Laboratoire de Météorologie Dynamique, CNRS, Université Pierre et Marie Curie, Paris, France

An atmospheric general circulation model is used to investigate the influence of the North Atlantic Ocean on atmospheric variability. The study covers the period from 1950 to 1994. The observed sea surface temperature and sea ice extension are used to force the atmospheric model. Several configurations of the oceanic boundary conditions were made to isolate the role of the North Atlantic and to study its non-linear interaction with forcings from other oceanic basins. The multi-realization character of the experiments distinguishes between the internal random part and the external forced part of the total variability. The potential predictability can thus be evaluated. The response of the atmosphere is also studied with a modal approach in terms of hemispheric teleconnection patterns. The North Atlantic Ocean has a direct influence on both the Northern Hemisphere annular mode and the Pacific-North-America pattern, leading to a weak predictability. However the direct response is largely modulated by forcings from other oceanic basins. The non-linearity of the system compensates the predictable component of the annular mode induced by the North Atlantic forcing. Furthermore it reduces the forced component of the Pacific-North-America pattern, increasing its chaoticity.

A 1000-year climate simulation is run with the ocean-atmosphere coupled model developed at the Institute Pierre- Simon Laplace (IPSL, Paris). No flux adjustment is used. The drift of the model is analyzed in terms of the seasurface temperature and deep ocean temperature. When the model's own equilibrium is reached, it is found that the Antarctic bottom water production experiences large-amplitude variation, oscillating between strong and weak episodes. This can yield oceanic temperature variation in the Southern Hemisphere and for the global mean.