Changes in the vigour of the Nordic Overflows over the last 3000 years

Abstract:

In today’s North Atlantic, warm tropical surface waters flow northwards into the Nordic Seas where they cool and sink to form deep south-flowing currents. This deepwater formation process is a major contributor to the Atlantic Meridional Overturning Circulation (AMOC) and is of great importance to the climate system as it distributes nutrients, heat and gasses throughout the world’s oceans. The Nordic Overflows are the deep waters that flow southwards over the submarine Greenland-Scotland Ridge and are the densest waters of the deep limb of the Atlantic Meridional Overturning Circulation. Changes in the vigour of these overflows may have had important climatic effects in the past and possibly in the future. Yet, evidence for multidecadal to millennial changes in the deep limb of the AMOC and their potential relationship to North Atlantic climate variability during our current interglacial is still scarce.

 Here we present grain size data (a current speed proxy) from sub to decadally resolved sediment cores located in the direct pathway of the two Nordic Overflows east and west of Iceland, the Iceland Scotland Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW), respectively. The results do not show a clear relationship between changes in the vigour of the Nordic Overflows and the well-known periods of centennial-scale climate variability recorded in the North Atlantic region. However, clear millennial-scale trends are found in both of the overflow strength records over the last 3000 years, possibly related to hydrographic reorganisations in the Nordic Seas driven by Northern Hemisphere insolation changes over the Neoglacial. A comparison between the near-bottom flow speed reconstructions from ISOW and DSOW suggest an antiphased relationship between the Nordic Overflows east and west of Iceland over the last 3000 years, a feature that has been observed in climate models as a result of shifts in atmospheric patterns over the Nordic Seas.