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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}1 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, followed by prolongation of the westerlies, is clearly visible in both reanalyses.
Although the broad representation of the SAO between 5 and 0.5 hPa is reasonably similar in the two reanalyses, with descending easterlies and westerlies at similar times of the year, the magnitude of the winds and the exact pattern of descent are not the same (Figure \ref{fig:qbo}1 c and d). However, between 0.5 and 0.1 hPa the agreement between the reanalyses is poor, with the two representations of the SAO being very different. In this region, ERA5 has westerlies that are up to 30 ms-1 larger than those in ERA-Interim and ERA5 also has a complete absence of descending easterlies. This predominance of westerlies in ERA5 is related to the spurious equatorial mesospheric jet that occurs in CY41R2 of the IFS and which peaks in the transition seasons.
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