TROPICAL CONVECTION AND PRECIPITATION REGIMES IN THE WESTERN UNITED-STATES

The authors have documented the relationship:between tropical convecti on and precipitation regimes in the western United States. Circulation patterns associated with precipitation regimes are described and phys ical mechanisms are-proposed. Contributions from intraseasonal and int erannual bands are examined. When enhanced convection is located in th e western Pacific, dry conditions in the Southwest (SW) and wet condit ions in the Pacific Northwest (PNW) are observed. Fluctuations in both intraseasonal and interannual bands contribute to the-rainfall variab ility. Enhanced convection in the western Pacific is accompanied by su ppressed convection in the central Pacific. The associated Rossby wave vorticity source (S) anomalies keep the Pacific jet west bf 150 degre es W A westward shift of the storm track to the North Pacific also con tributes to dry conditions in the SW and wetness in the PNW. When enha nced tropical convection is located near 150 degrees E, substantial co ntributions to outgoing longwave radiation anomalies are from fluctuat ions in the intraseasonal band. A wave train extends from the convecti ve area in the Tropics to North America, where negative 200-hPa stream function anomalies are consistent with wetness in California and dry c onditions in the PNW. When tropical convection is enhanced in the cent ral Pacific from the date line to 135 degrees W, most contributions ar e from the interannual band. The positive S anomalies associated with an enhanced local Hadley cell extending from the North Pacific to Cali fornia are responsible in part for the eastward shift of the subtropic al jet. The storm track moves southeast and is consistent with wet con ditions in the SW.