The representation of the Quasi-Biennial Oscillation (QBO) and Semi-Annual Oscillation (SAO) in the IFS depends on the model formulation in the tropical region, particularly the parameterisation of the effects of gravity waves. Therefore, the representation of the QBO and SAO vary with model cycle and resolution. Observations do provide constraints for the analyses, but poorly observed regions, such as the upper stratosphere and the mesosphere, will exhibit similar characteristics to those of the forecast model.
a | b |
c  | d |
The depiction of the QBO in the lower to mid stratosphere (from about 50 to 5 hPa) in ERA5 is very similar to that in ERA-Interim (Figure \ref{fig:qbo}a and b), at least for the period considered here, 2008 to 2017. The similarity is present in both the strength of the descending easterlies and westerlies and in their phase and periodicity. The disruption of the westerlies just above 50 hPa in early 2016 is clearly visible in both reanalyses.
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