Decomposition of synoptic eddy structure and its induced dynamical feedback onto NAO

The dynamical feedback of transient synoptic eddies onto the seasonal climatic variability of the North Atlantic Oscillation (NAO) are investigated in this study. By separating the synoptic eddy flow into basic stochastic eddy flow and anomalous eddy flow, it is demonstrated that the NAO flow can gain robust self-reinforcement by organizing synoptic eddies and harvesting eddy vorticity in the Atlantic storm track region. Using rescaled three-point covariance statistics of band-pass-filtered (2-8 days) synoptic eddy fields, the eddy structure changes associated with winter-mean NAO flow is examined. Observational evidences show clearly that the NAO flow anomalies systematically deform the structures of recurring synoptic eddies to generate seasonal-mean eddy-vorticity flux anomalies predominately directed to the left-hand side of the NAO flow. These anomalous eddy-vorticity fluxes converge into the cyclonic center and diverge from the anticyclonic center of the anomalous NAO flow, and thus in turn enhance the NAO flow. Moreover, the asymmetry in dynamical eddy feedback between the positive and negative NAO phases is shown, which can be reasonably linked to the so-called in-phase and in-quadrature feedback. This kinematic mechanism in this study for the NAO reflects the essential internal dynamical interactions between weather-storm activity and climate variability.