Transient Eddy Forcing of Low-Frequency Atmospheric Variability

Abstract

This paper is concerned with the forcing of the low-frequency variability (10 days up to 3 months) of the barotropic planetary waves (wavenumbers 1 to 4) for wintertime conditions in the Northern Hemisphere. In particular, the forcing by the transient vorticity fluxes arising from the interactions between the planetary waves and the synoptic-scale eddies is considered. This forcing effect is stochastically modeled in terms of a combined Markov-complex EOF approach. The performance of these stochastically modeled vorticity fluxes is evaluated in the framework of a simple barotropic dynamical model. It turns out that the forcing is highly coherent in wavenumber space, i.e., local in physical space, and that this organization is largely associated with its low-frequency components. The simple dynamical model with the modeled forcing is able to reproduce reasonably well the spatial structure of the observed low-frequency variance of the planetary waves. The performance is best if only the low-frequency components of the stochastically modeled forcing are prescribed. In this case the observed variance maximum over the Atlantic is well simulated, in position and in intensity, while the model variance over the Pacific is weaker than observed and shifted to the west.