The North Atlantic subpolar gyre experienced strong freshening in recent years starting around 2012. Here, we investigate the imprint of this freshwater anomaly on the water column hydrography and transport variability of the Irminger Current (IC). The IC transports warm and saline waters northward along the western flank of the Reykjanes Ridge as part of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). To investigate if the salinity anomaly spread and propagated downward, we used high-resolution mooring data from the IC covering the period 2014 – 2022 combined with hydrographic sections from the Irminger Sea and Iceland Basin. We found that the IC experienced a strong freshening starting in summer 2016. By 2018, this salinity anomaly covers the whole water column down to 1500 m depth and freshened the IC until 2022. In 2022, the IC was at its freshest state observed since the early 1990’s. Hydrographic sections across the adjacent basins showed that the recent freshening spread across the Irminger Sea and was also comparable to its fresh state in the early 1990’s. The salinity anomaly increased the freshwater transport of the IC by a factor of three from 2014-2015 to 2021-2022 and caused a decrease in density over much of the water column. This resulted in an increase in the transport of waters lighter than 27.55 kg m-3, potentially strengthening the upper limb of the AMOC.

The subpolar North Atlantic is a site of significant carbon dioxide, oxygen, and heat exchange with the atmosphere. This exchange, which regulates transient climate change and prevents large-scale hypoxia throughout the North Atlantic, is thought to be mediated by vertical mixing in the ocean's surface mixed layer. Here we present observational evidence that waters deeper than the conventionally defined mixed layer are affected directly by atmospheric forcing. When northerly winds blow along the Irminger Sea's western boundary current, the Ekman response pushes denser water over lighter water and triggers slantwise convection. We estimate that this down-front wind forcing is four times stronger than air--sea heat flux buoyancy forcing and can mix waters to several times the conventionally defined mixed layer depth. Slantwise convection is not included in most large-scale ocean models, which likely limits their ability to accurately represent subpolar water mass transformations and deep ocean ventilation.