  This paper describes stratospheric waves in ERA5 reanalysis and evaluates the
contributions of different types of waves to the driving of the quasi-biennial
oscillation (QBO). Because of its higher spatial resolution compared to its
predecessors, ERA5 is capable of resolving a broader spectrum of waves. It is
shown that the resolved waves contribute to both eastward and westward
accelerations near the equator, mainly by the way of the vertical flux of zonal
momentum. The eastward accelerations by the resolved waves are mainly due to
Kelvin waves and small-scale gravity (SSG) waves with zonal wavelengths smaller
than 2000 km, whereas the westward accelerations are forced mainly by SSG
waves, with smaller contributions from inertio-gravity and mixed-Rossby-gravity
waves. Extratropical Rossby waves propagate into the tropical region and impart
a westward acceleration to the zonal flow. They appear to be responsible for at
least some of the irregularities in the QBO cycle.
