Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 2 5 2006 10.5194/cpd-2-801-2006 http://www.clim-past-discuss.net/2/801/2006/ http://www.clim-past-discuss.net/2/801/2006/cpd-2-801-2006.html http://www.clim-past-discuss.net/2/801/2006/cpd-2-801-2006.pdf 801 830 2006-09-19 Low-frequency oscillations of the Atlantic Ocean meridional overturning circulation in a coupled climate model M. Schulz M. Prange A. Klocker Department of Geosciences, University of Bremen, Germany DFG Research Center "Ocean Margins", University of Bremen, Germany now at: CSIRO Marine and Atmospheric Research, Hobart, Australia Using a 3-dimensional climate model of intermediate complexity we show that the overturning circulation of the Atlantic Ocean can vary at multicentennial-to-millennial timescales for present-day boundary conditions. A weak and continuous freshwater input into the Labrador Sea pushes the overturning circulation of the Atlantic Ocean into a bi-stable regime, characterized by phases of active and inactive deep-water formation in the Labrador Sea. In contrast, deep-water formation in the Nordic Seas is active during all phases of the oscillations. The actual timing of the transitions between the two circulation states occurs randomly. The oscillations constitute a 3-dimensional phenomenon and have to be distinguished from low-frequency oscillations seen previously in 2-dimensional models of the ocean. A conceptual model provides further insight into the essential dynamics underlying the oscillations of the large-scale ocean circulation. The model experiments indicate that the coupled climate system can exhibit unforced climate variability at multicentennial-to-millennial timescales that may be of relevance for Holocene and future climate variations.