Department of Physics & Astronomy, Iowa State University, Ames, Iowa 50011
Chapter in RECENT ADVANCES IN STRATOSPHERIC DYNAMICS, T. R. Nathan and E. C. Cordero, Eds., Research Trends (1999), pp. 93-112..
The "4-day wave" is a ubiquitous, eastward moving quasi-nondispersive feature located in the polar winter stratosphere and mesosphere. The phenomenon is observed in meteorological satellite data as relatively warm and cold pools of air, one atop the other and with associated wind vortices, circling the winter pole at high altitudes with period near 4 days. This paper reviews the considerable amount of accumulated observational and theoretical evidence for the 4-day wave, and describes its characteristics. The phenomenon is attributed to a form of instability resulting from strong gradients in the winter polar jet stream of the stratosphere and mesosphere. The 3-D structure of the 4-day wave may be understood with the paradigm of potential vorticity "charge". Temperature and wind anomalies are induced by the potential vorticity charge, itself generated by fluid instabilities associated with strong polar night jet wind shear. The paradigm offers an elegant and conceptually satisfying explanation of 4-day wave physics. The 4-day wave modifies the climatology of the polar winter stratopause region, and furthermore provides an in situ source for mixing of chemical constituents in the otherwise isolated polar night middle atmosphere vortex. Implications of the latter for atmospheric chemistry remain to be fully explored.