Western U.S. Inversions and the link to Pacific Climate Variability

The common occurrence of some combination of clear skies, subsidence, low humidity, topographic trapping, and/or elevated surfaces leads to frequent temperature inversions over the western states. Marine inversions frequently occur along the West Coast. These affect dispersion of pollutants and other aerosols, and have important health effects. Region wide population growth is very rapid, including many retirees with health concerns. Efforts to integrate air quality management to include mobile and fixed sources of gasses and particles, from population concentrations and from variable sites (wild land fires, for example) have mainly tended to focus on synoptic behavior, but an examination of historical records reveals considerable variability over annual to decadal time scales. This temporal variability is apparent in a number of measures of low-level inversions, including total strength and position in the atmosphere of the base or the top. Furthermore, there is considerable spatial coherence in both the winter half and summer half-year, indicating the influence of regional and larger scale circulation. Significant associations are seen between leading EOF modes of variability of inversion frequency and ENSO and also the North Pacific Oscillation. These associations are found for inversion characteristics during both the winter and summer. In some cases predictive skill exists. In addition, the interpretation of climate records in western sites may be affected by differential behavior between elevated locations and valley bottoms, where most airports are found, with implications for climate change and variability studies.