THE 4-DAY WAVE AS OBSERVED FROM THE UPPER-ATMOSPHERE RESEARCH SATELLITE MICROWAVE LIMB SOUNDER

The ''4-day wave'' is an eastward moving quasi-nondispersive feature w ith period near 4 days occurring near the winter polar stratopause. Th is paper presents evidence of the 4-day feature in Microwave Limb Soun der (MLS) temperature, geopotential height, and ozone data from the la te southern winters of 1992 and 1993. Spacetime spectral analyses reve al a double-peaked temperature structure consisting of one peak near t he stratopause and another in the lower mesosphere, with an out-of-pha se relationship between the two peaks. This double-peaked structure is reminiscent of recent three-dimensional barotropic/baroclinic instabi lity model predictions and is observed here for the first time. The he ight variation of the 4-day ozone signal is shown to compare well with a linear advective-photochemical tracer model. Negative regions of qu asigeostrophic potential vorticity (PV) gradient and positive Eliassen -Palm flux divergence are shown to occur, consistent with instability dynamics playing a role in wave forcing. Spectral analyses of PV deriv ed from MLS geopotential height fields reveal a 4-day signal peaking n ear the polar stratopause. The three-dimensional structure of the 4-da y wave resembles the potential vorticity ''charge'' concept, wherein a PV anomaly in the atmosphere (analogous to an electrical charge in a dielectric material) induces a geopotential field, a vertically orient ed temperature dipole, and circulation about the vertical axis.