A document by Roberto Rondanelli . Click on the document to view its contents.

Atmospheric rivers (ARs) are known to produce both beneficial and extreme rainfall, leading to natural hazards in Chile. Motivated to understand moisture transport during AR events, this study performs a moisture budget analysis along 50 zonally elongated ARs reaching the western coast of South America. We identify the convergence of moist air masses of tropical/subtropical origin along the AR as the primary source of vertically integrated water vapor (IWV). Over the open ocean, moisture convergence is nearly balanced by precipitation. The advection of moisture along the AR, although smaller compared to mass convergence, significantly increases toward the landfalling region. The near conservation of IWV over the open ocean, observed by tracking a Lagrangian atmospheric column along the ARs, is the explanation behind the seemingly tropical origin of ARs in time-lapse visualizations of IWV

Rossby wave trains associated with the Madden-Julian Oscillation (MJO) tropical intraseasonal variability were analyzed using ERA5 composites arranged according to active phases of the MJO. During austral winter under neutral El Niño Southern Oscillation conditions, positive precipitation anomalies in Chile from 30$^\circ$ to 45$^\circ$S coincide with strong real-time multivariate MJO (RMM) phase 1 events. We found that a three-part mechanism, traced approximately 10 to 15 days before MJO phase 1 events, roughly corresponding to RMM phases 7 and 8, explains these precipitation anomalies. This mechanism is composed of three Rossby wave trains in response to convective perturbations: two interhemispheric teleconnections attributed to suppressed convection in the Indian Ocean and the Maritime Continent, and a cross-hemispheric Rossby wave linked to enhanced convection over the warm pool of the western hemisphere. Composite analyses of OLR and wave activity flux, complemented with Rossby wave ray tracing, support these findings. This research provides insights into the MJO’s influence on additional large-scale teleconnection patterns across the Southern Hemisphere besides the Pacific South American (PSA) teleconnection mode. The dispersion of the convective RMM phase at different lags and the interaction between multiple wave trains introduce uncertainty when associating a specific RMM phase with precipitation anomalies.