Rossby Wave Dynamics over South America Explored with Automatic Tropical–Extratropical Cloud Band Identification Framework

Abstract

AbstractDuring austral summer, persistent tropical–extratropical cloud bands, such as the South Atlantic convergence zone (SACZ) over South America (SAm), link the tropical humid areas to the subtropics. In this study, we utilize an automatic object-based methodology to identify synoptic cloud band events occurring over SAm that are responsible for almost 60% of the precipitation during the rainy season (November–March). In addition to identifying SACZ events as cloud bands persisting 4 or more days, the framework also highlights the relevance of transient events (i.e., events persisting for 3 days or less) to the climatology. The location and persistence of the cloud band events are modulated by the propagation of synoptic-scale extratropical disturbances interacting with intraseasonal variability in the basic-state upper-level zonal wind. During persistent events (i.e., lasting 4 or more days), upper-level westerly anomalies over the subtropics favor the propagation extratropical disturbances deeper into the tropics. Conversely, transient events occur when the Bolivian high is displaced/expanded southeastward, bringing upper-level easterly winds into subtropical latitudes and blocking the propagation of Rossby waves into lower latitudes. Subsequent anomalous subtropical convection from the cloud bands result in sources of Rossby waves that interact with the basic flow, resulting in downwind enhancement or damping of the extratropical disturbances. The adopted methodology proved to be a powerful framework in demonstrating this interaction between scales, with the basic state influencing and being modified by the synoptic disturbances.