the temperature anomalies even in an unfavorable basic state (as in winters with an extreme frequency of GF and emphasizes the importance of considering the MJO in the predictions of the temperature anomalies associated with GF over southeastern South America. [ABSTRACT FROM AUTHOR]

The South American Monsoon System (SAMS) is a major climatic feature of South America, and its domain extends from Amazon to La Plata basin. The SAMS region is vulnerable to variations of climate and precipitation patterns, which could impact economic activities and lead to potential societal consequences. In the face of a warming future scenario, the importance of the study of the past climate with numerical simulations is to evaluate the climate models and to assure the reliability of future projections. Here we investigate the Mid-Holocene SAMS, evaluating changes in strength, life cycle and associated dynamical mechanisms in ten Earth System Models simulations. Our results show that the SAMS was weaker in the Mid-Holocene than in the pre-industrial climate in December–January–February (DJF), but stronger in September–October–November (SON). This is probably a consequence of insolation variations in the Mid-Holocene, which contributed to changes in the moisture flux from the Atlantic Ocean to the continent, the strength of the upper-level atmospheric circulation, and the amount of precipitation over the SAMS region. Moreover, we suggest that the life cycle of the SAMS was altered during the Mid-Holocene, with an earlier onset and demise. Our results also indicate that Mid-Holocene SAMS changes are connected to precipitation variations near Northeast Brazil, in a dipole configuration of precipitation between western Amazon and Northeast Brazil, due to the influence of the Walker cell. Finally, this study highlights a need for improvement of the numerical models to better simulate the amount of precipitation over South America and the upper-level circulation over western Amazon in SON, which are crucial factors for a more realistic representation of the SAMS. [ABSTRACT FROM AUTHOR]