Sperber, K. R. , J. M. Slingo, P. M. Inness, W. K.-M.
Lau (1997) On the Maintenance and Initiation of the Intraseasonal Oscillation
in the NCEP/NCAR Reanalysis and in the GLA and UKMO AMIP Simulations. Clim.
Dynam., 13, 769-795.
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 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
previously analyzed. 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 is not the
dominant mechanism for promoting the eastward propagation of the intraseasonal
oscillation since evaporation to the west of the convection dominants.
The data suggest a wave-CISK (conditional instability of the second-kind)
type mechanism, although the contribution by frictional convergence is
not apparent. 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. This hypothesis is based upon the examination
of observed sea surface temperature (SST) and its relationship to the active
phase of the intraseasonal oscillation, which indicates that the IO may
evolve as a coupled ocean-atmosphere mode. The eastward propagation of
convection appears to be related to the gradient of SST, with above normal
SST to the east of the convection maintaining the eastward evolution, and
decreasing SST near the western portion of the convective envelope being
associated with the cessation of convection.