A comparison of reanalyses in the tropical stratosphere. Part 2: the quasi-biennial oscillation |
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| Authors: | Pawson S Fiorino M |
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| Institution: | (1) Institut für Meteorologie, Freie Universit?t Berlin, Germany Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, USA, DE |
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| Abstract: | Reanalysis datasets potentially offer the opportunity to examine the tropical quasi-biennial oscillation (QBO) in greater
detail than in the past, including the associated meridional circulation and the links with other parts of the atmosphere.
For such studies to be useful, the QBO represented by the reanalyses should be realistic. In this work, the QBO in the ERA
and NCEP reanalyses is validated against rawinsonde observations from Singapore. Monthly mean data are used. In the lower
stratosphere (at 50 hPa and 30 hPa) the ERA QBO is reasonable, although the wind extrema in both phases are too weak and the
vertical shear and the temperature anomalies are too small. The NCEP QBO is weaker still. At 10 hPa neither reanalysis system
performs well, both systems failing to reproduce the westerlies, possibly because of the proximity of the upper boundary.
The Singapore wind is representative of the zonal means in the reanalyses. The weak wind extrema in the reanalyses would not
support a wave-mean flow interaction theory of the QBO, because a large portion of the gravity wave spectrum which would be
absorbed in reality would be transmitted beyond 10 hPa. The stronger shear zones captured in the ERA data are associated with
larger, more realistic temperature perturbations near 30 hPa. The northward velocities in the NCEP data show a more realistic
structure than in the ERA reanalysis, where they are dominated by a vertical “gridpoint wave” structure in the lowermost stratosphere.
Despite the shortcomings of the reanalyses, the high correlations of the wind at 30 hPa and 50 hPa with the observations at
Singapore mean that the reanalyses could potentially be used to examine the effects of the QBO away from the tropical stratosphere.
Future reanalyses need to take full account of the wind shears evident in the rawinsonde observations and use models with
an adequate resolution to capture these vertical scales.
Received: 23 June 1997/Accepted 17 December 1998 |
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