Dynamics of the Stratospheric Semiannual Oscillation

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Abstract
This paper reports on an attempt to use observations to determine the nature of the eddy contributions to the mean zonal momentum balance in the region of the tropical stratosphere that is dominated by the semiannual oscillation (SAO). Since direct observations of the eddy wind fields in this region of the atmosphere are very limited, an indirect procedure was employed. The first step in this process was the computation of diabatic heating rates using observed temperatures and a sophisticated radiative transfer code. These heating rates were then employed to calculate the residual mean meridional circulation. The advection of mean zonal momentum and the Coriolis torque associated with the residual circulation could then be computed. The difference between this contribution to the zonal mean momentum balance and the actual observed acceleration of the zonally-averaged zonal wind was ascribed to the Eliassen-Palm (EP) flux convergence associated with eddies of all types. Meridional profiles of the inferred EP flux convergence were produced for each month of the year at the 1.0 and 0.4mb levels. At both levels there was an indication of the presence of an equatorially-trapped westerly contribution to the EP flux convergence. This is consistent with the suggestion of Hirota (1978) that the westerly accelerations in the SAO are provided by a dissipating equatorial Kelvin wave. In fact at 1mb the total EP flux convergence at the equator is always westerly; this suggests that the dominant contribution to the easterly acceleration near the equator comes from advection by the residual circulation, in accord with the model results of Holton and Wehrbein (1980) and Mahlman and Sinclair (1980). However, there is an indication of significant easterly eddy forcing of the mean flow away from the equator in the winter hemisphere (and in both hemispheres near the equinoxes). This forcing is presumably due to the equatorward propagation of planetary waves generated in the extratropics in the manner discussed by Hirota (1976, 1978, 1979). This easterly eddy forcing becomes more significant at the 0.4mb level, and the total EP flux convergence on the equator becomes easterly during the easterly acceleration phase of the SAO. It thus appears that both the Holton-Wehrbein (1980) and the Hirota (1978, 1979) mechanisms are important for the generation of the easterly accelerations in the SAO. The planetary wave contribution is most significant in the winter hemisphere and at higher levels, and the contribution from the residual circulation predominates in the summer hemisphere and at lower levels.