Response of The NCAR Climates System Model to Increased CO2
and The Role of Physical Processes
Gerald A. Meehl, William Collins, Byron Boville, Jeffrey T. Kiehl, T. M.
L. Wigley, and Julie M. Arblaster
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
Two physical processes that contribute to climate sensitivity and climate change
in global coupled climate models run with increasing CO2, the "El
Niño-like" response (slackening of the equatorial Pacific SST gradient) and
sea ice response at high latitudes, are examined in the NCAR Climate System
Model (CSM) and compared to results from a coupled model with greater climate
sensitivity, the NCAR DOE global coupled model. In an experiment where
atmospheric CO2 is increased 1% per year compound, globally averaged
surface air temperature increase around the time of CO2 doubling for
the CSM is 1.43C (3.50C for the DOE model), with globally averaged precipitation
increasing 2.0% (3.1% for the DOE model). The equilibrium sensitivity in a
CO2 doubling experiment with the atmospheric model from the CSM (the
CCM3) coupled to a non-dynamic slab ocean is 2.08C for globally averaged
temperature (4.58C for the DOE model), and 3.9% for globally averaged
precipitation (4.0% for the DOE model). The meridional overturning in the
Atlantic weakens by 3% in the CSM (41% for the DOE model), and the Indian summer
monsoon increases by 11.6% (1.4% for the DOE model) as seen in other
CO2-only coupled model experiments. The "El Niño-like" response,
a notable feature in the DOE and some other global coupled models, does not
occur in the CSM. We show that cloud responses are a major determining factor.
With increased CO2, there are negative net cloud forcing differences
in the western equatorial Pacific in the CSM and DOE model, but large positive
differences in the DOE model and negative differences in the CSM in the eastern
equatorial Pacific. This produces asymmetric cloud radiative forcing
contributing to an El Niño-like response in the DOE model and not in the
CSM. A slab ocean version of the atmospheric model in the CSM (CCM3) that
includes prognostic cloud liquid water shows a change in sign (from negative to
positive) of the net cloud forcing in the eastern equatorial Pacific, more
similar to the DOE model, compared to the CCM3 version with diagnostic cloud
liquid water. AMIP (prescribed SST) experiments show that all three atmospheric
models (DOE, CCM3 with diagnostic cloud liquid water and CCM3 with prognostic
cloud liquid water) perform poorly relative to observations, though CCM3 with
prognostic cloud liquid water is slightly superior to the others. Sea ice
retreat with increasing CO2 in the CSM is less than in the DOE model
in spite of identical sea ice formulations. Results from the North Atlantic and
GIN Sea region show that the surface energy budget response is controlled
primarily by surface albedo (related to ice area changes) and cloud changes.
However, a more important factor is the poleward ocean heat transport associated
with changes in meridional overturning in the GIN Sea. With increased
CO2, the transport of warmer water from the south into this region in
the DOE model is greater compared to the CSM. This leads to a larger ice
reduction in the DOE model, thus also contributing to the enhanced sensitivity
in the DOE model compared to the CSM.
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Hongjun Zhang:
zhangho@ucar.edu