The Coldest and Densest Overflow Branch Into the North Atlantic is Stable in Transport, But Warming.

The overflow of cold water through the Faroe Bank Channel (FBC) is the densest water crossing the Greenland‐Scotland Ridge and the densest source for the Atlantic Meridional Overturning Circulation (AMOC). Here, we show that the overflow volume transport remained stable from 1996 to 2022, but that the bottom water warmed at an average rate of 0.1°C per decade, mainly caused by warming of deep waters upstream. The salinity of the overflow water has increased as a lagged and reduced response to the salinity increase seen in the upper‐layer source waters. Therefore, the potential density of the bottom water over the FBC sill shows no statistically significant trend. After entrainment of warmer ambient waters downstream of the FBC, the nonlinear density dependence upon temperature implies, however, that the overflow contributed water of reduced density to the local overturning and the deep limb of the AMOC. Plain Language Summary: As part of the Atlantic Meridional Overturning Circulation—often called AMOC—dense water is formed north of the subsea ridge between Greenland and Scotland. On its route southward at depth, the dense water has to cross this subsea ridge called the Greenland‐Scotland Ridge. The densest water flows through the deepest trench, which is the Faroe Bank Channel with a sill depth of 840 m. Here, the strength of the flow has been monitored since 1995 together with its temperature and salinity characteristics. While the strength of the flow has been stable throughout the monitoring period, the dense water in the Faroe Bank Channel has warmed and also become more saline. Despite the warming, the density of the water, which is determined by the combination of temperature and salinity, has remained nearly unchanged. After leaving the Faroe Bank Channel, the dense water intensively entrains and mixes with warmer surrounding water and forms a larger volume of water, termed Iceland‐Scotland Overflow Water, that feeds the AMOC. The density of this large volume is sensitive to warming. The observed warming of the dense water may, therefore, have implications for the AMOC and thereby on the regional as well as global climate. Key Points: Observed Faroe Bank Channel overflow strength has been stable from 1996 to 2022, but the overflow waters warmed in the recent two decadesConcurrent salinity increase prevented the bottom water density from decreasing significantly from the effect of warmingAfter entrainment downstream, the overflow contributed water of reduced density to global overturning due to non‐linear effects of warming [ABSTRACT FROM AUTHOR]