Zonal asymmetries, teleconnections, and annular modes in a GCM

The influence of zonally asymmetric boundary conditions on the leading modes of variability in a suite of atmospheric general circulation models is investigated. The set of experiments consists of nine model configurations, with varying degrees of zonal asymmetry in their boundary conditions. For each configuration, the structure of the leading EOF of the surface pressure is found to vary strongly with the zonal asymmetry of the base state. In particular, a close relationship is found between the structure of the EOF and the model stormtracks. An approximately linear relationship is found to hold between the magnitude of the zonal asymmetry of the leading EOF and of the stormtracks in the models. It is shown that this linear relationship extends to the observations.

One-point correlation maps centered on the regions where the EOFs reach their maximum amplitude show similar structures for all configurations. These structures consist of a north-south dipole, resembling the observed structure of the NAO. They are significantly more zonally localized than the leading EOF, but do resemble one-point correlation maps and sector EOFs calculated for a simulation with zonally symmetric boundary conditions. Thus, the leading EOF for each simulation appears to represent the longitudinal distribution of individual events with the spatial structure of the NAO. This longitudinal distribution is strongly influenced by the structure of the stormtracks, both in the models and in the observations.