THE 4-DAY WAVE IN THE STRATOSPHERE AND MESOSPHERE AS OBSERVED FROM THE UPPER ATMOSPHERE RESEARCH SATELLITE MICROWAVE SOUNDER


D. R. Allen and J. L. Stanford, Department of Physics and Astronomy, Iowa State University, Ames, IA

L. S. Elson, E. F. Fishbein, L. Froidevaux, and J. W. Waters, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA


J. Atmos. Sci., 54, 420-434 (1997).


Abstract

The 4-day wave is a quasi-nondispersive feature which occurs near the winter polar stratopause, with eastward propagating zonal wavenumbers 1 through at least 4, all moving with the same phase speed. The feature has been documented in temperature, geopotential height, wind, and ozone. Synoptic temperature plots show the feature as a ``warm pool'' of air rotating eastward with period near four days. Theoretical studies show that barotropic (and sometimes baroclinic) instability of the polar night jet can produce quasi-nondispersive modes similar to the observed 4-day wave. The present paper presents evidence of the 4-day feature in MLS temperature, geopotential height, and ozone data from August and September, 1992 and 1993. Space-time spectral analyses and filteredsynoptic plots of temperature as a function of pressure and longitude reveal a meridional temperature structure consisting of two peaks, one near the stratopause and one in the lower mesosphere, with an out-of-phase relationship between the two peaks. These characteristics match those recently predicted theoretically by Manney and Randel (1993). The 4-day signal is also found in MLS ozone data. Negative regions of quasi-geostrophic potential vorticity gradient are shown to exist during times of 4-day wave growth, consist ent with instability dynamics playing a role in the developing stages of the feature. Spectral plots of quasi-geostrophic potential vorticity derived from MLS geopotential height fields reveal a 4-day signal peaking near the polar stratopause. The three-dimensional structure resembles the potential vorticity ``charge'' concept discussed recently by Bishop and Thorpe (1994).