Results from a 1400 year calculation using the HadCM3 coupled climate model without external forcings are used to show the existence of a quasi-periodic mode of internal climate variability with a characteristic time scale of about 100 years. This mode is manifested in the modelled thermohaline circulation (THC), defined as the peak meridional overturning streamfunction at 30N, which shows multidecadal variations in strength of about 2 Sv (10%) with a coherent phase evolution for periods of up to 50 years. Additionally, global, Northern Hemisphere and North Atlantic mean surface temperatures are found to vary in phase with the THC, with regressions of 0.05 degrees C/Sv, 0.09 degrees C/Sv and 0.12 degrees C/Sv respectively, suggesting that the mode is significant compared with twentieth-century climate variability. An influence of the THC-related mode on surface temperature in broad regions of the northern hemisphere is shown, particularly in the North Atlantic, Pacific and Asia. Wind and rainfall effects, notably in north-east Brazil and the Sahel, are also modulated.

In the natural climate system, a lack of sub-surface marine monitoring prevents knowledge of possible past fluctuations in THC strength, constraining us to use coupled ocean-atmosphere models to make inferences about its potential behaviour and its climate linkages. The results of this study lend weight to the hypothesis that the `Atlantic Multidecadal Oscillation' is a genuine quasi-cyclical climate phenomenon related to large-scale oceanic heat transport variations. The existence of such a mode would imply the potential for predicting climate variations for several decades into the future. In contrast, we also note that this THC-climate mode would imply uncertainty for predictions of future climate change. Additionally, we find that SST changes in the last two decades are consistent with THC strengthening, rather than the weakening commonly reported in model simulations of climate change or inferred from local marine observations.