Intraseasonal oscillations in the middle atmosphere forced by gravity waves

In our Numerical Spectral Model (NSM) that incorporates Hines’ Doppler Spread Parameterization, small-scale gravity waves (GW) propagating in the east/west direction can generate the essential features of the observed equatorial oscillations in the zonal circulation and in particular the QBO extending from the stratosphere into the upper mesosphere. We report here that the NSM also produces in the zonally symmetric (wavenumber=0) meridional circulation intraseasonal oscillations on time scales of a few months. A distinct but variable meridional wind oscillation (MWO) is generated, which appears to be the counterpart to the quasi-biennial oscillation (QBO). With a vertical grid-point resolution of about 0.5 km , the NSM produces the MWO through momentum deposition of GWs propagating in the north/south direction. The resulting momentum source represents a non-linear function of the meridional winds that is of third (generally odd) order, and this enables the oscillation, as is the case for the zonal winds in the QBO. Since the meridional winds are relatively small compared to the zonal winds, however, the vertical scale-length for the MWO is much smaller, i.e., only about 10 km instead of 40 km for the QBO. Consistent with the associated increase of the viscous stress, the period of the MWO is then short compared with that of the QBO, i.e., only about 1–4 months. Measurements with the HRDI instrument on UARS show evidence of the MWO with periods between 2 and 3 months (Huang and Reber, J. Geophys. Res. (2003)). Depending on the strength of the GW forcing, the computed amplitudes of the MWO are typically 5 m/ s in the upper stratosphere and mesosphere, and the associated temperature amplitudes are about 3 K . These amplitudes may be observable with the instruments on the TIMED spacecraft. Extended computer simulations with the NSM in 2D and 3D reveal that the MWO is modulated by and in turn influences the QBO.