The ability of 15 atmospheric GCM models (AGCM) to simulate the tropical intraseasonal oscillation has been studied as part of AMIP. Time series of the daily upper tropospheric velocity potential and zonal wind, averaged over the equatorial belt, were provided from each AGCM simulation. These data were analyzed using a variety of techniques such as time filtering and space-time spectral analysis to identify eastward and westward propagating 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 found in the analyses. Several models have peaks at intraseasonal time scales, but nearly all have relatively more power at higher frequencies

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SUMMARYA reference case of a Sahelian weather system observed during the Hydrological Atmospheric Pilot Experiment, HAPEX-SAHEL, in August 1992, is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints. It is shown that the case-study is representative of the climatology at all these scales and presents many interacting scales and physical processes. At intraseasonal scale, the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the African easterly jet (AEJ) and convection. At the month and day scales, the convective activity occurs in an apparent zonal break of the tropical easterly jet. The month of August 1992 exhibits intense synoptic activity. The vorticity eld is characterized by northerly (dry) and southerly (wet) components located at 850 hPa on each side of the AEJ. Their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity. Around 21 August, the 700 hPa vorticity eld features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation. Convective activity occurs mainly ahead of the 700 hPa vorticity maximum with the formation of a squall line on Aṏ r mountains propagating south-westward at 15 m s ¡1 . The convective system propagates about twice as fast as the vortex core, in contrast with the convection in the European Centre for Medium-Range Weather Forecasts re-analysis which stays in phase with the vorticity. The squall line corresponds to the largest contributor to the systems passing in August 1992 over the HAPEX-SAHEL region; its environmental conditions and its effects on the atmosphere including the surface parameters are presented.