The internal climate variability of HadCM3, a version of the Hadley Centrecoupled model without flux adjustments

We examine the internal climate variability of a 1000 year long integration of the third version of the Hadley Centre coupled model (HadCM3). The mode l requires no flux adjustment, needs no spin up procedure prior to coupling and has a stable climate in the global mean. The principal aims are (I) to validate the internal climate variability against observed climate variabi lity, (2) to examine the model for any periodic modes of variability, (3) t o use the model estimate of internal climate variability to asses the proba bility of occurrence of observed trends in climate variables, and (4) to co mpare HadCM3 with the previous version of the Hadley Centre model, HadCM2. The magnitude and frequency characteristics of the variability of the globa l mean surface temperature of HadCM3 on annual to decadal time scales is in good agreement with the observations. Observed upward trends in temperatur e over the last 20 years and longer are inconsistent with the internal vari ability of the model. The simulated spatial pattern of surface temperature variability is qualitatively similar to that observed, although there is an overestimation of the land temperature variability and regional errors in ocean temperature variability. The model simulates an El Nino Southern Osci llation with an irregular 3-4 year cycle, and with a teleconnection pattern which is much more like the observations than was found in HadCM2. The int erdecadal variability of the model ocean in the tropical Pacific, North Pac ific and North Atlantic is broadly similar to that in the real world with n one of the simulated patterns having any periodic behaviour. HadCM3 simulat es an Arctic Oscillation/North Atlantic Oscillation (NAO) in Northern Hemis phere winter which has a spatial pattern consistent with the observations i n the Atlantic region, but has too much teleconnection with the North Pacif ic. The recent observed upward trend in the NAO index is inconsistent with the model internal variability. The variability of the simulated zonal mean atmospheric temperature shows some marked differences to the observed zona l mean temperature variability, although the comparison is confounded by th e sparse observational network and its possible contamination by a climate change signal.