The GFDL/DERF GCM is analyzed for intraseasonal variations using AMIP results and compared with NMC observations. The GCM used in this study was spectrally truncated at triangular wave 42, has 18 sigma levels in the vertical and was integrated with the AMIP prescribed SSTs. For both the GCM and observations, velocity potential anomalies at 850 and 200 hPa arecalculated on a daily basis for the period January through June of each year from 1980 to 1988. In order to remove high frequency oscillations from these half year time series a five day running mean filter is applied.
Visual inspection via Hovmoeller plots and quantitative space-time analyses
applied in the equatorial region, indicate a significant difference between
the dominant mode seen in observations versus that seen in the GCM. The
observations clearly show a maximum intraseasonal oscillation with a period
in the 40 to 50 day range, quite consistent with the findings of other
investigators. The dominant period seen in the GCM is closer to 25 days.
There is also an indication that ENSO may interact with these modes, both
in the GCM and the observations. This is clearly evidenced from analyses
during the 1983 period.