Maintenance of the annular mode patterns in a simple atmospheric general circulation model

A simple atmospheric general circulation model is used to study the dynamics of the annular modes. The model, run with perpetual solstitial conditions, robustly produces an annular mode pattern in the winter hemisphere similar to that found in the real atmosphere (Kushner and Polvani, 2004).

Singular value decomposition is performed to examine the relationships between anomalies in zonal wind, the rate of change of zonal wind, and the divergence of Eliassen-Palm flux. The spatial pattern of leading variability in the zonal wind found through SVD analysis of its covariance with either the temporal change in zonal wind or the E-P flux divergence duplicates that of the leading EOF of the zonal wind. But the leading spatial patterns in the wind rate of change and E-P flux divergence found through the SVD analysis are different than those described by their leading EOFs.

Calculations using a forced, quasi-geostrophic, zonal-mean model show that the annular mode patterns are a result of variability in the divergence of the E-P flux. Moreover, it is the barotropic component of the E-P flux divergence which reproduces the annular zonal wind patterns.

The eddy feedback responsible for these annular mode patterns can act to amplify the response to external forcing, or produce an annular mode-like response to a forcing whose structure is different from that of the annular modes (Song and Robinson, 2004). In order to test further this amplification, we conduct a series of trials with artificial, external forces imposed in localized regions.