The vertical structure of wintertime climate regimes of the Northern Hemisphere extratropical atmosphere

A nonlinear generalization of principal component analysis (PCA), denoted nonlinear principal component analysis (NLPCA), is applied to Northern Hemisphere wintertime geopotential heights at 1000, 700, 500, 300, and 20 hPa. It is found that the optimal nonlinear approximation to the data at the four tropospheric levels is characterized by three equivalent-barotropic regimes of circulation. The NLPCA time series provides a kinematic description of variability within the regimes and transitions between them. The occupation frequencies of the regimes demonstrate substantial interannual and interdecadal variability, some of which can be associated with the influence of El Nino-Southern Oscillation. The stratospheric geopotential height field is also characterized by three circulation regimes. Two of these characterize a vacillation of polar vortex shape and polar temperature. The third regime, which is occupied episodically, is characterized by dramatic reductions in the zonal-mean zonal wind and meridional temperature gradient at high latitudes, and is associated with stratospheric sudden warmings. Evidence is presented for a connection between the stratospheric and tropospheric regimes, including an association between stratospheric sudden warmings and North Atlantic blocks. Finally, these results are compared to previous studies of regime structure and to the Arctic Oscillation paradigm.