CLEAR-SKY GREENHOUSE-EFFECT SENSITIVITY TO SEA-SURFACE TEMPERATURE-CHANGES - AN EVALUATION OF AMIP SIMULATIONS

Seasonal and interannual variations of the clear-sky greenhouse effect (G) are analysed for 10 general circulation models (GCMs) participati ng in the atmospheric model intercomparison project (AMIP). Results ar e compared to ERBE observations between 1985 and 1988. In order to tra ce the source of G variability, we also study modelled variability of the precipitable water content (W) and compare the results to ISCCP/TO VS and SSM/I estimates. For a subset of models, the variability of tem perature and water vapour profiles are also examined and compared to E CMWF analyses and NCEP/NCAR reanalyses for the year 1988. Large differ ences between models and a bias relative to observations are pointed o ut for annual mean values of W and G as well as for their seasonal and interannual variabilities. The model discrepancies are larger in the tropics. In these regions, there is a dry bias in all the GCMs conside red relative to both ISCCP/TOVS and SSM/I and factors of 2 or 3 are fo und in the variability of G and W compared to observations. The GCMs c onsidered tend to overestimate the seasonal variation of G in middle l atitudes and to underestimate it in the tropics. The opposite is found for interannual variations. Inspection of vertical temperature and re lative humidity profiles reveals that the origin of the clear-sky gree nhouse effect variability can be very different from one model to anot her.