Report 36:  On the Maintenance and Initiation of the Intraseasonal Oscillation in the NCEP/NCAR Reanalysis and the GLA and UKMO AMIP Simulations

Sperber, Kenneth R., Julia M. Slingo, Peter M. Inness, and William K-M. Lau
October 1996, 60 pp. 

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In this study, satellite derived outgoing longwave radiation (OLR) and the reanalysis from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) are used as verification data in a study of intraseasonal variability in the Goddard Laboratory for Atmospheres (GLA) and the United Kingdom Meteorological Office (UKMO) atmospheric general circulation models. These models simulated the most realistic intraseasonal oscillations (IO) of the 15 Atmospheric Model Intercomparison Project models examined by Slingo et al. (1995, 1996). During the active phase of the intraseasonal oscillation, convection is observed to migrate from the Indian Ocean to the western/central Pacific Ocean, and into the South Pacific Convergence Zone (SPCZ). The simulated convection, particularly in the GLA model, is most realistic over the western/central Pacific Ocean and the SPCZ. In the reanalysis, the baroclinic structure of the IO is evident in the eddy-streamfunction, and eastward migration of the anticyclone/cyclone pairs occurs in conjunction with the eastward development of convection. Both the GLA and UKMO models exhibit a baroclinic structure on intraseasonal time scales. The GLA model is more realistic than the UKMO model at simulating the eastward migration of the anticyclone/cyclone pairs when the convection is active over the western/central Pacific. In the UKMO model, the main heating is located off the equator, which contributes to the irregular structures seen in this model on intraseasonal time scales.

The maintenance and initiation of the intraseasonal oscillation has also been investigated. Analysis of the latent heat flux indicates that evaporative wind feedback and frictional wave-CISK (conditional instability of the second-kind) are not the dominant mechanisms for promoting the eastward propagation of the intraseasonal oscillation since evaporation to the west of the convection dominants. In the GLA model, enhanced evaporation tends to develop in-place over the west Pacific warm pool, while in the UKMO simulation westward propagation of enhanced evaporation is evident. It is suggested that lack of an interactive ocean may be associated with the models' systematic failure to simulate the eastward transition of convection from the Indian Ocean into the western Pacific Ocean since examination of observed sea surface temperature (SST) and its relationship to the active phase of the intraseasonal oscillation indicates that the intraseasonal oscillation may evolve as a coupled ocean-atmosphere mode. Above normal SST to the east of the convection may play a role in maintaining the eastward evolution of convection, while decreasing SSTs near the western portion of the convective envelope are associated with the cessation of convection. (pdf file).

UCRL-MI-123395

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