EAPS

Dependence of the Structure of Stratospheric Kelvin and Mixed Rossby-gravity waves on
the Background Flow
Equatorial Kelvin and mixed Rossby-gravity (MRG) waves are critical components in the
forcing the stratospheric Quasi-Biennial Oscillation (QBO), which is in turn an important
factor in modulating subseasonal tropospheric variability such as the Northern
Hemisphere storm track and the Madden-Julian Oscillation. Much past work has been
focused on the wave-mean flow interaction by stratospheric equatorial waves, but
numerical models used for both weather and climate simulations still have great difficulty
in maintaining and simulating the evolution of the QBO. Models also struggle to replicate
the structure, variability, and momentum forcing associated with equatorial waves,
therefore it is important to have good a priori knowledge of the characteristics of these
waves in the real atmosphere. Using ERA5 reanalysis data, we examine the structure,
scales, and propagation characteristics of Kelvin and MRG waves under extreme phases
of the QBO. It turns out that linear theory initially developed in the 1960s and 1970s by
Matsuno, Lindzen, and Holton, among others, provides a valuable guide in explaining at
least the qualitative structure and basic state dependence of stratospheric equatorial
waves. The equivalent depth and thus the phase speed of all waves increases with height,
as expected due to the effects of damping on upward propagating wave energy. Lower
stratospheric Kelvin waves are dominated by zonal wavenumber one and two structures,
but with height the lower frequency and wavenumber signals are damped, so the signals
become dominated by wavenumbers 3 and 4 above around 20 hPa. The structures of
Kelvin and MRG waves are greatly impacted by the sign of the zonal wind and its
vertical shear associated with the QBO. In general, Kelvin waves in QBO easterlies are
less equatorially trapped and so have much wider meridional structures and larger vertical
wavelengths than in westerlies. In QBO westerlies, MRG waves are characterized by the
classical structures predicted by Matsuno (1966), but in easterlies these structures are
highly altered, with the zonal scales increasing and their zonal phase speeds approaching
zero. These zonally standing features become dominant in this situation, with purely
downward phase speeds along with upward propagating energy dispersion. We show that
these structures can be explained by consideration of the impacts of the critical level,
where the phase speeds of the waves approach that of the easterly background flow, as
manifested by the expected impacts of scale dependent damping.