Common Excitation and/or Amplification Mechanisms of Rossby and Rossby‐Gravity Normal Modes Revealed by Long‐Term Reanalysis Data for the Whole Middle Atmosphere

Abstract Using the recently available reanalysis data set JAWARA, which covers the whole middle atmosphere over a long period of 19 years from September 2004 to December 2023, four Rossby‐gravity and 10 Rossby normal modes have been identified. In the upper mesosphere, the geopotential height (GPH) amplitudes of all Rossby‐gravity modes are maximized near the summer solstice except for the zonal wavenumber s = 1 Rossby‐gravity mode. For all Rossby modes, except for the s = 1 first symmetric Rossby mode, GPH amplitudes exhibit enhancement in the pre‐ and post‐summer solstice periods. It is inferred that the enhancements of the s = 3 and s = 4 Rossby‐gravity modes around the summer solstice as well as those of Rossby modes in the post‐summer solstice share a common feature: Eliassen‐Palm (EP) flux divergence is positive near the critical line over the region with a negative latitudinal gradient of potential vorticity . This feature strongly suggests that dynamical instability commonly plays a key role in the excitation and/or amplification of these modes. In contrast, characteristics of the Rossby modes in the pre‐summer solstice suggest a common but different mechanism from the above‐described acting to their enhancement: strong upward EP fluxes are observed from the middle stratosphere to the upper mesosphere on the equatorward side of the negative region at high summer latitudes in the mesosphere. Moreover, the slight difference in the enhancement timing of individual normal modes is probably attributable to that of the phase speed.