The Role of Equatorial Waves Forced by Convection in the Tropical Semiannual Oscillation

Abstract

Recent satellite observations suggest that convection over the tropical continents is capable of exciting wave motions over a wide range of spatial and temporal scales. An equatorial beta-plane model was used to investigate the forcing by convective heating of equatorial waves with zonal wavenumbers from 1 to 15 and a wide range of periods, including diurnal oscillations. Also studied are the propagation of these waves in the equatorial middle atmosphere and their role in driving the tropical semiannual oscillation (SAO). Specification of the heating distribution used to force the model is guided by observations and analyses of tropical convection. It was found that intermediate-scale Kelvin and inertia?gravity waves provide between 25% and 50% of the forcing necessary to drive the westerly phase of the SAO near the stratopause, while the remainder is supplied by planetary-scale Kelvin waves. In the mesosphere, intermediate-scale waves account for an even larger fraction of the force required to drive the westerly phase and they are solely responsible for driving the easterly phase. The resulting SAO agrees well with ground-based and satellite observations in both the stratosphere and mesosphere. The dependence of the simulated SAO on various model parameters has also been explored. A simulation wherein only planetary-scale waves (k = 1?3) are included yields a weaker than observed stratopause oscillation and fails to produce a mesospheric oscillation. If the full range of zonal wavenumbers (k = 1?15) is included but the diurnal component of the forcing is omitted, the stratopause oscillation is again weaker than observed, while the amplitude of the mesospheric oscillation is greatly diminished. These results suggest that strong excitation of intermediate-scale equatorial waves depends on the diurnal cycle of convection and that the waves thus excited play an important role in the forcing of the tropical semiannual oscillation.