Boundary current ventilation in the Irminger Sea

The subpolar North Atlantic is a hotspot of deep ocean ventilation. In other words, it is a region where mixed layer waters are oxygenated and then move into the deep interior North Atlantic where they are sheltered from the atmosphere. In this study, we use two years of densely-spaced moored oxygen observations in the Irminger Sea's western boundary current from 2020 to 2022, as well as Biogeochemical Argo floats, to investigate the processes governing ventilation in this region. We find that oxygen transport variability is dominated by upper Irminger Sea Intermediate Water (upper ISIW), a dense subpolar mode water, rather than denser waters formed by deep convection. While some of these waters are within the boundary current's winter mixed layer and are oxygenated by direct air-sea exchange, the denser portion appears to be oxygenated by along-isopycnal exchange with mixed layer waters just offshore of the boundary current. Spice and oxygen variability are independently suggestive of this lateral isopycnal exchange, highlighting the added value of direct oxygen measurements. While the classical view is that springtime re-stratification is what allows mixed layer oxygen to enter the stratified interior, our results suggest that eddy subduction processes in the wintertime could also play an important role in shaping the ventilation of the deep North Atlantic.