Zonal-Eddy Dynamics of the North Atlantic Oscillation

Abstract

This research is an attempt to understand the dynamical mechanisms that drive the wintertime North Atlantic oscillation (NAO) on monthly and longer timescales. In an earlier work by DeWeaver and Nigam, the authors showed that momentum fluxes from stationary waves play a large role in maintaining the zonal-mean zonal wind (u) perturbations associated with the NAO. In this paper, a linear stationary wave model is used to show that zonal-mean flow anomalies in turn play a large role in maintaining the NAO stationary waves. A strong two-way coupling thus exists between u and the stationary waves, in which each is both a source of and a response to the other. When forced by zonal-eddy coupling terms?terms that represent the interaction between NAO-covariant zonal-mean zonal wind anomalies and the climatological eddy flow?together with heating and transient fluxes, the model produces a realistic simulation of the observed stationary wave pattern. Zonal-eddy coupling terms make the largest contribution to the simulated stationary waves. Every feature of the stationary wave pattern is forced to some extent by zonal-eddy coupling, and the upper-level trough over Greenland is forced almost entirely by the coupling terms. The stationary waves generated by zonal-eddy coupling are well positioned to provide additional momentum to the u anomalies, demonstrating the strong positive feedback between zonal-mean and eddy flow components. The NAO is known for its effect on tropospheric temperatures over northern Eurasia, and the model produces a realistic simulation of these temperature changes at midtropospheric levels. Zonal-eddy coupling, including the zonal advection of land?sea thermal contrasts, is partly responsible for the temperature changes. However, diabatic heating anomalies associated with the displacement of the Atlantic storm track are also influential, causing more than half of the warming over Scandinavia and most of cooling from North Africa to the Caspian Sea.