Nonlinear Stationary Wave Maintenance and Seasonal Cycle in the GFDL R30 GCM

Abstract

In this study, the climatological stationary wave maintenance is examined from nonlinear perspective using the GFDL R30 GCM outputs, a fully nonlinear stationary wave model, and a linear stationary wave model. The primary focus of the study is on the nature of the stationary nonlinearity and relative contribution to the total nonlinearity by various factors, such as heating, orography, and the interaction between flows forced by heating and orography. It is found that both the nonlinear effect of the diabatic heating and the nonlinear interaction between flows forced by orography and diabatic heating are important contributors toward the total stationary nonlinearity in northern winter and summer. Some regional features, such as the anticyclone off the northwest coast of North America in winter and the southwestern U.S. summer anticyclone, are entirely due to the nonlinear interaction between flows forced by heating and orography. Consistent with the linear stationary wave maintenance, the diabatic heating is the most dominant forcing mechanism in the Tropics and the Southern Hemisphere (SH) throughout the seasonal cycle in the nonlinear framework. Over the Northern Hemisphere (NH) extratropics during northern winter, however, the role of the orographic forcing is comparable to that of diabatic heating due to its strong nonlinear interaction with flows forced by heating and transients. This contrasts significantly with the conclusion drawn from the direct nonlinear responses in which the orography is much less important than the diabatic heating. The regional feature of the ridge over northwestern North America in northern winter is found to be largely due to the presence of orography. The effect of transients in the nonlinear model, including the nonlinear interaction of transients with flows forced by heating and orography, shows a wave train over the Pacific?North American region (PNA) that resembles the atmospheric response to El Niño. This differs considerably from that in the linear view as well as that of the direct nonlinear response to transients. Furthermore, it is found that the inclusion of orography or transients in the total stationary wave forcing improves the spatial pattern simulation of the GCM stationary waves for both hemispheres in their respective winter months.