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<p>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.
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<br>Subjects: Meteorology; Climatology; Seasons; Madden-Julian oscillation</div>
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Stepanek, Adam J.</div>
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