Circulation and variability of the North Atlantic Current in the vicinity of the Mid-Atlantic Ridge

Abstract

Physical observations have been used to describe the circulation and water masses during a cruise whose primary objective was to study the effect of the Mid-Atlantic Ridge on benthic ecosystems at four sites along and across the Ridge between 49°N and 54°N. The cruise observations have been expanded using Argo floats and 16 years of satellite altimeter data to examine temporal variability over months to years. Water masses and transports are consistent with prior observations, showing subtropical water with large mesoscale eddy variability but weak mean eastward flow at the southern sites and cooler, subpolar water with low eddy variability and weak mean westward flow at the northern sites north of the Charlie Gibbs Fracture Zone. An interesting feature was high salinity bottom water at the southern sites, indicating a contribution of Iceland–Scotland Overflow Water mixed southwards through the central rift of the Mid-Atlantic Ridge. Another feature was that 27 Sv (1 Sv=10 6 m 3 s −1) eastward transport was mostly in the top 500 m of the water column except at the latitudes of the Charlie Gibbs Fracture Zone where two-thirds of the transport was deeper than 500 m. An intrusion of cold water extended 300 km southwards along a CTD section that paralleled the Mid-Atlantic Ridge, resulting in strong currents over the southern sites. The cold intrusion was flanked by a similar intrusion of warm water extending northwards. Each intrusion was present along the line of the section for about 6 months, controlling the pathway of 10 Sv of transport. Altimeter data reveal many similar intrusions, consisting of or spinning off mesoscale eddies which are slow moving and long lasting, often remaining identifiable near one vicinity for a year or more. Descriptions of the North Atlantic Current as a current with several branches which shift from time to time are a consequence of these long lasting, slowly changing eddies. Thus the North Atlantic Current is best described as a train of near-stationary eddies and meanders with the transport pathways and branching controlled by the slowly varying eddies.