INTRASEASONAL OSCILLATIONS IN 15 ATMOSPHERIC GENERAL-CIRCULATION MODELS - RESULTS FROM AN AMIP DIAGNOSTIC SUBPROJECT

The ability of 15 atmospheric general circulation models (AGCM) to sim ulate the tropical intraseasonal oscillation has been studied as part of the Atmospheric Model Intercomparison Project (AMIP). Time series o f the daily upper tropospheric velocity poential and zonal wind, avera ged over the equatorial belt, were provided from each AGCM simulation. These data were analyzed using a variety of techniques such as time f iltering and space-time spectral analysis to identify eastward and wes tward moving waves. The results have been compared with an identical a ssessment of the European Centre for Medium-range Weather Forecasts (E CMWF) analyses for the period 1982-1991. The models display a wide ran ge of skill in simulating the intraseasonal oscillation. Most models s how evidence of an eastward propagating anomaly in the velocity potent ial field, although in some models there is a greater tendency for a s tanding oscillation, and in one or two the field is rather chaotic wit h no preferred direction of propagation. Where a model has a clear eas tward propagating signal, typical periodicities seem quite reasonable although there is a tendency for the models to simulate shorter period s 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 capture d the dominance of the intraseasonal oscillation found in the analyses . Several models have peaks at intraseasonal time scales, but nearly a ll have relatively more power at higher frequencies (<30 days) than th e analyses. Most models underestimate the strength of the intraseasona l variability. The observed intraseasonal oscillation shows a marked s easonality in its occurrence with greatest activity during northern wi nter and spring. Most models failed to capture this seasonality. The i nterannual variability in the activity of the intraseasonal oscillatio n has also been assessed, although the AMIP decade is too short to pro vide any conclusive results. There is a suggestion that, the observed oscillation was suppressed during the strong El Nino of 1982/83, and t his relationship has also been reproduced by some models. The relation ship between a model's intraseasonal activity, its seasonal cycle and characteristics of its basic climate has been examined. It is clear th at those models with weak intraseasonal activity tend also to have a w eak seasonal cycle. It is becoming increasingly evident that an accura te description of the basic climate may be a prerequisite for producin g a realistic intraseasonal. In particular, models with the most reali stic intraseasonal oscillations appear to have precipitation distribut ions which are better correlated with warm sea surface temperatures. T hese models predominantly employ convective parameterizations which ar e closed on buoyancy rather than moisture convergence.