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Observations reveal that the North Atlantic Oscillation (NAO) exhibits a strong asymmetry: large amplitude, long persistence, and westward movement in its negative phase (NAO(-)) and conversely in its positive phase (NAO(+)). Further calculations show that blocking days occur frequently over the North Atlantic (Eurasia) after the NAO(-) (NAO(+)) peaks, thus indicating that North Atlantic blocking occurs because of the retrogression of the NAO(-), whereas blocking occurs over Eurasia because of enhanced downstream energy dispersion of the NAO(+).

Motivated by a unified nonlinear multiscale interaction (UNMI) model, the authors define dispersion, nonlinearity, and movement indices to describe the basic characteristics of the NAO. On this basis, the physical cause of the strong asymmetry or symmetry breaking of the NAO is examined. It is revealed that the strong asymmetry between the NAO(+) and NAO(-) may be associated with the large difference of the North Atlantic jet in intensity and latitude between both phases. When the NAO(+) grows, the North Atlantic jet is intensified and shifts northward and corresponds to reduced nonlinearity and enhanced energy dispersion because of an increased difference between its group velocity and phase speed related to enhanced meridional potential vorticity gradient. Thus, the NAO(+) has smaller amplitude, eastward movement, and less persistence. Opposite behavior is seen for the NAO(-) because of the opposite variation of the North Atlantic jet during its life cycle. Thus, the above results suggest that the NAO(+) (NAO(-)) tends to be a linear (nonlinear) process as a natural consequence of the NAO evolution because of different changes in the North Atlantic jet between both phases.