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In order to speed up production, ERA5 is being produced by several parallel experiments, each covering a different period, which are then appended together to create the final product. The disadvantage of this approach, is that there can be discontinuities in the final product at the transition points between the different experiments. Here, we consider the transition at the end of 2009 and the beginning of 2010. Each experiment begins with a warm up of typically a year, typically, which does not become part of the final product, but does overlap with the experiment for the preceding period. This overlap facilitates a comparison of the two experiments to see how well they have converged to a common solution.

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Fig. 4 shows the ERA5 zonal mean eastward wind (contours) and difference between the final product and the warm up (colours). In the troposphere and lower stratosphere, between 1000 hPa and 10 hPa, the only region with transition differences greater than 0.2 ms^-1 is a latitudinally confined region near the equator, above about 50 hPa, where the differences of alternating sign are no larger than about 1 ms^-1. From the mid-stratosphere to the stratopause (from 10 hPa to 1 hPa), the equatorial transition differences are slightly larger, with a larger latitudinal extent. However, in the mesosphere (from 1 hPa to 0.01 hPa), the differences are much larger, with peak values greater than 50 ms^-1 near the equator at about 0.1 hPa and values greater than 2 ms^-1 are widespread. These equatorial mesospheric differences in eastward wind, and possibly temperature too, are related to the spurious jet that this cycle of the IFS (CY41R2) suffers from, particularly in the transition seasons.

The equatorial transition differences in eastward wind in ERA-Interim in the lower stratosphere (Fig. 5) are slightly smaller than those in ERA5, though above in the mid to upper stratosphere, the differences are somewhat larger than those in ERA5. In the mesosphere, the transition differences in ERA-Interim are much smaller than those in ERA5. The spread in the eastward wind in the ERA5 EDA 10 member ensemble (Fig. 6) is generally larger than the ERA5 transition differences in the troposphere and stratosphere, except near the equator. In the mesosphere, the spread is smaller than the ERA5 transition differences.

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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 equatorial mesospheric jet that this cycle of the IFS (CY41R2) suffers from, particularly 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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