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GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L20708, doi:10.1029/2005GL024233, 2005

A signature of persistent natural thermohaline circulation cycles in observed climate

Jeff R. Knight

Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, UK


Robert J. Allan

Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, UK


Chris K. Folland

Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, UK


Michael Vellinga

Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, UK


Michael E. Mann

Department of Meteorology and Earth and Environmental Systems Institute (ESSI), Pennsylvania State University, University Park, Pennsylvania, USA


Abstract

Analyses of global climate from measurements dating back to the nineteenth century show an ‘Atlantic Multidecadal Oscillation’ (AMO) as a leading large-scale pattern of multidecadal variability in surface temperature. Yet it is not possible to determine whether these fluctuations are genuinely oscillatory from the relatively short observational record alone. Using a 1400 year climate model calculation, we are able to simulate the observed pattern and amplitude of the AMO. The results imply the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation (THC). This relationship suggests we can attempt to reconstruct past THC changes, and we infer an increase in THC strength over the last 25 years. Potential predictability associated with the mode implies natural THC and AMO decreases over the next few decades independent of anthropogenic climate change.

Received 1 August 2005; accepted 22 August 2005; published 25 October 2005.

Index Terms: 1616 Global Change: Climate variability (1635, 3305, 3309, 4215, 4513); 1626 Global Change: Global climate models (3337, 4928); 3339 Atmospheric Processes: Ocean/atmosphere interactions (0312, 4504); 4513 Oceanography: Physical: Decadal ocean variability (1616, 1635, 3305, 4215); 9325 Geographic Location: Atlantic Ocean.

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Citation: Knight, J. R., R. J. Allan, C. K. Folland, M. Vellinga, and M. E. Mann (2005), A signature of persistent natural thermohaline circulation cycles in observed climate, Geophys. Res. Lett., 32, L20708, doi:10.1029/2005GL024233.