  The dynamics and momentum budget of the quasi-biennial oscillation (QBO) are
examined in the ERA5 reanalysis and compared with those in ERA-I. Because of
ERA5's higher spatial resolution it is capable of resolving a broader spectrum
of atmospheric waves and allows for a better representation of the wave-mean
flow interactions, both of which are of crucial importance for QBO studies. It
is shown that the QBO-induced mean meridional circulation, which is mainly
confined to the winter hemisphere, is strong enough to interrupt the tropical
upwelling during the descent of the westerly shear zones. Since the momentum
advection tends to damp the QBO, the wave forcing is responsible for both the
downward propagation and for the maintenance of the QBO. It is shown that half
the required wave forcing is provided by resolved waves during the descent of
both westerly and easterly regimes. Planetary-scale waves account for most of
the resolved wave forcing of the descent of westerly shear zones and
small-scale gravity (SSG) waves with wavelengths shorter than 2000 km account
for the remainder. SSG waves account for most of the resolved forcing of the
descent of the easterly shear zones. The representation of the mean fields in
the QBO is very similar in ERA5 and ERA-I but the resolved wave forcing is
shown to be substantially stronger in ERA5. The contributions of the various
equatorially-trapped wave modes to the QBO forcing are documented in Part II.
