A Theory of Stationary Long Waves. Part II: Resonant Rossby Waves in the Presence of Realistic Vertical Shears

K. K. Tung Centre for Earth Planetary Physics, Harvard University, Camridge, MA 02138

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R. S. Lindzen Centre for Earth Planetary Physics, Harvard University, Camridge, MA 02138

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Abstract

In Part I a simple theory of resonant Rossby waves in a uniform zonal flow was developed. The present paper extends the previous results to the case of an atmosphere with winds varying with height. The wave responses to a large number of physically possible wind configurations are studied to help determine whether the observed wind fields in the winter atmosphere permit resonance of the large-scale waves and, in cases where resonance is possible, to search for the most favorable conditions for resonance. It is found that an increase in stratospheric jet strength and the descent of the stratospheric jet are both capable of exciting the resonant waves of zonal wavenumbers 1 and 2, with the latter (the descent of the stratospheric jet) being most effective in resonating the large-scale waves. The shorter waves (with zonal wavenumbers 3, 4 and up) are found to be insensitive to changes in wind conditions in the stratosphere as they are mostly trapped in the troposphere. These waves are easier to excite by changes in the wind conditions in the lower atmosphere. This finding may account for the higher frequency of occurrence of tropospheric blocking phenomena caused by the shorter waves (wavenumbers 3, 4 and up). The occurrence of large-scale (wavenumbers 1 and 2) blockings is seen to be relatively rare and is found to be usually accompanied by changes in stratospheric wind conditions.

Abstract

In Part I a simple theory of resonant Rossby waves in a uniform zonal flow was developed. The present paper extends the previous results to the case of an atmosphere with winds varying with height. The wave responses to a large number of physically possible wind configurations are studied to help determine whether the observed wind fields in the winter atmosphere permit resonance of the large-scale waves and, in cases where resonance is possible, to search for the most favorable conditions for resonance. It is found that an increase in stratospheric jet strength and the descent of the stratospheric jet are both capable of exciting the resonant waves of zonal wavenumbers 1 and 2, with the latter (the descent of the stratospheric jet) being most effective in resonating the large-scale waves. The shorter waves (with zonal wavenumbers 3, 4 and up) are found to be insensitive to changes in wind conditions in the stratosphere as they are mostly trapped in the troposphere. These waves are easier to excite by changes in the wind conditions in the lower atmosphere. This finding may account for the higher frequency of occurrence of tropospheric blocking phenomena caused by the shorter waves (wavenumbers 3, 4 and up). The occurrence of large-scale (wavenumbers 1 and 2) blockings is seen to be relatively rare and is found to be usually accompanied by changes in stratospheric wind conditions.

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