Climatology of synoptic-scale ascent for Western North America: a perspective on storm tracks

This study presents an ascent-based perspective on the climatology of storm tracks across western North America for the October-April cool seasons spanning 1989-2008. Data from the ECMWF ERA-interim reanalysis are used. Synoptic scale ascent is shown to be a physically significant measure of storm location and intensity that can be accumulated over seasonal time scales to represent storm tracks. Subsynoptic scale vertical motions, such as orographic ascent, are filtered by using an alternative balance version of the omega equation. This constraint is an important consideration to estimate storm tracks in regions of complex topography. The climatological mean distribution of ascent suggests that the primary storm track occupies a sinusoidal belt across the eastern Pacific and adjacent portions of North America. The seasonal cycle in the location and intensity of storms is pronounced and consistent with synoptic experience. Interannual variations in synoptic-scale ascent are examined by principal component analysis. Meridional shifts from year-to-year of the locations of greatest ascent can be related to amplification and weakening of the climatological sinusoidal storm track. These interannual variations are found to be related to the phase of the El Niño Southern Oscillation, although the sample size of 19 seasons is a limitation for this analysis. The link between precipitation and synoptic-scale ascentis shown to be modulated by the extent to which the atmosphere is saturated.