Recent studies have described the leading patterns of variability of the Northern Hemisphere extratropical circulation as an annular structure. The associated sea-level winter feature resembles the North Atlantic Oscillation pattern; however, it has been noticed that its centers of action, covering most of the Arctic, have a more zonally symmetric structure. This mode has been referred as the Arctic Oscillation (AO). By considering the vertical structure of the AO, some authors have suggested that a stratosphere-troposphere interaction mechanism may be the source of the interannual variability associated with the AO and, therefore, it could have some relevances also for the NAO. Thus, the NAO and the AO may be considered as different facets of the same phenomenon. In the present paper we analyze the interannual variability of 52 Northern hemisphere winters in the NCEP reanalysis. The study rests on a principal component analysis and singular value decomposition of the sea level pressure, the geopotential heights at 500 hPa and 50 hPa and the zonal wind at 200 hPa. Moreover, following Rossby earlier works, we compute the principal components also for the eddy fields. The analysis performed suggests that the eddy patterns of variability allow a better identification of the modes connected to the NAO in the middle troposphere and in the lower stratosphere. Two distinct stratospheric wave patterns are found to be related to NAO and PNA modes. The covariance analysis suggests a dynamical link with the Atlantic jet exit for the former mode, and a connection with the Atlantic jet entrance for the latter mode. In view of the NAO-AO debate, the results here presented seem to confirm that the NAO and PNA mechanisms contribute separately to the atmospheric eddies variability in the troposphere and in the lower stratosphere.
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|Titolo:||Tropospheric and stratospheric aspects of the North Atlantic Oscillation: An eddy perspective|
|Data di pubblicazione:||2003|
|Appare nella tipologia:||01a Articolo in rivista|