Variability of winter frosts in Central South America: quantifying mechanisms with decision trees

Abstract

Agricultural production in Central South America (CSA) is substantially influenced by frost events. This study charac¬terises and quantifies the physical processes leading to frost conditions in CSA from 1979 to 2022, focusing on three innovative aspects: regional frost properties, a novel multi-parametric upper-level jet description, and the quantification of underlying mechanisms through decision trees (DTs). The regionalisation analysis identifies five homogeneous frost regions in CSA. In all regions, the events tend to occur more frequently during the La Niña phase. Moreover, a significant increase in the frequency of widespread frost events has been observed in the Argentinean Pampas during the study period, primarily due to negative trends in minimum temperatures. Furthermore, the synoptic mechanisms triggering frosts, such as cold fronts and post-frontal anticyclones enhanced by subsidence near the subtropical jet (STJ) entrance, have not shown major long-term changes. To describe the jets, we compute six parameters for the STJ and seven for the polar front jet, including latitude, intensity, height, tilting, longitudinal extent, and branch number. DTs are used to identify key jet parameters linked to frost events, such as the latitude, longitudinal extent, and tilt of the Atlantic STJ. Frost likelihood increases when the STJ is north of 31°S, and the extension of the Atlantic STJ is longer than 35° and has a negative tilt. Finally, DTs focused on the onset and end of events highlight geopotential height anomalies and STJ extension as critical variables. These DTs provide concise and accessible information for agricultural decision-makers in CSA.