Revisiting the Description and Momentum Budget of the Quasi Biennial Oscillation as Seen in ERA5

Abstract:

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 modify the mean tropical stratospheric circulation: the descent in the westerly shear zone could interrupt the tropical upwelling. Since the momentum advection by the QBO-induced mean meridional circulation tends to damp the QBO, the wave forcing is responsible for both the downward propagation and for the maintenance of the QBO. Planetary scale waves account for more than half of the forcing for the descent of westerly shear zones, whereas the rest of forcing is provided by small-scale gravity waves with wavelengths shorter than 2000 km. Small-scale gravity waves provide the majority of forcing for the descent of the easterly shear zones. The unresolved wave forcing reinforces the resolved wave forcing by ~70% in the easterly shear zones. However, the role of unresolved wave forcing is not so much to promote the downward propagation during the descent of the westerly shear zones but rather to neutralize the instability of the flow. 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 than in ERA-I, in particular during the descent of the easterly shear zones.