The quasi-two-day wave in a middle atmosphere GCM

The quasi-two-day wave is a planetary-scale disturbance in the upper stratosphere and mesosphere. It occurs in both hemispheres throughout the year, although its amplitude peaks during solstice in the summer hemisphere. Its period is approximately two days but may vary between 44 and 56 hours. It is dominated by a zonal wavenumber 3 component, although a strong wavenumber 4 component is often present, and in some cases wavenumbers 2-5 have been observed.

This paper will examine the quasi-two-day wave in the upward extension of the Canadian Middle Atmosphere Model (CMAM). Characteristics such as the inter-seasonal and inter-annual dependence on wavenumber, frequency and amplitude will be compared with observations, and with other middle atmospheric models. The generation mechanism for the two-day wave in the CMAM will also be discussed. The two-day wave has been explained as a normal mode of the atmosphere, and also as a baroclinic instability of the shear zone at the top of the summer hemisphere jet. The extent to which these arguments apply to the two-day wave in the CMAM will be discussed.