The 2‐day wave in the middle atmosphere: Observations and theory

Abstract

A recurrent, global‐scale, 2‐day oscillation, in the middle atmosphere is well documented in a variety of data sources: meteor wind, partial reflection, satellite, and rocketsonde. The observed period is almost invariably near 2.1 days, and the zonal structure has been established as a westward propagating, wave number 3 mode. Observations also indicate that the meridional wind component maximizes near the equator and dominates the wind field at low latitudes. These features suggest the disturbance's identification with the wave number 3 Rossby‐gravity normal mode of a windless, isothermal atmosphere. Such modes are found to exist largely independent of the mean fields. The eigenperiods are only modestly altered by mean wind variations on the lowest three scale heights, and the influence on the mode structure is local. The vertical growth of amplitude is enhanced in regions of weak westerly wind relative to the wave and equatorward temperature gradient, while the vertical growth rate is retarded and amplitudes may even decay in regions of strong westerlies or poleward temperature gradient. In realistic mean fields, the third Rossby‐gravity mode has magnified responses very near 2.25 days in both equinox and solstice conditions. The horizontal structure remains largely intact in the lowest levels. However, considerable asymmetry is introduced in the solstice upper stratosphere and mesosphere by the magnified vertical growth of amplitude in the summer hemisphere. The latter results from the moderate easterlies and equatorward temperature gradient characteristic of the region. As for the simple eigenstructure, the meridional wind component in this region maximizes near the equator and dominates the wind field up to mid‐latitudes.