Multidecadal and NAO related variability in a numerical model of the North Atlantic circulation

Abstract

The natural variability of the North Atlantic and Arctic Ocean is still not completely understood. Especially on interannual to multidecadal time scales, where the amount of bservations is limited, many open questions remain. The objective of this study is therefore to investigate the reaction of a numerical model of the North Atlantic and Arctic Ocean to changes in the atmospheric surface forcing and to improve the understanding of the internal variability of the model and furthermore the real ocean-sea ice system. The ocean model is based on the MOM-2 code, which is coupled to a dynamic-thermodynamic sea ice model with a viscous-plastic rheology. A major mode of atmospheric variability over the North Atlantic Ocean is the North Atlantic Oscillation (NAO). Two 200 year integrations of the coupled ocean-sea ice model with surface boundary conditions which are related to periods of positive and negative NAO are compared to an integration with climatological atmospheric forcing over thesame number of years. The experiment with climatological forcing reveals a self-sustaining oscillation, with a period of 40 years. The interplay between the Deep Western Boundary Current (DWBC) and Gulf Stream is important to sustain the oscillation. A switch from a long time NAO state to an enduring NAO- situation is also investigated in this study. The sea ice extent increases quickly after this switch, whereas the changes in sea ice volume adapt slower to the changed forcing. The ocean adjusts with a fast barotropic circulation anomaly, accompanied by an enhancement of meridional overturning and northward heat transport at 48sup:o:/sup:N. The slow response is a substantial decrease of the northward heat transport, which is caused by a reduction of the strength of subpolar and subtropical gyres. One year of NAO- forcing shifts the area of convection in the Labrador Sea to a different position, which turns out to be a new state of equilibrium for the ocean.