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Ann. Geophys., 19, 99-114, 2001
www.ann-geophys.net/19/99/2001/
© European Geosciences Union 2001
The SAO and Kelvin waves in the EuroGRIPS GCMS and the UK Met. Office analyses
M. Amodei1, S. Pawson2,3, A. A. Scaife4, U. Langematz5, W. Lahoz6, D. M. Li4, and P. Simon1
1CNRM, Météo France, Toulouse, France
2Data Assimilation Office, NASA GSFC, Greenbelt, MD, USA
3Universities Space Research Association, Seabrook, MD, USA
4The Meteorological Office, Bracknell, UK
5Freie Universität Berlin, Germany
6Center for Global Atmospheric Modelling, University of Reading, UK
Correspondence to: M. Amodei, (marielle.amodei@meteo.fr)
Abstract. We compare the tropical
oscillations and planetary scale Kelvin waves in four troposphere-stratosphere
climate models and the assimilated dataset produced by the United Kingdom
Meteorological Office (UKMO). The comparison has been made in the GRIPS
framework "GCM-Reality Intercomparison Project for SPARC", where SPARC
is Stratospheric Processes and their Role in Climate, a project of the World
Climate Research Program. The four models evaluated are European members of
GRIPS: the UKMO Unified Model (UM), the model of the Free University in Berlin (FUB–GCM),
the ARPEGE-climat model of the French National Centre for Meteorological
Research (CNRM), and the Extended UGAMP GCM (EUGCM) of the Centre for Global
Atmospheric Modelling (CGAM). The integrations were performed with different,
but annually periodic external conditions (e.g., sea-surface temperature, sea
ice, and incoming solar radiation). The structure of the tropical winds and the
strengths of the Kelvin waves are examined. In the analyses where the SAO
(Semi-Annual Oscillation) and the QBO (Quasi-Biennal Oscillation) are reasonably
well captured, the amplitude of these analysed Kelvin waves is close to that
observed in independent data from UARS (Upper Atmosphere Research Satellite). In
agreement with observations, the Kelvin waves generated in the models propagate
into the middle atmosphere as wave packets, consistent with a convective forcing
origin. In three of the models, slow Kelvin waves propagate too high and their
amplitudes are overestimated in the upper stratosphere and in the mesosphere,
the exception is the UM which has weaker waves. None of the modelled waves are
sufficient to force realistic eastward phases of the QBO or SAO. Although the
SAO is represented by all models, only two of them are able to generate
westerlies between 10 hPa and 50 hPa. The importance of the role played in the
SAO by unresolved gravity waves is emphasized. Although it exhibits some
unrealistic features, the EUGCM, which includes a parametrization of gravity
waves with a non-zero phase speed, is able to simulate clear easterly to
westerly transitions as well as westerlies with down-ward propagation. Thermal
damping is also important for the westerly forcing in the stratosphere.
Key words. Meteorology and atmospheric dynamics (middle
atmosphere dynamics; tropical meterology; waves and tides)
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