Tropical Intraseasonal Oscilations Appearing in a GFDL General Circulation Model and FGGE Data. Part I: Phase Propagation

Abstract

Space-time spectrum and filter analyses are made of the tropical intraseasonal osciliations in the northern summer appearing in a GFDL 30-wavenumber spectral general circulation model and the FGGE IIIB data. The model exhibits major and minor wavenumber 1 spectral peaks in the equatorial zonal velocity at eastward moving periods of 40–50 and 25–30 day oscillations are associated with a similar spatial structure. In particular, both of these oscillations exhibit a phase reversal between the 200 and 800 mb zonal velocities. They propagate eastward with a node near the dateline and an antinode in the western hemisphere. Their wave patterns take the form of an eastward-moving Kelvin mode near the equator and an eastward-moving Rossby mode away from the equator. Both spectral peaks are also detectable in the model's precipitation, corresponding to those in the observed outgoing longwave radiation. The 40–50 and 25–30 day precipitation oscillations are in phase with the vertical velocity and propagate northeastward with major and minor antinodes in the eastern and western hemispheres respectively. The present results demonstrate that the intraseasonal oscillations can be simulated in a model without air-sea interactions and cloud-radiation feedbacks.