DECADAL VARIABILITY OF THE NORTH-ATLANTIC IN AN OCEAN GENERAL-CIRCULATION MODEL

Climatic fluctuations on a decadal timescale in the North Atlantic in a global ocean general circulation model were considered. The analysis was carried out for the 3800-year stochastic forcing simulation of Mikolajewicz and Maier-Reimer in which the Hamburg Large-Scale Geostrophic ocean model was driven by monthly climatologies of wind stress, air temperature, and freshwater flux with superimposed white noise freshwater fluxes with an amplitude of about 16 mm/month. We applied a Principal Oscillation Pattern analysis to the vector time series of the upper level salinity fields, so that the examined fluctuations appear as estimated eigenmodes of the system. In addition to an oscillation with a period of 320 years as already described by Mikolajewicz and Maier-Reimer, we found a broadband Principal Oscillation Pattern with a timescale of the order of 10 to 40 years. It describes the generation of salinity anomalies in the Labrador Sea and the following discharge into the North Atlantic. In sensitivity experiments we clarified that the source of the variability lies in the Labrador Sea and showed that the generation of the salinity anomalies is mainly due to an undisturbed local integration of the white noise freshwater fluxes.