Revisiting the Quasi-Biennial Oscillation as Seen in ERA5. Part I: Description and Momentum Budget

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