LOW-FREQUENCY VARIABILITY OF SURFACE AIR-TEMPERATURE IN A 1000-YEAR INTEGRATION OF A COUPLED ATMOSPHERE-OCEAN-LAND SURFACE MODEL

This study analyzes the variability of surface air temperature (SAT) a nd sea surface temperature (SST) obtained from a 1000-yr integration o f a coupled atmosphere-ocean-land surface model, which consists of gen eral circulation models of the atmosphere and oceans and a heat and wa ter budget model of land surface. It also explores the role of oceans in maintaining the variability of SAT by comparing the long-term integ ration of the coupled model with those of two simpler models. They are 1) a ''mixed layer model,'' that is, the general circulation model of the atmosphere combined with a simple slab model of the mixed layer o cean, and 2) a ''fixed SST model,'' that is, the same atmosphere model overlying seasonally varying, prescribed SST. With the exception of t he tropical Pacific, both the coupled and mixed layer models are capab le of approximately simulating the standard deviations of observed ann ual and 5-yr-mean anomalies of local SAT. The standard deviation tends to be larger over continents than over oceans, in agreement with the observations. Over most continental regions, the standard deviations o f annual, 5-yr- and 25-yr-mean SATs in the fixed SST model are slightl y less than but comparable to the corresponding standard deviations in the coupled model, suggesting that a major fraction of low-frequency local SAT variability over continents of the coupled model is generate d in situ. Over the continents of both the coupled and the mixed layer models, the spectral density of local SAT is nearly independent of fr equency. On the other hand, the spectral density of local SAT over mos t of the oceans of both models increases very gradually with decreasin g frequency apparently influenced by the thermal inertia of mixed laye r oceans. However, both SST and SAT spectra in the coupled model are s ubstantially different from those in the mixed layer model near the De nmark Strait and in some regions in the circumpolar ocean of the South ern Hemisphere where water mixes very deeply. In these regions, both S ST and SAT are much more persistent in the coupled than in the mixed l ayer models, and their spectral densities are much larger at multi-dec adal and/or centennial timescales. It appears significant that not onl y the coupled model but also the mixed layer model without ocean curre nts can approximately simulate the power spectrum of observed, global mean SAT at decadal to interdecadal time-scales. However, neither mode l generates a sustained, long-term warming trend of significant magnit ude such as that observed since the end of the last century.