MJO-ENSO Influence on Extreme Rainfall over the Peruvian Andes

The combined influence of Madden-Julian Oscillation (MJO) and El Niño-Southern Oscillation (ENSO) on the occurrence of rainfall extremes in the central and southern Peruvian Andes were investigated. Forty-seven gauge stations showed interesting rainfall patterns during MJO-ENSO conditions of the period Nov-Mar 1979-2010. Positive standardized rainfall anomalies and greater frequency of rainfall extremes (above the 75th percentile) occurred during active MJO, and this pattern was enhanced during La Niña. Furthermore, MJO-ENSO modulation of extreme rainfall varied according to the location of stations. For instance, during ENSO-neutral conditions, extreme rainfall in stations located over the upper Amazon basin were more sensitive to MJO phases 4, 5 and 8; the frequency of extremes in stations located over the coastal Pacific basin slightly increased during MJO phase 4 whereas phase 2, 4 and 8 modulated extreme rainfall events in the High-Andean Puna plateau stations. Lastly, dynamical mechanisms linking the MJO-ENSO to the occurrence of extreme rainfall in the Peruvian Andean stations were investigated using composites of integrated moisture flux and upper level winds anomalies. Among the main mechanisms that explained the increase in extreme rainfall events in the Peruvian Andes was the intensification of the easterly moisture flux anomalies, which was favored during certain MJO-ENSO conditions.