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