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Abstract

NCEP reanalysis data are analyzed globally in the phase space of normal modes of the equations of motion in a layered atmosphere at rest. If the regime character of stratospheric circulation in boreal winter months is used to stratify the data set into months with strong and months with weak polar vortex, several interesting results appear: 1. Changes in the stratospheric regimes lead to structural changes of variability in the troposphere. The 4 clusters of 500 hPa geopotential height identified by Corti et al. (1999) can be interpreted as two pairs of variability structures under the conditions of weak and strong northern polar vortex, respectively. 2. A significant anticorrelation is found in sea level pressure between the Icelandic Low and the Aleutian Low in the strong polar vortex regime, but not in the weak one. It is found also in "control runs" of climate models since these often are biased towards the strong polar vortex regime. 3. The well known pattern of the "Arctic Oscillation" is mainly the result of the mean surface pressure difference between the two stratospheric regimes. 4. The Southwest-Northeast tilt of the node line of the North Atlantic Oscillation pattern is the result of the undue application of linear statistics to non-linear variability. It is due to the superposition ofthe mean difference in atmospheric pressure between the two stratospheric circulation regimes with a strictly meridional NAO pattern. Only the latter is a physical mode. The zonal North Pacific-North Atlantic connection (suggested to work via planetary waves) also influences the Southwest-Northeast tilt of the node line in the strong vortex regime.