The 1978 shif in the NCEP reanalysis stratospheric quasi-biennial oscillation

Amihan S. Huesmann and Matthew H. Hitchman

Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, Wisconsin, USA

The National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP) reanalysis exhibits interdecadal changes in the structure of the quasi-biennial oscillation (QBO) and annual mean fields in the tropical stratosphere during 1958-2001. A sudden temperature increase occured around 1978, with equatorial maximal of ~3 K at 10 hPa and near the tropopause. When this 1978 jump is removed, a moderate cooling trend is found throughout the tropical lower stratosphere, with an equatorial maximum cooling of 3 K over 40 years near 30 hPa. While the QBO in zonal wind is similar in both the 1958-1978 and 1979-2001 periods, the QBO in temperature is significantly reduced in amplitude and latitudinally contracted after 1978, and the QBO meridional circulation is stronger. Vertical temperature variations in the annual mean and QBO have been underestimated since the introduction of nadir-sounding satellites. A mechanism is suggested for how this may affect meridional circulations in data assimilation modls.

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