Wave-Maintained Annular Modes of Climate Variability*

The leading modes of month-to-month variability in the Northern and Southern Hemispheres are examined by comparing a 100-yr run of the Geophysical Fluid Dynamics Laboratory GCM with the NCEP-NCAR reanalyses of observations. The model simulation is a control experiment in which the SSTs are fixed to the climatological annual cycle without any interannual variability. The leading modes contain a strong zonally symmetric or annular component that describes an expansion and contraction of the polar vortex as the midlatitude jet shifts equatorward and poleward. This fluctuation is strongest during the winter months. The structure and amplitude of the simulated modes are very similar to those derived from observations, indicating that these modes arise from the internal dynamics of the atmosphere.