Bridging the Annular Mode and North Atlantic Oscillation paradigms.

Castanheira J. M., Liberato M. L. R., Marques C. A. F., Graf, H.-F.
Journal of Geophysical Research Atmospheres, 112, D19103, 8pp. doi:10.1029/2007JD008477

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The annular nature of the leading patterns of the Northern Hemisphere winter extratropical circulation variability is revisited. The analysis relies on principal component analysis (PCA) of tropospheric geopotential height fields and lagged correlations with the stratospheric polar vortex strength and with a proxy of midlatitude tropospheric zonal mean zonal momentum anomalies. Results suggest that two processes, occurring at different times, contribute to the Northern Annular Mode (NAM) spatial structure. Polar vortex anomalies appear to be associated with midlatitude tropospheric zonal mean zonal wind anomalies occurring before the stratospheric anomalies. After the polar vortex anomalies, zonal mean zonal wind anomalies of the same sign are observed in the troposphere at high latitudes. The timescale separation between the two signals is about 2 weeks. It is suggested that the leading tropospheric variability patterns found in the literature represent variability associated with both processes. The tropospheric variability patterns which appear to respond to the polar vortex variability have a hemispheric scale but show a dipolar structure only over the Atlantic basin. The dipole resembles the North Atlantic Oscillation pattern (NAO), but with the node line shifted northward.