Modelling the Denmark Strait overflow during the last interglacial cycle : from regional dynamics to basin wide impacts

The Denmark Strait overflow is the major gateway for dense water from the Nordic Seas into the North Atlantic and thus it constitutes an important part of the thermohaline circulation under present-day climate conditions. This study focuses on the regional to basin wide dynamics of the Denmark Strait overflow for present-day and Last Glacial Maximum conditions. It contributes to the Research Unit Ocean Gateways located at the University of Kiel which addresses the question how ocean gateways might have affected climate in the past.The volume transport of the overflow is likely to be controlled by hydraulic constraints, thus the transport is set by the relative density contrast between the water masses north and south of the Denmark Strait and the height of dense water above sill level. By using high resolution numerical models it is investigated how the volume transport can be best described using existing hydraulic theories. Hydraulic constraints are shown to be valid over a wide parameter range which allows to transfer this theory to LGM conditions. During the LGM both bathymetry and hydrography have been different and the relative importance of the individual effects is disentangled using a high-resolution numerical model. The drop in eustatic sea-level and glaciated shelves in this area had a comparable smaller effect than the reduced density contrast. In addition to buoyancy forcing wind stress might play a role in setting the overflow transport. In a regional model of the Greenland-Scotland Ridge an influence of wind stress on the overflow can be shown. Therefore, wind stress changes due to different phases of the NAO are expected to imprint a signal on the overflow transport. The same holds for wind field changes from Last Glacial Maximum to present-day conditions. Even though the wind stress was increased the density difference between Nordic Seas and North Atlantic supports almost no buoyancy forcing of the overflows thus the Denmark Strait overflow was probably reduced to less than 0.5 Sv during Last Glacial Maximum summer. However, it is proposed that the increased LGM wind stress is an important factor in keeping the Nordic Seas ice free during summer. The heat transport associated with the volume transport is generally reduced for the Last Glacial Maximum with a pronounced reduction if the Denmark Strait is additionally closed e.g. by iceberg blocking. The relation connecting hydraulic constraints and volume transport was implemented in a global climate model to improve the representation of the Denmark Strait overflow in that model. It results in an increased Atlantic meridional overturning and the associated northward heat transport. A close correlation between overflow representation and climate in northern Europe can be shown. This emphasises the impact of the Denmark Strait overflow on climate even though the response in overturning due to the representation is less than expected from previous studies. There is nearly no stabilising effect of the overflow for large scale freshwater perturbations but a weak effect if the system is perturbed locally in the Labrador Sea.

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