The sensitivity of the atmospheric seasonal cycle to ocean conditions and solar forcing is investigated using a low-resolution spectral atmospheric general circulation model (GCM). The external forcing of the model is grouped into two basic categories: (1) ocean conditions (sea surface temperature and sea ice extent) and (2) radiation specifications (incoming solar radiation, albedo parameters involving cloud cover and land snow cover, and long-wave emissivity coefficients). These forcing parameters are all prescribed with smoothly time-varying seasonal cycle variations in the control simulation. In two sensitivity experments, parameters in only one of the two forcing categories are allowed to vary seasonally; the others are fixed at their annual mean values. Analysis shows that model sensitivity to radiation specifications is effectively a solar-forcing sensitivity. These experiments show that on a zonally averaged basis at mid-latitudes in the northern hemisphere, the seasonal variations in the direct heating of the atmosphere and land surfaces by incoming solar radiation are generally more important than seasonal variations in the ocean forcing for determining the fundamental annual harmonic variations of temperature, pressure, zonal wind, and precipitation in the model.

In fact, for many of these variables, even in the southern hemisphere, the simulation of the annual harmonics with the radiation forcing alone is not statistically different from simulation of the annual harmonics with the control case. Sea surface temperature and sea ice extent do play an important role in the fundamental harmonic variations in the southern hemisphere for sea level pressure and lower tropospheric temperature on a zonally averaged basis. At tropical latitudes, seasonal variation of sea surface temperature is critical for capturing the fundamental annual harmonic variations of low-troposphere temperatures, sea level pressure, and precipitation, presumably due to the contributions of convective and boundary layer processes. The results presented in zonally averaged, horizontal map and vertical cross-section formats help to describe the variables and specific areas for which the nature of the formulation of the ocean component in an interactive ocean-atmosphere model, especially in terms of its seasonal variation, will be an important consideration for simulation of the atmospheric annual harmonic cycle climatology.