Two‐Dimensional Model Studies of Equatorial Dynamics and Tracer Distributions

A parametrization of Kelvin wave absorption in the equatorial stratosphere has been included in a two‐dimensional model of the atmosphere in order to improve the simulation of the semi‐annual oscillation. Additionally, a more detailed treatment of the momentum deposition associated with gravity wave breaking in the mesosphere has replaced the previous Rayleigh friction scheme. the model exhibits a semi‐annual reversal of the zonal winds at the equator in the stratosphere, with the westerly phase descending with time, as observed. the inclusion of the improved gravity wave parametrization has led to a number of significant improvements; in particular, the model also exhibits a semi‐annual oscillation in the mesosphere which is out of phase with that in the stratosphere. Meridional circulations associated with the westerly phase of the semiannual oscillation produce ‘double peak’ structures in a number of modelled tracer fields, including N2O, CH4 and H2O which compare well with observations by the SAMS and LIMS satellite experiments. These structures consist of a distinct minimum at the equator and double maxima in the subtropics; they are present for several months around each equinox. Inclusion of the more realistic westerly forcing has pointed to the need for additional easterly forcing in the model around each solstice. Further supporting evidence for the importance of momentum deposition associated with planetary waves in the easterly phase of the semi‐annual oscillation comes from the difference in double peaks between the northern hemisphere spring and autumn, as suggested in an earlier paper.