Maintenance of Northern Summer Stationary Waves in a GCM

The maintenance of northern summer stationary waves is studied with data from a 15-year integration of the general circulation model (GCM) experiment performed at the Geophysical Fluid Dynamics Laboratory. The model has relatively high resolution (rhomboidal 30 wavenumbers, 9 vertical levels) and simulates the summertime stationary waves reasonably well. A steady, linear, baroclinic model is used to understand the various forcing mechanisms for the northern summer stationary waves. The linear model response to global diabatic heating is found to play a dominant role in maintaining the summertime stationary waves in the GCM, especially in the subtropics. This response to diabatic heating shows a baroclinic structure in the vertical with a node at about σ = 0.5. On the other hand, stationary nonlinear interaction terms are found to be largely responsible for the extratropical, equivalent barotropic stationary wave features. It is hypothesized that this nonlinear interaction is a result of the thermally induced stationary waves interacting with the local orography. The direct linear response to orography is found to be rather insignificant, however. Transient vorticity and heat fluxes also tend to play a negligible role in explaining the summer stationary wave patterns. Further decomposition of the linear model response to global diabatic heating indicates that the response to the Indian monsoon and the western Pacific heat source is of primary importance in determining the global stationary wave pattern. This large heat source not only determines the stationary flow features locally, but also remotely controls the flow structure over the whole Pacific, North America, and the Atlantic region. Thus, variabilities in the Indian monsoon and the western Pacific heating may exert a strong influence on the global climate variability.