ESTIMATES OF MOMENTUM FLUX ASSOCIATED WITH EQUATORIAL KELVIN AND GRAVITY-WAVES

A new indirect method is proposed to estimate momentum flux based on t he theory of slowly varying gravity waves and equatorial waves in vert ical shear by Dunkerton [this issue] which explains the discovery by S ate et al. [1994] that the cospectra of temperature and zonal wind flu ctuations at Singapore (1.4 degrees N, 104.0 degrees E) are synchroniz ed with the quasi-biennial oscillation (QBO) of mean zonal wind in the stratosphere. The indirect estimates obtained from cospectra correspo nd to the summation of absolute values of momentum flux; associated wi th each wave, whereas direct estimates from quadrature spectra give th e summation of momentum flux. An analysis was made for twice daily raw insonde data at Singapore, The direct estimate for Kelvin waves (5-20 day components) is 2-9 x 10(-3) m(2) s(-2) and accords with the indire ct estimate to within the estimation error. This result supports the v alidity of the indirect method. Although the indirect estimate depends on an assumed wave structure, large values of momentum flux are obtai ned for all possible equatorial modes having short periods (1-3 days). The indirect estimate for westerly shear is 20-60 x 10(-3) m(2) s(-2) based on the theory of two-dimensional gravity waves, while the direc t estimate is only 0-4 x 10(-3) m(2) s(-2). The reduction of indirect estimate under the assumption of equatorial waves is about 30-70%. The discrepancy between direct and indirect estimates indicates a large c ancelation of positive and negative momentum fluxes. This is the case also for easterly shear. The indirect estimate for westerly shear is a lmost twice as large as that for easterly shear. The characteristics o f waves near the source in the troposphere are thought to be independe nt of the QBO in the stratosphere, so that the difference in wave acti vity should be attributed to the differing characteristics of wave pro pagation under the strong QBO shear. Several possible explanations are discussed. Parameters such as phase velocity and zonal wavelength are estimated from the ratio of potential to kinetic energies assuming th at the 1-3 day components are due to equatorial waves. The estimates i n this paper were made assuming that the observed frequencies are actu al ground-based wave frequencies. If there is aliasing from higher fre quencies than 1 day, the actual momentum fluxes can be significantly l arger than the estimated values.