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Fig. 4 shows the ERA5 zonal mean zonal 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 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 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 values of order 100 mspeak values greater than 50 ms-1 CHECK near the equator at about 0.1 hPa and values greater than 2 ms-1 being are widespread.
The equatorial differences in zonal wind in ERA-Interim in the lower stratosphere (Fig. 5) are 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 differences in ERA-Interim are much smaller than those in ERA5. The spread in the wind in the ERA5 10 member ensemble (Fig. 6) , is generally below 2 ms-1 except at low latitudes above about 0.2 hPa, where values can exceed 5 ms-1.
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