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Vacillations Induced by Interference of Stationary and Traveling Planetary Waves

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  • 1 Department of Astrophysical, Planetary, and Atmospheric Sciences, University of Colorado, Boulder, CO 80309
  • | 2 National Center for Atmospheric Research, Boulder, CO 80307
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Abstract

We explore the interference Pattern Produced when a traveling planetary wave propagates over a stationary forced wave. The interference signature is examined in a variety of diagnostics, ranging from instantaneous local wave amplitudes to cross sections of Eliassen–Palm flux and synoptic maps of Ertel potential vorticity. Results capture the salient characteristics of quasi-Periodic disturbances in the stratosphere reported by Madden and others.

The interference process results in a modulation of all the transport Properties of the stationary wave, even if the traveling component is purely barotropic as is typical of transient planetary waves in the troposphere and lower stratosphere. Locally, the Eliassen–Palm flux involves a transient vector which orbits about the time-mean component, causing the instantaneous flux of wave activity to vary in both magnitude and direction. For stationary and traveling waves of comparable amplitude, the EP flux vector F can readily be driven through zero, completely altering its strength and direction. This temporal fluctuation actually arises out of the spatial modulation of the wave field and the migration of the resulting pattern with time. In this manner, the steady uniform stream of wave activity associated with the stationary wave is organized into a series of capsules or wavepackets which propagate upward and equatorward. Consequently, the signature at a particular location emerges as a series of bursts in wave activity.

Because of rising values of F at the leading edge of each of these wavepackets and opposite behavior at the trailing edge, the mean flow experiences an alternating succession of eddy forcing. This gives rise to a fluctuating response in the zonal-mean which, under typical amplitudes, may be considerable. Greatest influence is exerted in the polar stratosphere due to the veering of F to and away from the pole. Convergence of meridians at high latitudes leads to repeated focusing and spreading of wave activity and thereby a magnified response in the mean flow. For amplitudes representative of January 1979, a mean wind reversal occurs in the polar stratosphere and mesosphere, attended by substantial warming over the polar cap and out-of-phase behavior in the tropics and mesosphere.

The synoptic signature of wavenumber 1 interference consists of two basic elements: (i) displacement and wobbling of the vortex about the pole, as a ridge builds in from below; (ii) distortion of the vortex into a comma-like shape, its axis spiraling anticyclonically and equatorward about the ridge. Both features are widely documented in observations. They are introduced here by the eddy field as a capsule of wave activity propagates through a given level. Potential vorticity on isentropic surfaces exhibits similar but exaggerated behavior. The results suggest an alternate, perhaps complementary, interpretation to planetary wave breaking of the evolution of potential vorticity during January 1979.

Abstract

We explore the interference Pattern Produced when a traveling planetary wave propagates over a stationary forced wave. The interference signature is examined in a variety of diagnostics, ranging from instantaneous local wave amplitudes to cross sections of Eliassen–Palm flux and synoptic maps of Ertel potential vorticity. Results capture the salient characteristics of quasi-Periodic disturbances in the stratosphere reported by Madden and others.

The interference process results in a modulation of all the transport Properties of the stationary wave, even if the traveling component is purely barotropic as is typical of transient planetary waves in the troposphere and lower stratosphere. Locally, the Eliassen–Palm flux involves a transient vector which orbits about the time-mean component, causing the instantaneous flux of wave activity to vary in both magnitude and direction. For stationary and traveling waves of comparable amplitude, the EP flux vector F can readily be driven through zero, completely altering its strength and direction. This temporal fluctuation actually arises out of the spatial modulation of the wave field and the migration of the resulting pattern with time. In this manner, the steady uniform stream of wave activity associated with the stationary wave is organized into a series of capsules or wavepackets which propagate upward and equatorward. Consequently, the signature at a particular location emerges as a series of bursts in wave activity.

Because of rising values of F at the leading edge of each of these wavepackets and opposite behavior at the trailing edge, the mean flow experiences an alternating succession of eddy forcing. This gives rise to a fluctuating response in the zonal-mean which, under typical amplitudes, may be considerable. Greatest influence is exerted in the polar stratosphere due to the veering of F to and away from the pole. Convergence of meridians at high latitudes leads to repeated focusing and spreading of wave activity and thereby a magnified response in the mean flow. For amplitudes representative of January 1979, a mean wind reversal occurs in the polar stratosphere and mesosphere, attended by substantial warming over the polar cap and out-of-phase behavior in the tropics and mesosphere.

The synoptic signature of wavenumber 1 interference consists of two basic elements: (i) displacement and wobbling of the vortex about the pole, as a ridge builds in from below; (ii) distortion of the vortex into a comma-like shape, its axis spiraling anticyclonically and equatorward about the ridge. Both features are widely documented in observations. They are introduced here by the eddy field as a capsule of wave activity propagates through a given level. Potential vorticity on isentropic surfaces exhibits similar but exaggerated behavior. The results suggest an alternate, perhaps complementary, interpretation to planetary wave breaking of the evolution of potential vorticity during January 1979.

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