Theseasurfacesalinity (SSS)variability in theNorth Atlantic is investigatedusing numericalmodelsimulationsfor the last 50 yearsbasedon atmosphericforcing variability from ComprehensiveAtmosphereOcean Data Set (COADS) and National Center for EnvironmentalPrediction

Theseasurfacesalinity (SSS)variability in theNorth Atlantic is investigatedusing numericalmodelsimulationsfor the last 50 yearsbasedon atmosphericforcing variability from ComprehensiveAtmosphereOcean Data Set (COADS) and National Center for EnvironmentalPrediction / National Center for Atmospheric Research (NCEP/NCAR) Reanalysis. The largest interannual and longer term variability occurs in two regions: the Labrador Sea and the North Equatorial Countercurrent (NECC) region. In both regions the seasonality of the surface salinity variability is prominent with the maximum standard deviation occurring in the summer/fall period. In the Labrador Sea the summer SSS anomalies far exceed those of wintertime in amplitude. The interannual SSS variability in the subpolar gyre can be attributed to two factors: excess ice melt and heat flux (i.e. deep mixing) variations. On the other hand, heat flux variability can also lead to meridional overturning changes on decadal time scales such that weak overturning is manifested in fresh surface conditions in the subpolar gyre. The overturning changes also influence the NECC region SSS variability. Moreover, the subpolar freshening events are expected to occur during the negative phase of North Atlantic Oscillation which is associated with a weak wintertime surface heat loss in the subpolar gyre. No excess sea ice melt or precipitation is necessary for the formation of the fresh anomalies, because with the lack of wide-spread deep mixing, the fresh water that would be expected based on climatology, would accumulate at the surface. Thus, the fresh water 'conveyor' in the Atlantic operates via the overturning circulation such that deep mixing inserts fresh water while removing heat from the water column. 1. lntro_duction North of 40°N and south of 40°S, there is net precipitation over evaporation. This fact must be reconciled in the closure of the global salinity budget to understand the surface salinity distribution and its variability. Assuming that the total fresh water resources are nearly in equilibrium, i.e. no major accumulation or depletion over the ice sheets occurs, the question arises how does this 'excess' freshwater return to the lower latitudes where it can again evaporate and be transported by the a_osphere to the higher latitudes ? In the North Adantic there are no major surface ocean cu_ents that transport the cold and fresh waters from the polar/subpolar oceans to the latitudes around 20N where most of the evaporation takes place. Certainly, the 'fresh' coastal current along the eastern seaboard in the North Atlantic cannot penetrate southward due to the blockage by the Gulf Stream. During summertime subpolar waters are known to Sp? ead southward in the north-eastern side of the basin and to be eventually mixed down and Subducted in the following winter (Bauer et al. 199 I). Thus, no effective surface route exists for southward fresh water transport, so the only route back to the lower latitudes has to take place through the meridional circulation. The subpolar/polar regions are source regions for the intermediate and deep/bottom waters, thus a part of the fresh water route back to global system coincides with the thermohaline circulation. In the present climate the conversjqn of light water to bottom water in the North Atlantic occurs mainly through heat loss rendering the opposing fresh water 0aaline) flux less important. Wide spread fresh anomalies in the surface salinity in the northern North Atlantic have been occurring nearly once a decade (Reverdin et. al. 1997, Belkin et al. 1998). Individual 'fresh' events vary considerably from location to another, but by far the largest and most wide spread occurrence was the Great Salinity Anomaly (GSA) at the end of 1960's and early 1970's (Dickson et al 1988), Another large event took place in the early 1980' (Belkin et al. 1998). These events sere as a reminder about the delicate balance between the thermal and haline component of the surface buoyancy flux. Dickson et al 1988