Relationships between the 5 Day External Rossby Mode and Extratropical Storm Tracks

Free Rossby modes of the atmosphere have traditionally been identified through the projection of data onto their theoretical structures. While this approach has yielded much valuable information about the temporal variability of these disturbances, the projection approach constrains the data to have only the largest scale structures associated with each mode. Here we employ an alternate methodology, using the well-known "5-day wave" as a proof of concept. The space time spectral signature of the 5 day wave is very strong in geopotential height and zonal wind at all levels from the surface up through the upper stratosphere. This enables us to filter specifically for the scales of the wave, then perform a Principal Component analysis of those fields that yields a pair of EOFs representing the propagating disturbance. This method reveals that the 5 day wave has a classical structure expected of this normal mode in the stratosphere, but in the troposphere a strong modulation of the storm tracks in both hemispheres is superimposed upon the westward traveling zonal wavenumber 1 structure. The patterns are robust to performing the EOF analysis on tropospheric or stratospheric data first, then projecting the data from other levels onto these PCs. Strong modulation of the 5 day wave occurs on intraseasonal time scales, and some of this can be related to variations in tropical convective forcing. Other external Rossby modes can also be analyzed in a similar manner, as long as they exhibit strong spectral signals and are not substantially affected by temporal variations in Doppler shifting effects.