Tropical Convection and Precipitation Regimes in the Western United States

Abstract

The authors have documented the relationship between tropical convection and precipitation regimes in the western United States. Circulation patterns associated with precipitation regimes are described and physical mechanisms are proposed. Contributions from intraseasonal and interannual bands are examined. When enhanced convection is located in the western Pacific, dry conditions in the Southwest (SW) and wet conditions in the Pacific Northwest (PNW) are observed. Fluctuations in both intraseasonal and interannual bands contribute to the rainfall variability. Enhanced convection in the western Pacific is accompanied by suppressed convection in the central Pacific. The associated Rossby wave vorticity source (S) anomalies keep the Pacific jet west of 150°W. A westward shift of the storm track to the North Pacific also contributes to dry conditions in the SW and wetness in the PNW. When enhanced tropical convection is located near 150°E, substantial contributions to outgoing longwave radiation anomalies are from fluctuations in the intraseasonal band. A wave train extends from the convective area in the Tropics to North America, where negative 200-hPa streamfunction anomalies are consistent with wetness in California and dry conditions in the PNW. When tropical convection is enhanced in the central Pacific from the date line to 135°W, most contributions are from the interannual band. The positive S anomalies associated with an enhanced local Hadley cell extending from the North Pacific to California are responsible in part for the eastward shift of the subtropical jet. The storm track moves southeast and is consistent with wet conditions in the SW.