| Description |
This study uses multivariate statistical methods to analyze the predominant modes of Rossby wave breaking (RWB) observed over the Pacific sector during winter, and relates these modes to atmospheric circulation and downstream precipitation over western North America. An existing RWB detection algorithm was enhanced to calculate a more detailed geometry of the potential vorticity (PV) overturning, refining prior climatologies of Pacific RWB. The leading mode of variability in anticyclonic RWB captures enhanced frequency of breaking over the north central Pacific, and occurs in conjunction with displacement of cyclonic RWB to the west and east. This mode is tightly coupled to the El Ni ̃no-Southern Oscillation (ENSO), with more frequent anticyclonic RWB occurring during the La Ni ̃na phase, which features a slower and broader Pacific jet stream. The second mode of variabil- ity is a northward shift in anticyclonic RWB in the east Pacific-North American sector, and is linked to a northward shift of the jet stream upstream over east Asia. This second mode is a stronger driver of western U.S. precipitation than ENSO because of the influence on the storm track, producing a dry-northwest / wet-southeast precipitation dipole. A linear model using indices of the first and second anticyclonic RWB modes accounts for up to two-thirds of winter precipitation variability over Mexico and the western U.S. |
