The Nature and Predictability of Tropical Intraseasonal Oscillations

R.Smith         W. Stern

D. Waliser (Marine Sciences Research Center, SUNY)

Results from both AMIP (1540) and coupled GCM integrations are being used to investigate tropical intraseasonal oscillations (TIO). As part of the Monsoon Modelling Intercomparison Project, a comparison of TIO/MJO behavior in the 1982-83 and 1997-98 ENSOs was performed using 10 member ensembles of AGCM integrations forced by observed SSTs. Longitude-time plots and power spectra indicate a difference in MJO behavior between the onset and mature phases of both ENSO events. Applying statistical tests indicates that some of the MJO interannual variability appears to be significant, especially in the west-central equatorial Pacific ocean. Considerable TIO activity is also seen in the Asian monsoon region and appears to be associated in part with northward propagation from the equatorial Indian ocean. An association between equatorial MJO activity and Monsoon rainfall intensity is being sought.


Results from Coupled Model Ensemble Prediction (CMEP) coupled GCM are being analyzed to better understand TIO as a coupled atmosphere-ocean phenomenon. Figure 4.3 shows evidence of a robust TIO in the model using the first extended EOF of bandpassed (20-100 day) 200 hPa velocity potential. This EOF compares favorably to those from the NCEP/NCAR reanalysis, but it is stronger. By forcing the atmospheric AGCM with SSTs generated by the CGCM and comparing these results to those obtained  using a weakly interacting slab mixed-layer ocean, a better understanding will be gained of how interactive ocean processes impact the behavior of TIO.

Fig. 4.3First mode extended EOF time-longitude diagrams of bandpassed (20-100 day) equatorial (10N-10S) 200 hPa velocity potential from the GFDL CGCM (top) and the NCEP/NCAR reanalysis (bottom). This plot suggests that the propagating features of the MJO in the GFDL model are consistent with observations while the strength appears to be somewhat greater.