Tropical Response to Lateral Forcing with a Latitudinally and Zonally Nonuniform Basic State

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  • 1 Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801
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Abstract

The tropical response to a lateral forcing is investigated with a quasi-linear numerical model of the tropical atmosphere that has a basic flow consisting of a steady planetary wave as well as a zonal shear flow. The basic flow is prescribed in accordance with the observed seasonal or annual mean flow at the 200 mb level in the tropics. The forcing at a middle latitude boundary is formulated in terms of a single incoming wave with known self-compatible parametric values. Each of the induced waves is required to satisfy the radiation condition at both boundaries. A moderately truncated set of governing equations is found to be adequate for accurately describing the forced motions. It is shown that the zonal shear flow can give rise to a favorable condition for trapping wave modes in an equatorial zone between two turning points, provided that the parameters of the wave modes have appropriate values. In conjunction with this, the interaction between the basic wave and the forcing wave may provide a dynamical mechanism of exciting such trapped waves in situ. AS a result, quasi-resonant modes with readily identifiable structure are found over a wide range of parametric conditions on the forcing wave and the basic state. This mechanism of exciting equatorially trapped wave motions has an intrinsic scale selection and is believed to be particularly relevant to the mixed Rossby-gravity waves in the atmosphere.

Abstract

The tropical response to a lateral forcing is investigated with a quasi-linear numerical model of the tropical atmosphere that has a basic flow consisting of a steady planetary wave as well as a zonal shear flow. The basic flow is prescribed in accordance with the observed seasonal or annual mean flow at the 200 mb level in the tropics. The forcing at a middle latitude boundary is formulated in terms of a single incoming wave with known self-compatible parametric values. Each of the induced waves is required to satisfy the radiation condition at both boundaries. A moderately truncated set of governing equations is found to be adequate for accurately describing the forced motions. It is shown that the zonal shear flow can give rise to a favorable condition for trapping wave modes in an equatorial zone between two turning points, provided that the parameters of the wave modes have appropriate values. In conjunction with this, the interaction between the basic wave and the forcing wave may provide a dynamical mechanism of exciting such trapped waves in situ. AS a result, quasi-resonant modes with readily identifiable structure are found over a wide range of parametric conditions on the forcing wave and the basic state. This mechanism of exciting equatorially trapped wave motions has an intrinsic scale selection and is believed to be particularly relevant to the mixed Rossby-gravity waves in the atmosphere.

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