A Mechanism for Excitation of Ultralong Rossby Waves

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Abstract
Ultralong Rossby waves (low zonal and meridional wavenumbers on the sphere) have been studied for many years. Extensive observational evidence tends to identify these waves with the gravest normal modes of the atmosphere. Due to dissipative processes, these disturbances have to be generated by some forcing mechanism, even though they have phase speeds consistent with free oscillations. In this paper, we consider a mechanism for the excitation of these waves based on temporal changes of the zonal wind in the tropics, in the presence oforography and stationary thermal foming. The waves are excited as a consequence of the adjustment of the quasi-stationary component to the new background configuration. A barotropic calculation is carried out in order to test the mechanism in a minimal model resolving ultralong Rossby waves, namely, the shallow-water equations over the sphere, including topographic and thermal forcing. Even in this simplified model, the predominance of the 16-day wave is suggested. Our results indicate that thermal foming may be more important than orography, but the limitations of the model do not allow one to assess the relative contributions in the atmosphere with any meaningful accuracy. The proposed mechanism is quite general and may be tested in more sophisticated models, including effects such as baroclinicity and a more realistic representation of forcing and dissipation.

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