Simulation of the semi‐annual oscillation of the equatorial middle atmosphere using the Extended UGAMP General Circulation Model

The semi‐annual oscillation (SAO) is examined in two runs of a middle atmosphere general circulation model. The first (run PGW) includes a parametrization of gravity waves with zero phase speed only, whilst the second (run AGW) also includes gravity waves with phase speeds of 20 m s−1. Run PGW successfully simulates equatorial SAOs near the stratopause and in the mid to upper mesosphere. However, the zonal winds at all heights are easterly and the amplitude of the oscillation is too weak by up to 15 m s−1 near the stratopause and too strong by about 45 m s−1 in the upper mesosphere. The corresponding SAOs in run AGW, however, have amplitudes that are within 5 m s−1 of those observed by rocketsondes, and have a pattern of easterly and westerly winds that are in reasonable agreement with the observations. Zonal mean momentum‐budget studies show that both at the stratopause and in the mid to upper mesosphere, the westerly phase of the oscillation in run AGW is stronger than in run PGW owing to an increase in total eddy forcing associated with the introduction of the 20 m s−1 phase speed gravity wave. The easterly phase of the stratopause oscillation on the summer side of the equator is driven by advection of easterlies across the equator. However, on the winter side of the equator, the net zonal forcing is westerly in May but easterly in November, since easterly eddy forcing associated with extratropical planetary waves is stronger in November than in May. The easterly phase of the mid to upper mesosphere oscillation is forced by contributions from both the residual circulation and eddy forcing. The eddy‐forcing contribution is more easterly than in other modelling studies owing to weaker Kelvin‐wave activity and to easterly forcing associated with model diurnal tides. Introduction of the 20 m s−1 gravity waves also improves the simulation of an N2O‐like tracer. In run AGW a ‘double peak’ of tracer is present in April, in reasonable agreement with observations, but this feature is absent in run PGW.