LOW-FREQUENCY VARIABILITY AND CO2 TRANSIENT CLIMATE-CHANGE .2. EOF ANALYSIS OF CO2 AND MODEL-CONFIGURATION SENSITIVITY

We used empirical orthogonal function (EOF) analysis to examine the mo nthly variance structure of several general circulation model (GCM) si mulations to look for possible systematic changes of variability, not only due to increased carbon-dioxide (CO2) concentration in the atmosp here but also due to model configuration. We evaluated four simulation s which were present-day and doubled CO2 experiments with the same atm ospheric GCM coupled to (1) a simple nondynamic mixed-layer ocean (ter med ''mixed-layer model'') and (2) an ocean GCM (termed ''coupled mode l''). Model-generated variability, as represented by EOFs of 700-mb he ight, is similar in all cases for global analyses and is mainly charac terized by an opposition of sign between mid- and high latitudes in bo th hemispheres. This overall pattern does not appreciably change with a doubling of CO2 in the models. However, there are regional changes b etween 1 X CO2 and 2 X CO2 runs which are similar for the mixed-layer and coupled models. These changes include shifts of centers of variabi lity in the Pacific and Atlantic sectors of the Northern Hemisphere th at are similar to changes in persistent height anomalies or ''blocking '' noted in a previous study. Changes in model configuration give rise to more extensive changes in the overall pattern of variation, with v ariability in Northern and Southern Hemispheres more tightly linked in the coupled model than in the mixed-layer model. We also computed EDF s using only model data for the tropics (between 30 degrees N and 30 d egrees S). In these EOFs, differences between the two model smaller-am plitude annual harmonic and larger-amplitude half-year harmonic of sur face air temperature. Similar reductions in amplitude of the annual ha rmonic at high northern latitudes are noted in observed surface air te mperature data in a recent period compared to an earlier period in thi s century. configurations in terms of geographic centers of variabilit y and time series power spectra are greater than between 1 X CO2 and 2 X CO2 cases. This is because the coupled model simulates some aspects of the El Nino-Southern Oscillation (ENSO) while the mixed-layer vers ion does not. Consequently, different model configuration has a strong er effect on simulated interannual variability globally than does alte red CO2 forcing. Because ENSO is not represented in the mixed-layer mo del, CO2-induced changes in variability are not credible in that model . For the coupled model, regional increases in variability, such as ov er the monsoon region of south Asia, are consistent with results from other analyses. We also evaluated CO2 sensitivity of the coupled model 's seasonal cycle of surface air temperatures using a harmonic analysi s. Strongly different seasonal cycles appear in the high latitudes of the Northern Hemisphere in the coupled model under different CO2 condi tions. This phenomenon, noted in earlier studies with mixed-layer mode ls, is apparently mostly due to snow and ice reductions with increased CO2 that contribute to a smaller-amplitude annual harmonic and larger -amplitude half-year harmonic of surface air temperature. Similar redu ctions in amplitude of the annual harmonic at high northern latitudes are noted in observed surface air temperature data in a recent period compared to an earlier period in this century.