Evolution of the Zonal Mean State in the Equatorial Middle Atmosphere during October 1978-May 1979

Abstract

The evolution of the zonal mean state in the equatorial middle atmosphere is investigated with the use of daily mapped temperatures derived from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. These quasi-global, high vertical resolution data cover the pressure range 100-05 mb and the period 25 October 1978-28 May 1979. The equatorial semiannual oscillation (SAO) in zonal mean temperature, derived zonal wind and meridional shear of the zonal wind is described in detail. Rocket profiles are used to validate features seen in LIMS data. These include ranges in temperature and zonal wind of 20 K and 100 m s?1, and cross-equatorial shears of at 3 day?1 Consistent with the theory that the wave-mean flow interaction is essential to the SAO, flow acclerations over the equator exhibit strong week-to-week variations. While easterly accelerations are moderate and occur in deep cool layers, westerly accelerations are generally stronger and occur in shallow warm layers which descend with time at a mean rate of about 0.3 cm s?1. A detailed heating algorithm is used to estimate residual circulations. Wave-driven residual mean circulation cells associated with the SAO are found to extend well into midlatitudes their latitudinal scale expanding from December through February as newly formed SAO westerlies descend in the lower mesosphere. In the descending branch of the SAO circulation over the equator, estimated downward advection is very similar to observed westerly acceleration in pattern and magnitude. Cross-equatorial shear and mean meridional wind both maximize beneath the descending zero wind line, which is also the level of maximum penetration of easterlies into the winter hemisphere. Inertial instability may enhance meridional circulation especially during November through mid-January in the lower mesosphere. The vertical distribution of wave driving over the equator, inferred as a residual in the zonal momentum equation, is compatible with expectations from gravity wave theory.