A Observational Study of Wave-Mean Flow Interaction in the Equatorial Middle Atmosphere.
The evolution of the zonal mean state and the behavior of planetary scale waves in the equatorial middle atmosphere are 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 span the period 25 October 1978 - 28 May 1979 and the pressure range 100-0.05 mb. The following phenomena are described in detail and validated by comparison with rocket profiles: the equatorial semiannual oscillation (SAO), the diurnal tide, Kelvin waves, Rossby waves of midlatitude origin, and vertically layered structures which are interpreted to be manifestations of inertial instability. An SAO warm westerly shear layer descends at about 0.4 cm/s. This is comparable to the dynamically-induced descent required to maintain the warm layer against radiative damping. Inferred downward advection is comparable to observed westerly accelerations. Associated mean meridional circulation cells extend to midlatitudes. Strong meridional shears occur at the base of the SAO westerlies, indicating that the low latitude winter hemisphere is inertially unstable. Inertial acceleration, local or remote Rossby wave driving, or Kelvin wave driving can each act to intensify SAO meridional circulations. SAO easterlies can be explained by Rossby wave driving and thermally-driven meridional advection. However, Kelvin wave driving can account for only 20-50 percent of SAO westerly momentum changes. A source of westerly momentum is required within 25 degrees of the equator. Propagation and absorption of Kelvin wave packets can be described fairly well by the slowly-varying theory of 2-dimensional internal gravity waves. In support of the linear critical line theory of Rossby wave absorption, Rossby wave action flux generally penetrates as far as the zero wind line. Rossby waves appear to excite meridional circulations in local longitude bands during November - mid-January in the equatorial mesosphere, when meridional shear is particularly strong. These quasi-stationary features are centered at the level of maximum easterlies and have 8-14 km vertical wavelengths. They act to restore a stable latitudinal profile of angular momentum.
- Pub Date:
- Physics: Atmospheric Science