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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 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 30 ms-1, or more, larger than those in ERA-Interim and . At these levels, ERA5 also has a complete absence of descending easterlies around the time of the solstices, though, due to a dearth of observations, it is not certain what happens in reality. 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. For more information on this spurious equatorial mesospheric jet, see Section 7 in the ECMWF Technical Memorandum No. 824 Report on Stratosphere Task, Force by T.G. Shepherd, I. Polichtchouk, R.J. Hogan and A.J. Simmons.
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