Water mass modification of the warm Atlantic Inflow towards the Arctic across the Iceland-Faroe Ridge

Abstract. The warm Atlantic inflow across the Iceland-Faroe Ridge (IFR) is the strongest of the three branches carrying warm water to the Nordic Seas and further into the Arctic. This branch (IF-inflow) carries almost 50 % of the total Atlantic water inflow. The Atlantic water crossing the IFR mainly flows through the Iceland Basin with no indication of a significant contribution from the Rockall Trough. The salinity variations of the Atlantic water approaching the IFR are well explained as a delayed response to the intrusion of the Subpolar Gyre into the Iceland Basin while the temperature variations may be seen as a combined response to the gyre and global warming. The flow corridor feeding the IFR with Atlantic Water gets narrower and migrates southeastward to the rim of the Iceland Basin when the Subpolar Gyre extends into the basin. As the Atlantic water crosses the IFR, it is cooled by at least 1 °C and freshened by at least 0.1 g kg^-1. Satellite-tracked drifters that crossed the IFR exhibited prolonged residence times on the western flank of the IFR together with a high level of eddy kinetic energy. This indicates that transformation occurs in this area. We show that transformation of the water mainly is caused by mixing with Arctic water masses, rather than by air-sea-interactions, and that most of the modification takes place upstream of the ridge crest. The Atlantic water changes character from an almost barotropic to a much more baroclinic flow over and northeast of the ridge enabling water masses, otherwise constrained to follow isobaths, to cross the ridge. We find that the cooling and freshening of the Atlantic water across the IFR are relatively constant throughout the whole period from 1993 to 2023, except for the last few years. The associated freshening implies that the salinity difference between IF-inflow water and the deep waters northeast of the ridge has been reduced by roughly 20–30 % after crossing the ridge, which weakens the potential for dense-water formation. Updated transport estimates show a slight strengthening for this AMOC branch. From 1993 to 2023, the volume transport of the IF-inflow increased by (12±7) %, while the heat transport relative to 0 °C increased by (16±8) %.