Nonlinear Forcing of Planetary Scale Waves by Amplifying Unstable Baroclinic Eddies Generated in the Troposphere

Abstract

A global distribution of amplifying intermediate scale baroclinic modes can cause growth of planetary scale modes through wave-wave coupling, such that the growth of the long waves is comparable to the growth of the most unstable baroclinic modes. A global, spectral, primitive equation circulation model is used to study this phenomenon for two initial mean zonal wind fields having idealized meridional distributions, but realistic vertical variations. For the cases treated here, both the direct transfer of kinetic energy from intermediate scales of motion, and baroclinic conversion of available potential energy to kinetic energy within a given planetary wave, are important. It appears that direct kinetic energy transfer to the long waves can be inhibited by heat transport. Planetary wave amplitudes are considerably enhanced by wave-wave coupling over what they would be due to baroclinic instability of the zonally averaged state to planetary wave disturbances. Nonlinear forcing in the troposphere may be an important mechanism for generating eastward propagating planetary waves observed in the Southern Hemisphere. The planetary wave forcing terms have relative maxima near the tropopause. Such nonlinear forcing near the tropopause may explain the lack of coherence between the troposphere and stratosphere observed in eastward traveling waves with zonal wavenumbers 1 and 2.