| Summary: | Tese de doutoramento em Física (Meteorologia), apresentada à Universidade de Lisboa através da Faculdade de Ciências, 2008 The annular nature of the leading patterns of the Northern Hemisphere winter extratropical circulation variability is analysed by means of a Principal Component Analysis (PCA) of tropospheric geopotential height fields followed by lagged correlations of relevant Principal Components with the stratospheric polar vortex strength as well as with a proxy of midlatitude tropospheric zonal mean zonal momentum anomalies. Results suggest that two processes, occurring with a time scale separation of about two weeks, contribute for the Northern Annular Mode (NAM) spatial structure. Polar vortex anomalies appear to be associated with midlatitude tropospheric zonal momentum anomalies leading the vortex. Zonal mean zonal wind anomalies of the same sign lagging the vortex are then observed in the troposphere at high latitudes. Tropospheric variability patterns which seem to respond to the polar vortex variability have a hemispheric scale. A dipolar structure may be observed only over the Atlantic basin that resembles the North Atlantic Oscillation pattern (NAO), but with the node line shifted northward. The association between zonal symmetric components of the tropospheric and stratospheric circulations is further confirmed by a PCA on the barotopic component of the Northern winter atmospheric circulation. Results show that a zonally symmetric component of the middle and lower tropospheric circulation variability exists at high latitudes. At the middle latitudes obtained results suggest that the zonally symmetric component, as identified in other works, is artificially overemphasized by the usage of PCA on single isobaric tropospheric levels. The 3-Dimensional normal mode expansion of the atmospheric general circulation was also used to relate the variability of the stratospheric polar vortex to the energy variability of the forcing planetary waves. Positive (negative) anomalies of the energy associated ...
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