North Pacific - North American circulation and precipitation anomalies associated with the Madden-Julian oscillation

Abstract

The Madden-Julian Oscillation (MJO) has been associated with extreme precipitation events in western North America. However, the mechanisms for, and predictability of, these associations are not clear. We have examined the influence of the MJO on North Pacific - North America (NPNA) circulation and precipitation anomalies during the boreal winter. We constructed composites of MJO events during 1979-2005 determined from the Wheeler RMM1/RMM2 index of MJO activity. Our analyses of NPNA anomalies were based primarily on the National Centers for Environmental Prediction reanalysis data set. We focused our investigations on the impacts on NPNA circulation and precipitation of: (1) the location and amplitude of the convective and subsidence components of the MJO; (2) the season of MJO occurrence; and (3) concurrent El Nino (EN) or La Nina (LN) events. We found that the NPNA response to the MJO is sensitive to the location of both the convective and subsidence components of the MJO, the season of MJO occurrence, and to the existence of concurrent EN or LN events. EN or LN events affect the extratropical response to the MJO by altering the equatorial Rossby-Kelvin wave response to the components of the MJO. This in turn affects the anomalous extratropical wave trains initiated by the MJO, and alters the strength and location of the resulting NPNA precipitation anomalies. Our results have allowed us to identify characteristic patterns associated with the MJO that can be related to the location and intensity of extreme NPNA precipitation. MJO events are relatively persistent phenomena. Thus, increased understanding of the mechanisms by which they impact the extratropics has the potential to improve extratropical extended range forecasting. Our results provide a substantial foundation for improving forecasts of NPNA circulation and precipitation.