Shifting surface currents in the northern North Atlantic Ocean

Analysis of surface drifter tracks in the North Atlantic Ocean from the time period 1990 to 2007 provides evidence that warm subtropical waters have recently increased their penetration toward the Nordic seas. Prior to 2000, the warm water branches of the North Atlantic Current fed by the Gulf Stream turned southeastward in the eastern North Atlantic. Since 2001, these paths have shifted toward the Rockall Trough, through which the most saline North Atlantic waters pass to the Nordic seas. These surface drifters are able to overcome the Ekman drift, which would force them southward under the westerly winds dominating the subpolar Atlantic, yet the changes in path cannot be accounted for by changes in Ekman drift. Eddy kinetic energy from satellite altimetry shows increased energy along the shifted drifter pathways across the Mid-Atlantic Ridge since 2001. These near-surface changes have occurred during continual weakening of the North Atlantic subpolar gyre, as seen by altimetry. They are also consistent with the observed increase in temperature and salinity of the waters flowing northward into the Nordic seas. These findings suggest the changes in the vertical structure of the northern North Atlantic Ocean, its dynamics, and exchanges with the higher latitudes. Wind stress and its curl changes are discussed as a possible forcing of the changes in the pathways of the subtropical waters.