Internal Equatorial Planetary-Scale Waves in Shear Flow

Open Access

Abstract

The theory of internal equatorial Planetary-scale waves as developed by Matsuno and by Lindzen is extended to include the effects of shear in the basic state. By means of a numerical study we find that both the equatorial Kelvin wave (the gravest symmetric westerly mode) and the Yanai wave (the gravest anti- symmetric easterly wave), the two most commonly observed internal equatorial waves, are absorbed shortly before reaching critical levels where their frequencies are Doppler shifted to zero. Away from such levels, these waves retain their identity within shear zones. However, they become more closely confined to the equator as their Doppler-shifted frequencies are reduced. It is found, moreover, for a given frequency and vertical wavelength, that the vertical group velocity of the Yanai wave is smaller than that of the Kelvin wave. In addition, as the Doppler-shifted frequency ωˆ is reduced, the vertical group velocity of the Yanai wave diminishes as ωˆ³ while the vertical group velocity of Kelvin wave diminishes only as ωˆ². As a result of these two features, Yanai waves ten dto be more significantly affected by dissipation than are Kelvin waves.

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