Slingo, J. M., K. Sperber, J. Boyle, J.-P. Ceron, M. Dix,
B. Dugass, W. Ebisuzaki, J. Fyfe, D. Gregory, J.-F. Gueremy, J. Hack, A.
Harzallah, P. Innes, A. Kitoh, W. K.-M. Lau, B. McAvaney, R. Madden, A.
Matthews, T. Palmer, C.-K. Park, D. Randall, N. Renno, 1995b: Intraseasonal
oscillations in 15 atmospheric general circulation models: Results from
an AMIP diagnostic subproject. WCRP-88, WMO/TD-No. 661, World Climate Research
Programme Report, Geneva, 32 pp.
The ability of 15 Atmospheric General Circulation Models
(AGCM) to simulate the tropical intraseasonal oscillation has been studied
as part of the Atmospheric Model Intercomparison Project (AMIP). Time series
of the daily upper tropospheric velocity potential and zonal wind, averaged
over the equatorial belt, were provided from each AGCM. These data were
analyzed using a variety of techniques such as time filtering and space-time
spectral analysis to identify eastward and westward moving waves. The results
have been compared with an identical assessment of ECMWF analyses for the
period 1982-1991. The models display a wide range of skill in simulating
the intraseasonal oscillation. Most models show evidence of an eastward
propagating anomaly in the velocity potential field, although in some models
there is a greater tendency for a standing oscillation, and in one or two
the field is rather chaotic with no preferred direction of propagation.
Where a model has a clear eastward propagating signal, typical periodicities
seem quite reasonable although there is a tendency for the models to simulate
shorter periods than in the ECMWF analyses, where it is near 50 days. The
results of the space-time spectral analysis have shown that no model has
captured the dominance of the intraseasonal oscillation in the analyses.
Several models have peaks at intraseasonal time scales, but nearly all
have relatively more power at higher frequencies (< 30 days) than the
analyses. Most models underestimate the strength of the intraseasonal variability.
The observed intraseasonal oscillation shows a marked seasonality in its
occurrence with greatest activity during northern winter and spring. Most
models failed to capture this seasonality. The interannual variability
in the activity of the intraseasonal oscillation has also been assessed,
although the AMIP decade is to short to provide any conclusive results.
There is a suggestion that the observed oscillation is suppressed during
an El Nino, and this relationship is also reproduced by some models. The
relationship between a model's intraseasonal activity, its seasonal cycle
and characteristics of its basic climate has been examined. It is clear
that those models with weak intraseasonal activity tend also to have a
weak seasonal cycle. It is becoming increasingly evident that an accurate
description of the basic climate may be a prerequisite for producing a
realistic intraseasonal oscillation. In particular, models with the most
realistic intraseasonal oscillations appear to have precipitation distributions
which are well correlated with warm sea surface temperatures. These models
predominantly employ convective parameterizations which are closed on buoyancy
rather than moisture convergence.