Tropical stratospheric zonal winds in ECMWF ERA-40 reanalysis, rocketsonde data, and rawinsonde data

The quasi-biennial oscillation (QBO) is well defined by rawinsonde observations up to 10 hPa. Above that level, measuring the QBO is problematic because the most accurate wind measurements (rocketsondes) were not made at the equator and have large data gaps in time. We will show that the ECMWF ERA-40 reanalysis zonal winds, which provide a continuous data record since 1958, are very close to tropical rocketsonde and rawinsonde observations up to 10 hPa. Above 10 hPa, agreement with rocketsonde data decreases, with the ERA-40 data provide a good representation of tropical winds up to 2–3 hPa. The amplitudes of the QBO and the semi-annual oscillation (SAO) derived from ERA-40 data match the rawinsonde and rocketsonde observations up to 2–3 hPa. We conclude that zonal-mean ERA-40 equatorial winds could be used in place of rawinsonde station observations.