Estimation of the Kelvin Wave Contribution to the Semiannual oscillation

Mathew H. Hitchman Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Conway B. Leovy Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

The behavior of Kelvin waves in the equatorial middle atmosphere is investigated with the use of daily mapped temperature derived from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. Diagrams of wave activity per unit mass and wave activity flux density concisely illustrate bulk properties of Kelvin waves and facilitate tracing of packets to source times near the tropopause. Kelvin wave packets of different zonal wavenumbers propagate separately and appear to be forced separately. During the LIMS data period (25 October 1978–28 May 1979) two Kelvin wave regimes are found. Packets of wave one, wave two, or wave three Kelvin waves occur at irregular intervals prior to April. During April and May a nearly continuous upward flux of wave one activity dominates.

For very tall Kelvin waves the observed dependence of vertical wavelength on zonal wind is weaker than predicted by the slowly-varying theory for internal gravity waves. However, most properties of the observed waves are consistent with slowly-varying theory, and the zonal mean body force per unit mass due to Kelvin waves is estimated from observed temperatures and application of the WKBJ approximation. Both a flux convergence and radiative damping formulation yield westerly wave driving which is smaller than that required to satisfy the zonal momentum budget. A comparison of the residual of terms in the zonal momentum equation, estimated from LMS data, with gravity wave driving, estimated by Lindzen's breaking parameterization, suggests that gravity waves may contribute significantly to the equatorial stratopause semiannual oscillation.

Abstract

The behavior of Kelvin waves in the equatorial middle atmosphere is investigated with the use of daily mapped temperature derived from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. Diagrams of wave activity per unit mass and wave activity flux density concisely illustrate bulk properties of Kelvin waves and facilitate tracing of packets to source times near the tropopause. Kelvin wave packets of different zonal wavenumbers propagate separately and appear to be forced separately. During the LIMS data period (25 October 1978–28 May 1979) two Kelvin wave regimes are found. Packets of wave one, wave two, or wave three Kelvin waves occur at irregular intervals prior to April. During April and May a nearly continuous upward flux of wave one activity dominates.

For very tall Kelvin waves the observed dependence of vertical wavelength on zonal wind is weaker than predicted by the slowly-varying theory for internal gravity waves. However, most properties of the observed waves are consistent with slowly-varying theory, and the zonal mean body force per unit mass due to Kelvin waves is estimated from observed temperatures and application of the WKBJ approximation. Both a flux convergence and radiative damping formulation yield westerly wave driving which is smaller than that required to satisfy the zonal momentum budget. A comparison of the residual of terms in the zonal momentum equation, estimated from LMS data, with gravity wave driving, estimated by Lindzen's breaking parameterization, suggests that gravity waves may contribute significantly to the equatorial stratopause semiannual oscillation.

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