Predictability of ultralong waves in global and hemispheric quasi-geostrophic barotropic models

A global quasi-geostrophic barotropic model, including orography, zonal forcing, and frictional dissipation, is compared to two hemispheric models, one with antisymmetric equatorial boundary conditions and one with symmetric boundary conditions. The stationary solutions in the global model and the hemispheric models are found to be different, because the hemispheric models lack either the symmetric or antisymmetric waves, and because the nonlinear feedbacks are much larger in the hemispheric models. Time-dependent calculations show that the hemispheric models can excite anomalous Rossby waves and can produce erroneous short-range forecasts in middle latitudes. It is concluded that global models are preferred for making both short-range and long-range forecasts for middle latitudes.