Geography and water masses The study addresses the circulation and water masses of the northern North Atlantic Ocean. The region is dominated by two major circulation patterns (or gyres) of key importance: The Subpolar Gyre and the Subtropical Gyre, indicated as blue and red arrows in Figure G1. These two gyres transport distinct water masses: Warm and saline waters flow with the Subtropical Gyre, while colder and less saline waters are transported by the Subpolar Gyre. This is clearly seen in modeled distribution of ocean salinity (Figure G2) and temperature (Figure G3) in the North Atlantic. The Gulf Stream is the northeastward branch of the Subtropical Gyre. The two water masses flow northeastwards side by side and mix in the northern North Atlantic. It is this mixture of water masses - orange and yellow colours in Figures G2 and G3 - that is the focus of the present study. Figure G1. Schematic illustration of the circulation in the northern North Atlantic. R is the Rockall Trough, F is the Faroe Current, and I is the Irminger Current. From Hátún et al. (2005). Figure G2. Modelled distribution of salinity in the upper waters of the North Atlantic. Red colours show saline waters associated with the Subtropical Gyre, whereas the bluish colours show fresh waters associated with the Subpolar Gyre. [For experts: Shown is the mean salinity averaged over the time period 1997-2002 and over the uppermost 11 layers of the model, i.e. the waters of density less than 1027.52 kg m-3 relative to surface pressure. These are the main water masses that flow into the Nordic Seas in the model]. Figure G3. As Figure G2, but for temperature. References: Hátún, H., A. B. Sandř, H. Drange, B. Hansen, and H. Valdimarsson (2005), Influence of the Atlantic Subpolar Gyre on the Thermohaline Circulation, Science, 309, 1841-1844 [article]