Rapid transitions and ultra-low frequency behaviour in a 40?kyr integration with a coupled climate model of intermediate complexity

 A 40 kyr integration with the coupled atmosphere/ocean/sea-ice model of intermediate complexity ECBilt for present boundary conditions has been performed. The climate of ECBilt displays quasi-periodical behaviour with a period of approximately 13 kyr. The quasi-periodical behaviour is characterized by large changes in the overturning cell in the Southern Ocean. The southern cell fluctuates between two quasi-stationary states, with accompaning changes in the atmospheric circulation in the Southern Hemisphere. The transition between these states is rapid and resembles the polar halocline catastrophes and flushes as observed in ocean general circulation models under mixed boundary conditions. The sea-ice influence on both the surface heat and fresh water flux appears to be crucial for the existence and the prolongation of the quasi-stationary states. The atmospheric circulation of those two quasi-stationary states displays large regional differences over Antarctica, resulting in even opposite surface air temperature trends for certain locations during the transition from one state to another. Received: 7 October 1999 / Accepted: 28 August 2000