Stratospheric and Mesospheric Kelvin Waves Simulated by the GFDL “SKYHI” General Circulation Model

Abstract

A space-time spectral analysis is made of large-scale equatorial disturbances simulated by the 40-level, 5° latitude GFDL “SKYHI” general circulation model with annual mean conditions. Three kinds of eastward moving waves with wavenumbers 1–2 are found in the lower and upper stratosphere and mesosphere. These waves are characterized by small meridional winds and an eastward tilt with height and are identifiable with observed Kelvin waves. A time-height section reveals their vertical group propagation. The lower stratospheric Kelvin wave is associated with periods of 10–30 days (eastward phase speed 15–46 m s⁻¹) for wavenumber 1 and a vertical wavelength of ∼10 km, corresponding to that observed in 1968 by Wallace and Kousky. The upper stratospheric Kelvin wave is associated with periods of 5–7 days (66–92 m s⁻¹) for wavenumber 1 and a vertical wavelength of ∼20 km, corresponding to that observed by Hirota. The mesospheric Kelvin wave is associated with periods of 3–4 days (115–154 m s⁻¹) for wavenumber 1 and a vertical wavelength of ∼40 km, corresponding to that recently discovered by Salby and others. All these Kelvin waves transport energy and eastward momentum upward and contribute to the maintenance of the eastward flow. In addition, gravity waves of zonal wavenumbers 1–30 and periods of 0.7–2 days have been found, particularly in the model's equatorial stratosphere and mesosphere. Their eastward and westward moving components transport eastward and westward momentum upward and contribute to the momentum balance as much as, or even more than, Kelvin waves with periods longer than two days.