Cyclone-Scale Forcing of Ultralong Waves

Abstract

A numerical experiment is carried out with a simplified general circulation model. In this experiment, instabilities of all wavelengths are allowed to develop simultaneously from small perturbations on a zonally symmetric flow. The initial development of the ultralong waves in this experiment is apparently forced by the interaction between the cyclone-scale waves and the basic flow in which they are embedded. Because the spectrum of the developing baroclinic waves is not monochromatic, the interaction between the cyclones and the basic flow varies with longitude, and waves longer than the cyclone scale are forced. The structure of the ultralong waves in the numerical experiment is consistent with this forcing mechanism. One implication for numerical weather prediction is that errors in forecasts of ultralong waves may be due in part to errors in the cyclone scale.