Role of multiple-level tropospheric circulations in forcing ENSO winter precipitation anomalies

Regionally organized winter (DJF) precipitation anomalies over North Americ a are presented in association with cold and warm phases of the El Nino-Sou thern Oscillation. Variations in low-level flow from moisture sources and i n upper-level dynamic properties are diagnosed for each anomalous precipita tion region using DJF composites of upper- and lower-tropospheric winds, se a level pressure, divergence, and vorticity advection. Variance analysis de termines which precipitation and circulation patterns are robust and evalua tes the predictability of the extreme phase composites with respect to the climatology. In cold phases, the Pacific Northwest and the lower Mississippi Valley regi ons 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 equa torward entrance region of a west-east-oriented jet core combines with anom alous positive vorticity advection (PVA) and a convergent low-level flow of f the Gulf of Mexico, resulting in heavier precipitation. Drier than normal conditions occur in coastal Alaska and California due to negative vorticit y advection anomalies aloft, low-level divergence, and weaker onshore how. The increased precipitation over the southern Mississippi Valley and the Pa cific Northwest, and the drier conditions over Florida, are statistically r obust. During warm phases, more precipitation occurs in the coastal regions of Ala ska, California, and the Gulf of Mexico, whereas drier conditions persist o ver the Pacific Northwest. Alaskan precipitation is enhanced by strong, con vergent, onshore flow associated with a deeper Aleutian low, upper-level PV A, and the northern branch of a split jet pattern. A strong southern subtro pical branch of the split jet provides dynamic support for uplift of moistu re along the Gulf Coast. The drier conditions in the Pacific Northwest and the wet conditions in Texas and Florida are predictable relative to the cli matology and are statistically robust.