Precipitation Producing Synoptic-Scale Flow and El Niño-Southern Oscillation

Abstract:

The differences in winter synoptic-scale flow during precipitation events between phases of El Niño–Southern Oscillation are established for the coastal southwest United States. Principal component analysis in T-mode with Varimax rotation was performed on fields of 500-hPa, 700-hPa, and 850-hPa geopotential height and specific humidity (from the North American Regional Reanalysis) during days with precipitation. These precipitation days were grouped through a k-means cluster analysis of the loadings on the extracted components from all atmospheric variables. Eight clusters were ultimately analyzed based on separation and cohesion statistics. Analyses of cluster membership and cluster composite maps reveal synoptic-scale patterns that are most common during particular phases of ENSO. Distinctly El Niño patterns are charactized by a broad offshore upper level trough with relatively moist low-level onshore flow. Distinctly La Niña patterns display a deep southwest-northeast oriented trough over the western United States with drier low-level onshore flow. Analysis of 250-hPa potential vorticity suggests that the La Niña (El Niño) patterns are linked to anticyclonic (cyclonic) baroclinic wave breaking events. This agrees with a recent line of investigations which find anticyclonic (cyclonic) wave breaking in the Pacific North American region to be more common during La Niña (El Niño) events. Knowledge of interannual variability in winter precipitation event characteristics can improve seasonal runoff forecasts and advance understanding of interdecadal variability and the effects of climate change in the western United States.