10:45 AM - 11:00 AM
[AAS06-06] Common excitation and/or amplification mechanisms of Rossby and Rossby-gravity normal modes revealed by long-term reanalysis data for the whole middle atmosphere
Keywords:middle atmosphere, normal modes
Using the recently available reanalysis dataset JAWARA (Koshin et al., 2025), which covers the whole middle atmosphere over a long period of 19 years from September 2004 to December 2023, four Rossby-gravity normal modes and ten Rossby normal modes have been identified. In the upper mesosphere, the geopotential height (GPH) amplitudes of all Rossby-gravity modes reach a maximum 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 (the 5-day wave), GPH amplitudes exhibit enhancement in the pre-summer solstice and post-summer solstice periods.
The observed enhancement of the s=3 and s=4 Rossby-gravity modes around the summer solstice, as well as that of Rossby modes in the post-summer solstice period, suggests the presence of a common excitation and/or amplification mechanism. During these periods, outward Eliassen-Palm (EP) fluxes originating from a region of positive EP flux divergence are consistently observed near the critical line, within a region characterized by a negative latitudinal gradient of potential vorticity (PV). This pattern strongly suggests that baroclinic and/or barotropic instability plays a key role in the amplification and/or excitation of these modes. In contrast, for the pre-summer solstice period, a different mechanism appears to be responsible for the enhancement of Rossby modes, as strong upward EP fluxes are commonly observed extending from the lower stratosphere to the upper mesosphere on the equatorward side of the region where the latitudinal gradient of PV is negative at high summer latitudes in the mesosphere.
Although the timing of enhancement varies slightly among the normal modes, it consistently aligns with the characteristic distribution of the critical line corresponding to the phase speed of each mode. Previous studies, such as McCormack et al. (2010) and Yamazaki et al. (2021), have analyzed only specific normal modes and proposed similar mechanisms, particularly for enhancements occurring around and in the post-summer solstice period. The present study extends their findings by showing that the enhancement mechanism is common to multiple normal modes. We have also shown that the mechanism responsible for pre-summer solstice enhancements is different but shared among multiple normal modes.
The observed enhancement of the s=3 and s=4 Rossby-gravity modes around the summer solstice, as well as that of Rossby modes in the post-summer solstice period, suggests the presence of a common excitation and/or amplification mechanism. During these periods, outward Eliassen-Palm (EP) fluxes originating from a region of positive EP flux divergence are consistently observed near the critical line, within a region characterized by a negative latitudinal gradient of potential vorticity (PV). This pattern strongly suggests that baroclinic and/or barotropic instability plays a key role in the amplification and/or excitation of these modes. In contrast, for the pre-summer solstice period, a different mechanism appears to be responsible for the enhancement of Rossby modes, as strong upward EP fluxes are commonly observed extending from the lower stratosphere to the upper mesosphere on the equatorward side of the region where the latitudinal gradient of PV is negative at high summer latitudes in the mesosphere.
Although the timing of enhancement varies slightly among the normal modes, it consistently aligns with the characteristic distribution of the critical line corresponding to the phase speed of each mode. Previous studies, such as McCormack et al. (2010) and Yamazaki et al. (2021), have analyzed only specific normal modes and proposed similar mechanisms, particularly for enhancements occurring around and in the post-summer solstice period. The present study extends their findings by showing that the enhancement mechanism is common to multiple normal modes. We have also shown that the mechanism responsible for pre-summer solstice enhancements is different but shared among multiple normal modes.