Influence of entrainment on the mean and seasonal variability in the Denmark Strait Overflow Water (DSOW) properties in the West Irminger Sea

Formed by deep convection in the Nordic Seas, cold, oxygen-rich Denmark Strait Overflow Water (DSOW) is the densest water mass in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). As DSOW spreads southward from the Denmark Strait into the Irminger Sea, the overflow water entrains ambient water masses. This entrainment is critical for the transfer of climate signals and biogeochemical tracers from the surface to the deep ocean. Although the impact of entrainment on DSOW properties in the immediate vicinity of the Denmark Strait (< 100 km downstream of the sill) has been studied extensively, the entrainment dynamics contributing to DSOW transformation further downstream in the Irminger Sea and its influence on variability in DSOW properties remain unclear. Here, we use observations from BGC-Argo floats and moorings along with an idealized numerical model of entrainment mixing to investigate the dynamics contributing to the observed DSOW transformation in the western Irminger Sea. We find that DSOW experiences continual warming and deoxygenation as it spreads along the Deep Western Boundary Current in the Irminger Sea. Furthermore, novel dissolved oxygen measurements from moored instruments offshore of Cape Farewell, Greenland reveal seasonality in the oxygen content of DSOW for the first time. Numerical model simulations of supercritical entrainment dynamics predict DSOW properties offshore Cape Farewell that are cooler and more oxygenated than in the observations. This disagreement suggests that sub-critical entrainment influences DSOW transformation in the western Irminger Sea, which we test by accounting for this process in the model. Model sensitivity experiments further suggest that the observed seasonal variation in the oxygen content of DSOW likely originates from the source overflow water upstream of the Denmark Strait. Overall, this work is the first to explore the entrainment dynamics of DSOW in the western Irminger Sea and its influence on the oxygen content of the overflow water, with implications for the response of the AMOC structure and dissolved gas uptake to changes in the climate system.