Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsa51b..05s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SA51B-05
Other
3334 Middle Atmosphere Dynamics (0341, 0342)
Scientific paper
The 2-day wave is a prominent feature of the middle and upper atmosphere, amplifying twice-yearly around solstice. Its period, structure, and reproducibility have led to its association with the gravest planetary normal mode of zonal wavenumber 3, the so called Rossby-gravity mode. On the other hand, its amplification around solstice has also led to its association with baroclinic instability of summer easterlies.To explore the relationship between the Rossby-gravity mode and instability, calculations have been performed with the linearized primitive equations which have been generalized to account for that mode's interaction with a generally-unstable mean flow. The mode's eigenfrequency is then complex, the imaginary component representing amplificationand decay. The behavior recovered, including structural differences between solstice and equinox, is consistent with major features of the 2-day wave. Under solstitial conditions, the Rossby-gravity mode amplifies by extracting energy from the mean flow, with e-folding times as short as 5 days. Even though its eigenfrequency is then complex, the mode's period remains close to the theoretical value, consistently lying at westward periods of 2.0 -- 2.2 days. Equally robust is its eigenstructure, which extends into both hemispheres. It mirrors the modal structure isolated earlier in the response over real frequency.In contrast, the mode's amplification depends sensitively upon details of the zonal-mean state. Changes of mean flow that are modest, in some instances subtle, are sufficient to remove instability. They sharply alter the mode's growth rate, but have little effect on its eigenperiodand structure.
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