Greenhouse warming and changes in the seasonal cycle of temperature: Model versus observations

Thomson [1995] argues that an enhanced green-house effect may be altering the seasonal cycle in temperature. We compare trends in the amplitude and phase of the seasonal cycle in observational temperature data in the northern hemisphere with the response of two general circulation models to increased CO₂ concentrations. Sizeable amplitude decreases are observed in both models and observations. Significant phase delays (ie, later seasonal transitions) are found in the simulations, opposite to the phase advances isolated in the observations. The retreat of winter sea ice in high-latitude regions appears to explain the models' response to CO₂ increase. Much of the variability in the observational data is not predicted by the models.