Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 5 2006 10.5194/acpd-6-9259-2006 http://www.atmos-chem-phys-discuss.net/6/9259/2006/ http://www.atmos-chem-phys-discuss.net/6/9259/2006/acpd-6-9259-2006.html http://www.atmos-chem-phys-discuss.net/6/9259/2006/acpd-6-9259-2006.pdf 9259 9271 2006-09-26 Differences between the QBO in the first and in the second half of the ERA-40 reanalysis H. J. Punge M. A. Giorgetta Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany IMPRS on Earth System Modelling, Bundesstr. 53, 20146 Hamburg, Germany The representation of the quasi-biennial oscillation (QBO) is investigated in the ERA-40 reanalysis. In the lower stratosphere, where there is a good number of observations, the representation of the QBO is equally well throughout the record. However, strong differences between the first and the second half of the zonal wind data set are found in the upper stratosphere, with a typical offset of −10 m/s in the equatorial zonal wind in the earlier part versus the later part of the ERA-40 data set. The strength of the QBO is also affected. Possible explanations are discussed. The identified change of the assimilated wind profiles over time in ERA-40 requires a careful use of equatorial upper stratospheric winds from the reanalysis for validation purposes.