ON THE MAINTENANCE AND INITIATION OF THE INTRASEASONAL OSCILLATION INTHE NCEP NCAR REANALYSIS AND IN THE GLA AND UKMO AMIP SIMULATIONS/

In this study, satellite-derived outgoing longwave radiation (OLR) and the reanalysis from the National Centers for Environmental Prediction /National Center for Atmospheric Research are used as verification dat a in a study of intraseasonal variability in the Goddard Laboratory fo r Atmospheres (GLA) and the United Kingdom Meteorological Office (UKMO ) atmospheric general circulation models. These models simulated the m ost realistic intraseasonal oscillations (IO) of the 15 Atmospheric Mo del Intercomparison Project models previously analysed. During the act ive phase of the intraseasonal oscillation, convection is observed to migrate from the Indian Ocean to the western/central Pacific Ocean, an d into the South Pacific Convergence Zone (SPCZ). The simulated convec tion, particularly in the GLA model, is most realistic over the wester n/central Pacific Ocean and the SPCZ. In the reanalysis, the baroclini c structure of the IO is evident in the eddy-stream function, and east ward migration of the anticyclone/cyclone pairs occurs in conjunction with the eastward development of convection. Both the GLA and UKMO mod els exhibit a baroclinic structure on intraseasonal time scales. The G LA model is more realistic than the UKMO model at simulating the eastw ard migration of the anticyclone/cyclone pairs when the convection is active over the western/central Pacific. In the UKMO model, the main h eating is located off the equator, which contributes to the irregular structures seen in this model on intraseasonal time scales. The mainte nance and initiation of the intraseasonal oscillation has also been in vestigated. Analysis of the latent heat flux indicates that evaporativ e wind feedback is not the dominant mechanism for promoting the eastwa rd propagation of the intraseasonal oscillation since evaporation to t he west of the convection dominants. The data suggest a wave-CISK (con ditional instability of the second-kind) type mechanism, although the contribution by frictional convergence is not apparent. In the GLA mod el, enhanced evaporation tends to develop in-place over the west Pacif ic warm pool, while in the UKMO simulation westward propagation of enh anced evaporation is evident. It is suggested that lack of an interact ive ocean may be associated with the models systematic failure to simu late the eastward transition of convection from the Indian Ocean into the western Pacific Ocean. This hypothesis is based upon the examinati on of observed sea surface temperature (SST) and its relationship to t he active phase of the intraseasonal oscillation, which indicates that the IO may evolve as a coupled ocean-atmosphere mode. The eastward pr opagation of convection appears to be related to the gradient of SST, with above normal SST to the east of the convection maintaining the ea stward evolution, and decreasing SST near the western portion of the c onvective envelope being associated with the cessation of convection.