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In summary, the ERA5 transition differences for 2009/2010 in the troposphere and stratosphere are generally smaller than transition differences in ERA-Interim and are generally smaller than the ensemble spread. However, in the mesosphere the transition differences are much larger, exceed those in ERA-Interim and are larger than the ensemble spread. The large differences in the mesosphere are particularly apparent in the polar and equatorial regions. For the latter region, the differences in eastward wind, and possibly temperature too, are related to the spurious mesospheric jet that this cycle of the IFS (CY41R2) suffers from, particularly in the transition seasons.
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