Influence of seasonal variation upon the sensitivity of a model climate

This study investigates the influences of the seasonal variation of solar radiation based upon the results of numerical experiments with a mathematical model of climate. The model consists of (1) a general circulation model of the atmosphere, (2) a heat‐and water‐balance model of continents, and (3) a simple mixed layer model of the ocean. It has a limited computational domain and idealized geography. Two versions of the model are constructed. In the first version of the model (the seasonal model), a seasonal variation of insolation is imposed at the top of the model atmosphere. On the other hand, an annual mean insolation is prescribed for the second version of the model (the annual model). The response of the seasonal model to the quadrupling of CO2‐concentration in air is compared to the corresponding response of the annual model. It is found that the response of the annual mean surface air temperature of the seasonal model is significantly less than the corresponding response of the annual model. The smaller sensitivity of the seasonal model is attributed to the absence of strongly reflective snow cover (or sea ice) during the summer when the insolation has a near‐maximum intensity. A comparison between the hydrologic responses of the seasonal and the annual models indicates that the latitudinal distributions of these responses have qualitatively similar zonal mean features. However, the zonal mean response of the seasonal model is found to have considerable seasonal variations. For example, in summer the zonal mean soil wetness is reduced extensively over two separate zones of middle and high latitudes in response to the CO2 increase, respectively. Owing to the seasonal variation mentioned above, the latitudinal variation of the annual mean hydrologic response of the seasonal model is less than that of the corresponding response of the annual model.