Climate variability of persistent deep inversions in the Intermountain West of the United States

Persistent temperature inversions are a frequent wintertime phenomenon in areas of the Intermountain West of the United States. Their presence is often blamed for poor air quality in some areas of the region. Prolonged inversions have been linked to some of the dangerous smog episodes in urban valleys or basins such Salt Lake City, Utah. In this study, climate and climate variability of persistent inversions are analyzed using historical data from North American Regional Reanalysis or NARR. The results reveal a widespread occurrence of short-lived (1-2 days) temperature inversions throughout the region. Prolonged inversion events, however, tend to occur mainly in large basins and valleys. EOF analyses are applied to identify dominant patterns of the interannual variability of these persistent inversion events. The first EOF mode, which explains about 30% of the total variance and is in-phase across the region, is linked to the Pacific Decadal Oscillation and the El Niño-Southern Oscillation. Corresponding to the positive phase of the first mode is an anomalous 500-hPa ridge induced by a Rossby wave-train over the Western United States, which contributes to more frequent occurrence of persistent inversion episodes by blocking cold-air invasions and producing subsidence warming. The second EOF mode, which accounts for approximately 10% of the total variance and shows an out-of-phase variability between the Columbia Basin and the other large basins/valleys in the Intermountain West, is associated with a negative 500-hPa geopotential height anomaly that corresponds to more frequent disturbances and cold air invasions and thus fewer persistent inversion events.