UC Davis

Zonally long tropicalwaves in the presence of a large-scalemeridional and vertical overturning circulation are studied in an idealized model based on the intraseasonal multiscalemoist dynamics (IMMD) theory. Themodel consists of a systemof shallow-water equations describing barotropic and first baroclinic vertical modes coupled to one another by the zonally symmetric, time-independent background circulation. To isolate the effects of the meridional circulation alone, an idealized background flow is chosen to mimic the meridional and vertical components of the flow of theHadley cell; the background flowmeridionally converges and rises at the equator. The resulting linear eigenvalue problem is a generalization of the long-wave-scaled version of Matsuno's equatorial wave problem with the addition of meridional and vertical advection. The results demonstrate that the meridional circulation couples equatorially trapped baroclinic Rossby waves to planetary, barotropic free Rossby waves. The meridional circulation also causes the Kelvin wave to develop an equatorially trapped barotropic component, imparting a westward-tilted vertical structure to the wave. The total energy of the linear systemis positive definite, so all waves are shown to be neutrally stable.Acritical layer exists at latitudes where themeridional background flowvanishes, resulting in aminimumfrequency cutoff for physically feasible waves. Therefore, linear Matsuno waves with periods longer than the vertical transport time of the meridional circulation do not exist in the equatorial waveguide. This implies a low-frequency cutoff for long equatorial waves.