Causes of Changes in the Denmark Strait Overflow

The warming Nordic seas potentially tend to decrease the overflow across the Greenland–Iceland–Scotland Ridge (GISR) system. Recent observations by Macrander et al. document a significant drop in the intensity of outflowing Denmark Strait Overflow Water of more than 20% over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0.4°C. A simulation of the exchange across the GISR with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the Denmark Strait overflow and its relations to changed forcing condition. On seasonal and longer time scales, the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill. On annual and shorter time scales, the wind stress curl around Iceland determines the barotropic circulation around the island and thus the barotropic flow through Denmark Strait. For the overlapping time scales, the barotropic and overflow component interactively determine transport variations. Last, a relation between sea surface height and reservoir height changes upstream of the sill is used to predict the overflow variability from altimeter data. Estimated changes are in agreement with other recent transport estimates based on current-meter arrays.