Evolution and formation of North Atlantic Eighteen Degree Water in the Sargasso Sea from moored data

Two profiling subsurface moorings were deployed as part of the CLIvar MOde Water Dynamics Experiment (CLIMODE) to study the formation and evolution of Eighteen Degree Water (EDW) from November 2006 to November 2007. Both moorings were deployed south of the Gulf Stream in the EDW outcrop region, the northwestern part of the subtropical gyre of the North Atlantic. The two moorings captured the seasonal evolution of EDW characterized by gradual mixed layer deepening and wintertime outcrop, rapid restratification from May to June and slower dissipation during the rest of the year. Superimposed on this seasonal cycle, the moored records are characterized by high frequency passing of eddies with a characteristic time scale of ∼10 days, i.e. it took about 10 days for eddies to pass the mooring sites. The net impact of these eddy fluxes is evaluated by analyzing one-dimensional heat and salt budgets of the upper ocean at the moorings and comparing them to the local air–sea fluxes. It is shown that oceanic lateral fluxes converge heat and salt into the formation region during winter thus offsetting the heat loss to the atmosphere and influencing the formation of EDW. A comparison with results from a one-dimensional model shows that without the lateral fluxes EDW would outcrop earlier and it would be colder and fresher. The warm, salty waters transported into the region originate from the Gulf Stream and this suggests that frontal processes likely play a fundamental role in EDW formation and its evolution.