Recent modulation of the seasonal cycle

Changes in the nature of the seasonal cycle of temperature are analysed by fitting sinusoidal waves (Wilks, 1995) to observed and model-simulated time series. Previous studies have revealed a recent decrease in the seasonal temperature range and a variety of modifications to the phase of the annual cycle (Thompson, 1995; Mann and Park, 1996). Many observed modifications to the seasonal cycle are inconsistent with model projections forced with an enhanced greenhouse effect. This study extends earlier work by (i) using the global and hemispheric temperature series of Jones et al. (1999) updated to 1999; (ii) using climate model simulations that also have changing sulphate aerosol forcing; and (iii) using ensembles of climate model simulations to better identify the signal of climate change. Comparisons are made between Northern and Southern hemispheric changes, with respect to the observed data and model predictions. Conclusions are drawn regarding potential forcing mechanisms and model accuracy.