Low-frequency variability and CO2 transient climate change.
Part 3. Intermonthly and interannual variability
Gerald A. Meehl, Matthew Wheeler, and Warren M. Washington
1994: Clim. Dyn., 10, 277-303.
Abstract
Components of interannual, intermonthly, and total monthly
variability of lower troposphere temperature are calculated from
a global coupled ocean-atmosphere general circulation model (GCM)
(referred to as the coupled model), from the same atmospheric model
coupled to a nondynamic mixed-layer ocean (referred to as the mixed-layer
model), and from microwave sounding unit (MSU) satellite data.
The coupled model produces most features of intermonthly and interannual
variability compared to the MSU data, but with somewhat reduced
amplitude in the extratropics and increased variability in the tropical
western Pacific and tropical Atlantic. The relatively short 14-year
period of record of the MSU data precludes definitive conclusions about
variability in the observed system at longer time scales (e.g., decadal
or longer). Different 14-year periods from the coupled model show variability
on those longer time scales that were noted in Part 1 of this series. The
relative contributions of intermonthly and interannual variability that
make up the total monthly variability are similar between the coupled
model and the MSU data, suggesting that similar mechanisms are at work
in both the model and observed system. These include El Nino-Southern
Oscillation (ENSO)-type interannual variability in the tropics, Madden-Julian
Oscillation (MJO)-type intermonthly variability in the tropics, and
blocking-type intermonthly variability in the extratropics.
Manifestations of all these features have been noted in various versions
of the model. Significant changes of variability noted in the coupled
model with doubled carbon dioxide differ from those in our mixed-layer
models. In particular, in our mixed-layer model intermonthly and
interannual variability changes are similar with a mixture of
regional increases and decreases, but with mainly decreases in the zonal
mean from about 20°S to 60°N and near 60°S. In the
coupled model, intermonthly and interannual changes of variability
with doubled CO2 show mostly increases of tropical interannual
variability and decreases of intermonthly variability near 60°N.
These changes in the tropics are related to changes in ENSO, the
south Asian monsoon, and other regional hydrological regimes, while
the alterations near 60°N are likely associated with changes in
blocking activity. These results point to the important contribution
from ENSO seen in the coupled model and the MSU data that are not present
in the mixed-layer model.