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 Ocean. This branch (IF-inflow) carries almost 50 % of the total Atlantic water inflow. After crossing the ridge, a large part of this water is converted to dense water that returns to the Atlantic as cold overflow, which contributes to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). A better understanding of the temperature and salinity variations of the IF-inflow is therefore important for assessing both regional and global climate. Based on satellite altimetry and drifter observations, we document the pathway of the Atlantic water through the Iceland Basin to the IFR and show how this pathway is affected by variable intrusion of the Subpolar Gyre into the basin with a narrower, faster and southward-shifted pathway during periods of strong Subpolar Gyre intrusion. The variable intrusion is furthermore shown to be the main cause of the long-term salinity variations upstream as well as downstream of the IFR while the temperature has increased due to the general warming of the oceans in addition to the variations caused by variable intrusion. 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. We show that this transformation 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 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 freshening across the ridge 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 downstream. 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 %.