Occurrence of extreme precipitation events in California and relationships with the Madden-Julian Oscillation

California receives most of its annual precipitation during the winter season, with large spatial contrasts and temporal variations being observed in the amount of total rainfall. Increasing observational evidence indicates that large-scale tropical circulation anomalies on intraseasonal time scales can modulate wet and dry conditions in the Pacific Northwest. The Madden-Julian Oscillation (MJO), which is the main mode of tropical intraseasonal variation, has been shown to be related to precipitation anomalies in the western U.S.. Some previous observational studies have demonstrated, for instance, that when equatorial convective anomalies associated with the MJO are located near 150°E (120°E) above (below) normal precipitation conditions are observed over California.

This study investigates the occurrence of extreme precipitation events in California and a possible modulation by the MJO. Daily observed precipitation from gridded hourly station data (2.0º lat x 2.5º lon) from January 1958 to December 1996 is used to define extreme precipitation events. Anomalies (20-90 days) of 5-day averages of Outgoing Longwave Radiation (OLR) from January 1979 to December 1996 are used to identify tropical convective anomalies associated with the MJO. In addition, anomalies (20-90 days) of 5-day averages of zonal winds (U) at 200 hPa and 850 hPa derived from the NCEP/NCAR reanalysis from January 1958 to December 1996 are employed to characterize large-scale circulation anomalies related to the MJO.

Based on 5-day precipitation totals, three types of extreme events are defined: type I if the 5-day total precipitation exceeds 5% of the total annual precipitation, type II if the 5-day total precipitation exceeds 10% of the total annual precipitation and type III if the 5-day total precipitation exceeds 15% of the total annual precipitation. The results show that nearly twice as many extreme precipitation events of type I and II in central and southern California occur when tropical convective and circulation anomalies are large and associated with the MJO.