Interannual variability in the pathways of the North Atlantic current over the Mid-Atlantic Ridge and the impact of topography

Recent studies have indicated that the North Atlantic Ocean subpolar gyre circulation undergoes significant interannual-to-decadal changes in response to variability in atmospheric forcing. There are also observations, however, suggesting that the southern limb of the subpolar gyre, namely, the eastward-flowing North Atlantic Current (NAC), may be quasi-locked to particular latitudes in the central North Atlantic by fracture zones (gaps) in the Mid-Atlantic Ridge. This could constrain the current's ability to respond to variability in forcing. In the present study, subsurface float trajectories at 100-1000 m collected during 1997-99 and satellite-derived surface geostrophic velocities from 1992 to 2006 are used to provide an improved description of the detailed pathways of the NAC over the ridge and their relationship to bathymetry. Both the float and satellite observations indicate that in 1997-99, the northern branch of the NAC was split into two branches as it crossed the ridge, one quasi-locked to the Charlie-Gibbs Fracture Zone (CGFZ; 52 degrees-53 degrees N) and the other to the Faraday Fracture Zone (50 degrees-51 degrees N). The longer satellite time series shows, however, that this pattern did not persist outside the float sampling period and that other branching modes persisted for one or more years, including an approximately 12-month time period in 2002-03 when the strongest eastward flow over the ridge was at similar to 49 degrees N. Schott et al. showed how northward excursions of the NAC can temporarily block the westward flow of the Iceland-Scotland Overflow Water through the CGFZ. From the 13-yr time series of surface geostrophic velocity; it is estimated that such blocking may occur on average 6% of the time, although estimates for any given 12-month period range from 0% to 35%.