Observed subpolar gyre transports at the Mid-Atlantic Ridge

Abstract

The subpolar gyre of the North Atlantic Ocean is an integral component for the climate relevant oceanic circulation. To measure the variability of the North Atlantic Current (NAC) and thus the strength of the subpolar gyre, an array of four inverted echo sounders with bottom pressure sensors (PIES) was deployed along the Mid-Atlantic Ridge between 47° and 53°N in August 2006. The locations of the individual PIES allow the separation of the main spreading paths of the NAC. The array was deployed at ground track crossing points of altimetry satellites. The PIES delivered daily data that were retrieved by acoustic telemetry each year, while the array remained at the seafloor. The first four year long time series is analysed regarding the transport variability of the NAC and the underlying water masses. The transports are calculated using the Gravest Empirical Mode technique, which has been adapted and tested. The daily uncertainties of transports across the full PIES array are 1.9 Sv. The mean transport across the full array is 29.2 Sv with a standard deviation of 8.6 Sv. Two thirds of the observed mean transport can be assigned to the NAC, while the other third is made up of Labrador Sea Water and a mixture of Denmark Strait Overflow Water and Iceland Scotland Overflow Water. The power spectral density reveals that most of the variability of the barotropic component is between 2 and 20 days. The baroclinic component has its largest energy in the period of 100 to 300 days, which seems to be generated by north-eastward carried eddies. The favoured position of the NAC within the array is corresponding to a crossing of the Mid-Atlantic Ridge at the Faraday and Maxwell Fracture Zones. The surface velocities from altimetry were analysed and used to calculate a baroclinic transport time series for the entire 19 years of satellite measurements. The analysis of the surface velocities confirms the results from the PIES time series and adds an interannual signal to the variability. The preference of the Faraday Fracture Zone as the crossing point for the NAC across the Mid-Atlantic Ridge could be further confirmed by considering the mean and standard deviation of Eddy Kinetic Energy as a direct indicator for the position of the NAC. The 19 year long transport time series of the three subsections present an opposing trend in the two northern sections. No trend for the transport across the full section can be found but only a hint to a slight southward shift of the NAC. The baroclinic mean transport across the full section is 27.6 Sv with a standard deviation of 4.6 Sv. The comparison of the 3-year low-pass filtered satellite based transport time series with the mean Eddy Kinetic Energy (EKE) across the full section and the winter North Atlantic Oscillation (NAO) index proves that the NAO leads the EKE and transports by 2 to 3 years (R = 0.5) and modulates the transport in the range of 6 to 7 Sv (peak-to-peak).