A DECADAL CLIMATE CYCLE IN THE NORTH-ATLANTIC OCEAN AS SIMULATED BY THE ECHO COUPLED GCM

In this paper a decadal climate cycle in the North Atlantic that was d erived from an extended-range integration with a coupled ocean-atmosph ere general circulation model is described. The decadal mode shares ma ny features with the observed decadal variability in the North Atlanti c. The period of the simulated oscillation, however, is somewhat longe r than that estimated from observations. While the observations indica te a period of about 12 yr, the coupled model simulation yields a peri od of about 17 yr. The cyclic nature of the decadal variability implie s some inherent predictability at these timescales. The decadal mode i s based on unstable air-sea interactions and must be therefore regarde d as an inherently coupled mode. It involves the subtropical gyre and the North Atlantic oscillation. The memory of the coupled system, howe ver, resides in the ocean and is related to horizontal advection and t o the oceanic adjustment to low-frequency wind stress curl variations. In particular, it is found that variations in the intensity of the Gu lf Stream and its extension are crucial to the oscillation. Although d iffering in details, the North Atlantic decadal mode and the North Pac ific mode described by M. Latif and T. P. Barnett are based on the sam e fundamental mechanism: a feedback loop between the wind driven subtr opical gyre and the extratropical atmospheric circulation.