Internal variability, external forcing and climate trends in multi-decadal AGCM ensembles

An atmospheric general circulation model of intermediate complexity is used to investigate the origin and structure of the climate change in the second half of the twentieth century. The variability of the atmospheric flow is considered as a superposition of an internal part, due to intrinsic dynamical variability, and an external part, due to the variations of the sea surface temperature (SST) forcing. The two components are identified by performing a 50-member ensemble of atmospheric simulations with prescribed, observed SSTs in the period 1949 - 2002. The large number of realizations allows the estimation of statistics of the atmospheric variability with a high confidence level. The analysis performed focuses on interdecadal and interannual variability of 500 hPa geopotential height in the Northern Hemisphere (NH) during winter. The model reproduces well the structure of the observed trend (defined as the difference in the two 25-year intervals 1977 - 2001 and 1952 - 1976), particularly in the Pacific region, and about half of the amplitude of the signal. The trend in 500 hPa height projects mainly onto the second empirical orthogonal function (EOF), both in the observations and in the model ensemble. However, differences between the modelled and the observed variability are found in the pattern of the second EOF in the Atlantic sector. SST changes associated with the El Nino southern oscillation (ENSO) are responsible for about 50% of the signal of the 500 hPa height trend in the Pacific. A second 50-member ensemble is used to evaluate the sensitivity of interdecadal variability to an increase in CO2 optical depth compatible with observed concentration changes. In this second experiment, the simulated trend includes a statistically significant contribution from the positive phase of the Arctic oscillation (AO). Such a contribution is also found in observations. Furthermore, the additional CO2 forcing accounts for part of the NH trend in near-surface temperature, and brings the zonal-mean temperature changes in the stratosphere and uppertroposphere closer to observations.