There is mounting evidence of the importance of the transports of heat and freshwater by the North Atlantic Subpolar Gyre for impacts on European and global climate. The principal objective of the Overturning in the Subpolar North Atlantic Program (OSNAP) is to provide a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic and relate those to water-mass transformations and AMOC variability. This seminar will focus on the results from the UK led Eastern Boundary (EB) array, which is designed particularly to quantify the flux of northward-flowing warm and saline water through the Rockall Trough and across the Rockall Plateau.
Transports over the Rockall Plateau are estimated from sixteen glider sections. Absolute transport is computed from the baroclinic shear and the depth average current measured by the gliders. The mean northward transport in the upper 1000m appears to be stronger (6.7 +/- 2.6 Sv) in summer (May to October), and accounts for 45\% of the total NAC transport of upper-ocean waters (σ0 < 27.55 kg.m-3) estimated by Sarafanov et al.  along 59.5°N. Two quasi-permanent northward-flowing branches of the NAC are identified: (i) the Hatton Bank Jet (6.3 +/- 2.1Sv) and (ii) the Rockall Bank Jet (1.5 +/- 0.7Sv). In addition, a southward flow of 1.1 +/- 1.4 Sv is observed over the western flank of the Hatton-Rockall Basin.
In the Rockall Trough, transports at the boundaries are estimated from direct current measurements, whilst fluxes in the interior are calculated from end-point dynamic height moorings. We discuss the limitations of satellite absolute dynamic topography to reference the mid-basin geostrophic shear. We choose to reference our geostrophic velocities to the bottom (1800m), as no net transport is expected at that depth due to a close bathymetric contour. The resulting transports are the first continuous, direct measurements for the warm water path through the Rockall Trough. The 3-year mean transport (3.8 +/- 0.5 Sv) is similar to previous hydrographic estimates based on several decades of observations. The majority of the transport occurred in mid-basin, in the upper 1000m. One unexpected result is the high variability of the transport across the Rockall Trough (standard deviation of 4.1 Sv, minimum of -5.6 Sv and maximum of +12.9 Sv). Using complementary datasets (Argo float trajectories, altimetry), we suggest that this highly variable circulation could be linked to significant exchanges occurring between the Iceland Basin and the Rockall Trough in the Banks region.