The effect of excessive snow mass over the Eurasian continent on the spring and summer climate is investigated by using the MRI * GCM. The ensemble mean of the four runs (SNOW runs) with the excessive snow mass of 5 cm (water equivalent) at the beginning of March over the snow cover area of the continent is compared with that of the control runs, to deduce the effect of the snow mass on the climatic parameters in the later seasons.The main results are summarized as follows:(1) In spring, the albedo effect is dominated in the lower latitudes particularly over the Tibetan Plateau. The reduced net radiation by the anomalous snow cover balances the reduced surface sensible and latent heat fluxes, which account for the significant decrease of surface temperature, cloudiness and total diabatic heating over there in the SNOW runs.(2) In summer, in contrast, the snow-hydrological effect is significant, particularly in the mid-latitudes. The increase of ground wetness in the SNOW runs causes anomalous cooling and higher pressure near the surface. A moderate signal of the weakened Asian summer monsoon is also obtained. However, the increase of evaporation activates cumulus convection, which partly compensates for the decrease of total diabatic heating by the cooling near the surface. This evaporation/convection feedback seems to work, on the other hand, to sustain the increased ground wetness throughout the summer. (3) The atmospheric teleconnection patterns induced by the anomalous snow cover over the Tibetan Plateau and east Asia significantly appear over the north Pacific and the North American continent in spring through late summer. These anomalous circulations cause the considerable decrease of surface temperature over the northeastern part of North America. (4) The implication of these results for the Ice Age issue is also briefly discussed.

Characteristics of the equatorial intraseasonal oscillation are studied with the use of a general circulation model which includes the Arakawa-Schubert (abbreviated as AS hereafter) model of penetrative cumulus convections. The AS model is modified by introducing the minimum value of cumulus entrainment rate of the environmental air, min, as * min=a/D, where D is the depth of the planetary boundary layer (PBL) and a is a non-negative constant. The introduction of a positive a in the AS model suppresses the activity of deep penetrative cumulus in the area where D is not sufficiently thick, which allows, in turn, an accumulation of moist air in the large-scale low level convergence zone. This process is essential in maintaining the equatorial 30-60 day oscillations, and also in simulating the Pacific subtropical high during the northern summer. Experiments are performed by changing * from 0 (ie., the original AS model), to 00 (i.e., no penetrative cumulus convection) under an aqua-planet condition.When *=0, the 30-60 day oscillation does not appear in the tropics. Instead, there exists a quasi-10 day eastward propagating oscillation with zonal wavenumber 1, which resembles a neutral Kelvin wave. Moist air is not accumulated in the `low level east-west convergence longitude associated with the flow of zonal wavenumber 1 (LLCL)' due to the rapid response of the AS model to the evaporation and moisture flux convergence by small scale motions and also due to the resulting upward transport of water vapor by penetrative cumuli to the west of the LLCL before the moist air can be accumulated in the LLCL. When *=0.1, a quasi-30 day eastward propagating oscillation with zonal wavenumber 1 grows in the model, with the moist air and the major heating found around the LLCL. The change in the heating is mostly due to the increase of middle-level convection (i.e., moist convection between adjacent vertical layers within the free atmosphere) and to the decrease of deep penetrative cumuli to the west of the LLCL. Overall characteristics of the mode are close to the observed ones. When a=oo, a quasi-45 day eastward propagating oscillation grows in the model. The structure of the heating associated with the oscillation is similar to that of the quasi-30 day oscillation.Associated with the increase of *, the static stability decreases in the low latitudes. The maximum level of the zonally averaged heating lowers due to the suppression of deep penetrative cumulus and the increase in both the middle-level convection and large-scale condensation. The maximum amplitude level of the heating associated with the equatorial intraseasonal oscillation also lowers from 300mb for *=0 to 500mb for *=0.1-*. These changes seem to provide favorable conditions for the occurrence of the equatorial intraseasonal oscillation, in agreement with linear stability studies of a CISK model.

Some climatological information from 14 atmospheric general circulation models is presented and compared in order to assess the ability of a broad group of models to simulate current climate. The quantities considered are cross sections of temperature, zonal wind, and meridional stream function together with latitudinal distributions of mean sea level pressure and precipitation rate. The nature of the deficiencies in the simulated climates that are common to all models and those which differ among models is investigated; the general improvement in the ability of models to simulate certain aspects of the climate is shown; consideration is given to the effect of increasing resolution on simulated climate; and approaches to understanding and reducing model deficiencies are discussed. The information presented here is a subset of a more voluminous compilation which is available in report form (Boer et al., 1991). This report contains essentially the same text, but results from all 14 models are presented together with additional results in the form of geographical distributions of surface variables and certain difference statistics.

An intercomparison is undertaken of the tropical behavior of 17 coupled ocean-atmosphere models in which at least one component may be termed a general circulation model (GCM). The aim is to provide a taxonomy--a description and rough classification-of behavior across the ensemble of models, focusing on interannual variability. The temporal behavior of the sea surface temperature (SST) field along the equator is presented for each model, SST being chosen as the primary variable for intercomparison due to its crucial role in mediating the coupling and because it is a sensitive indicator of climate drift. A wide variety of possible types of behavior are noted among the models. Models with substantial interannual tropical variability may be roughly classified into cases with propagating SST anomalies and cases in which the SST anomalies develop in place. A number of the models also exhibit significant drift with respect to SST climatology. However, there is not a clear relationship between climate drift and the presence or absence of interannual oscillations. In several cases, the mode of climate drift within the tropical Pacific appears to involve coupled feedback mechanisms similar to those responsible for El Nifio variability. Implications for coupled-model development and for climate prediction on seasonal to interannual time scales are discussed. Overall, the results indicate considerable sensitivity of the tropical coupled ocean-atmosphere system and suggest that the simulation of the warm-pool/cold-tongue configuration in the equatorial Pacific represents a challenging test for climate model parameterizations.

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