LOW-FREQUENCY VARIABILITY AND CO2 TRANSIENT CLIMATE-CHANGE .3. INTERMONTHLY AND INTERANNUAL VARIABILITY

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 nondynami c mixed-layer ocean (referred to as the mixed-layer model), and from m icrowave sounding unit (MSU) satellite data. The coupled model produce s most features of intermonthly and interannual variability compared t o the MSU data, but with somewhat reduced amplitude in the extratropic s and increased variability in the tropical western Pacific and tropic al Atlantic. The relatively short 14-year period of record of the MSU data precludes definitive conclusions about variability in the observe d 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 con tributions of intermonthly and interannual variability that make up th e total monthly variability are similar between the coupled model and the MSU data, suggesting that similar mechanisms are at work in both t he model and observed system. These include El Nino-Southern Oscillati on (ENSO)-type interannual variability in the tropics, Madden-Julian O scillation (MJO)-type intermonthly variability in the tropics, and blo cking-type intermonthly variability in the extratropics. Manifestation s of all of these features have been noted in various versions of the model. Significant changes of variability noted in the coupled model w ith doubled carbon dioxide differ from those in our mixed-layer model and earlier studies with mixed-layer models. In particular, in our mix ed-layer model intermonthly and interannual variability changes are si milar with a mixture of regional increases and decreases, but with mai nly decreases in the zonal mean from about 20-degrees-S to 60-degrees- N and near 60-degrees-S. In the coupled model, intermonthly and intera nnual changes of variability with doubled CO2 show mostly increases of tropical interannual variability and decreases of intermonthly variab ility near 60-degrees-N. These changes in the tropics are related to c hanges in ENSO, the south Asian monsoon, and other regional hydrologic al regimes, while the alterations near 60-degrees-N are likely associa ted with changes in blocking activity. These results point to the impo rtant contribution from ENSO seen in the coupled model and the MSU dat a that are not present in the mixed-layer model.