Mechanisms for the influence of the MJO on precipitation in Southwestern
South America
Abstract
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.