12th Conference on Middle Atmosphere

Monday, 4 November 2002: 10:30 AM
The stratospheric QBO in the NCEP reanalyses
Amihan S. Huesmann, University of Wisconsin, Madison, WI; and M. H. Hitchman
The reanalysis products from the National Centers for Environmental Prediction (NCEP), which now span the years 1948-present, are a useful tool for scientists who require a long, stable, and complete data record. Here, we study global quasi-biennial variation in the lower stratosphere and tropopause region using 44 years (1958-2001) of NCEP wind, temperature, and geopotential height data. A new quasi-biennial oscillation (QBO) indexing method is presented, which is based on anomalies of equatorial zonal mean zonal wind shear. The vertical-meridional structure of the QBO as it descends is examined using indices for four different levels, while "composite phases" and "composite years" allow a closer examination of the time progression phase relationships, and relationship to the annual cycle. Subtracting the time mean and annual cycle removes ~2/3 of the asymmetry in wind (and wind shear) zone descent rate. The NCEP data validate previous findings that both the easterly and westerly QBO anomalous wind regimes in the lower stratosphere change sign preferentially during northern summer.

It is noteworthy that the QBO structure and relationship among zonal wind, temperature, and meridional circulation undergo a substantial change around 1978 in the NCEP data. While the equatorial zonal wind amplitude is similar in both periods, the equatorial temperature anomalies are smaller, the meridional scale of the circulation diminishes, and the meridional circulation strengthens. This change is interpreted as a response of the temperature and continuity equations to the introduction of nadir-sounding satellite temperature profiles into the data stream, which are smoother than radiosonde profiles.

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