Role of Multiple-Level Tropospheric Circulations in Forcing ENSO Winter Precipitation Anomalies

Abstract

Regionally organized winter (DJF) precipitation anomalies over North America are presented in association with cold and warm phases of the El Niño?Southern Oscillation. Variations in low-level flow from moisture sources and in upper-level dynamic properties are diagnosed for each anomalous precipitation region using DJF composites of upper- and lower-tropospheric winds, sea level pressure, divergence, and vorticity advection. Variance analysis determines which precipitation and circulation patterns are robust and evaluates the predictability of the extreme phase composites with respect to the climatology. In cold phases, the Pacific Northwest and the lower Mississippi Valley regions have above-normal precipitation totals. The heavier precipitation over the Pacific Northwest is the result of a multilevel onshore flow forced by an amplified Pacific high off the California coast and a single zonal upper-tropospheric jet. In the Lower Mississippi and Tennessee Valleys, the equatorward entrance region of a west?east-oriented jet core combines with anomalous positive vorticity advection (PVA) and a convergent low-level flow off the Gulf of Mexico, resulting in heavier precipitation. Drier than normal conditions occur in coastal Alaska and California due to negative vorticity advection anomalies aloft, low-level divergence, and weaker onshore flow. The increased precipitation over the southern Mississippi Valley and the Pacific Northwest, and the drier conditions over Florida, are statistically robust. During warm phases, more precipitation occurs in the coastal regions of Alaska, California, and the Gulf of Mexico, whereas drier conditions persist over the Pacific Northwest. Alaskan precipitation is enhanced by strong, convergent, onshore flow associated with a deeper Aleutian low, upper-level PVA, and the northern branch of a split jet pattern. A strong southern subtropical branch of the split jet provides dynamic support for uplift of moisture along the Gulf Coast. The drier conditions in the Pacific Northwest and the wet conditions in Texas and Florida are predictable relative to the climatology and are statistically robust.