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The associated anomaly patterns of the stratospheric geopotential height field and the tropospheric geopetential and temperature height fields of the Northern Hemisphere are determined applying the canonical correlation analysis. With this linear multivariate technique the coupled modes of variability of lime series of two fields are isolated in the space of empirical orthogonal functions. The one dataset is the 50-hPa geopotential height field; the other set consists of different height fields of the tropospheric pressure levels (200, 500, 700, and 850 hPa) and the temperature of the 850-hPa pressure level. For the winter months (December, January, February) two natural coupled modes, a barotropic and a baroclinic one, of linear relationship between stratospheric and tropospheric circulation are found. The baroclinic mode describes a connection between the strength of the stratospheric cyclonic winter vortex and the tropospheric circulation over the North Atlantic. The corresponding temperature pattern for an anomalously strong stratospheric cyclonic vortex is characterized by positive temperature anomalies over higher latitudes of Eurasia. These “Winter Warmings” are observed, for example, after violent volcanic eruptions. The barotropic mode is characterized by a zonal wavenumber 1 in the lower stratosphere and by a PNA-like pattern in the troposphere. It was shown by van Loon and Labitzke that this mode can be enhanced, for example, by El Niños via the intensification of the Aleutian low.
Perlwitz et al. (Sun,) studied this question.