The preferred structure of variability of the Northern Hemisphere atmospheric circulation

A nonlinear generalisation of Principal Component Analysis (PCA) is applied to the 500mb geopotential height field of the Northern Hemisphere extratro pical atmosphere. It is found that the low-frequency variability of the mid -troposphere is characterised by three distinct quasistationary states. The states are described and compared to those obtained from applications of c luster analyses and linear PCA to the height field. Evidence is provided th at modes obtained through PCA (notably the Arctic Oscillation (AO)) are not independent dynamical modes of variability of the Northern Hemisphere extr atropics. Rather they arise as the optimal linear compromise between the pr eferred quasi-stationary states of the circulation.