ON A GENERATING MECHANISM FOR YANAI WAVES AND THE 25-DAY OSCILLATION

A spectral. Chebyshev-collocation method applied to the linear, 1.5 la yer reduced-gravity ocean model equations is used to study the dynamic s of Yanai (or mixed Rossby-gravity) wave packets. These are of intere st because of the observations of equatorial instability waves (which have the characteristics of Yanai waves) and their role in the momentu m and heat budgets in the tropics. A series of experiments is performe d to investigate the generation of the waves by simple cross-equatoria l wind stress forcings in various configurations and the influence of a western boundary on the waves. They may be generated in the interior ocean as well as from a western boundary. The observations from all t he oceans indicate that the waves have a preferential period and wavel ength of around 25 days and 1000 km respectively. These properties are also seen in the model results and a plausible explanation is provide d as being due to the dispersive properties of Yanai waves.