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The northern limb of the Atlantic thermohaline circulation and itstransport of heat and salt towards the Arctic strongly modulate the climateof the Northern Hemisphere. The presence of warm surface waters prevents iceformation in parts of the Arctic Mediterranean, and ocean heat is directlyavailable for sea-ice melt, while salt transport may be critical for thestability of the exchanges. Through these mechanisms, ocean heat and salttransports play a disproportionally strong role in the climate system, andrealistic simulation is a requisite for reliable climate projections. Acrossthe Greenland–Scotland Ridge (GSR) this occurs in three well-definedbranches where anomalies in the warm and saline Atlantic inflow across theshallow Iceland–Faroe Ridge (IFR) have been shown to be particularlydifficult to simulate in global ocean models. This branch (IF-inflow) carriesabout 40 % of the total ocean heat transport into the ArcticMediterranean and is well constrained by observation during the last 2decades but associated with significant inter-annual fluctuations. Theinconsistency between model results and observational data is here explainedby the inability of coarse-resolution models to simulate the overflow acrossthe IFR (IF-overflow), which feeds back onto the simulated IF-inflow. Ineffect, this is reduced in the model to reflect only the net exchange acrossthe IFR. Observational evidence is presented for a substantial and persistentIF-overflow and mechanisms that qualitatively control its intensity. Throughthis, we explain the main discrepancies between observed and simulatedexchange. Our findings rebuild confidence in modelled net exchange across theIFR, but reveal that compensation of model deficiencies here through otherexchange branches is not effective. This implies that simulated ocean heattransport to the Arctic is biased low by more than 10 % and associatedwith a reduced level of variability, while the quality of the simulated salttransport becomes critically dependent on the link between IF-inflow andIF-overflow. These features likely affect sensitivity and stability ofclimate models to climate change and limit the predictive skill

Biased thermohaline exchanges with the Arctic across the Iceland–Faroe Ridge in ocean climate models