Long-term variations in Iceland–Scotland overflow strength during the Holocene

The overflow of deep water from the Nordic seas into the North Atlantic plays a critical role in global ocean circulation and climate. Approximately half of this overflow occurs via the Iceland–Scotland (I–S) overflow, yet the history of its strength throughout the Holocene (~ 0–11 700 yr ago, ka) is poorly constrained, with previous studies presenting apparently contradictory evidence regarding its long-term variability. Here, we provide a comprehensive reconstruction of I–S overflow strength throughout the Holocene using sediment grain size data from a depth transect of 13 cores from the Iceland Basin. Our data are consistent with the hypothesis that the main axis of the I–S overflow on the Iceland slope was shallower during the early Holocene, deepening to its present depth by ~ 7 ka. Our results also reveal weaker I–S overflow during the early and late Holocene, with maximum overflow strength occurring at ~ 7 ka, the time of a regional climate thermal maximum. Climate model simulations suggest a shoaling of deep convection in the Nordic seas during the early and late Holocene, consistent with our evidence for weaker I–S overflow during these intervals. Whereas the reduction in I–S overflow strength during the early Holocene likely resulted from melting remnant glacial ice sheets, the decline throughout the last 7000 yr was caused by an orbitally induced increase in the amount of Arctic sea ice entering the Nordic seas. Although the flux of Arctic sea ice to the Nordic seas is expected to decrease throughout the next century, model simulations predict that under high emissions scenarios, competing effects, such as warmer sea surface temperatures in the Nordic seas, will result in reduced deep convection, likely driving a weaker I–S overflow

Climate change impacts on ocean circulation relevant to the UK and Ireland

What is happening • Observations of the Atlantic Meridional Overturning Circulation or Gulf Stream System since the 1980s have shown a strengthening in the 1990s and a weakening in the 2000s, with no clear overall trend. • Shifts in North-east Atlantic circulation, leading to a greater influence of warmer subtropical-origin waters which can impact marine ecosystems and economically important fish species such as mackerel. The changing subpolar ocean circulation is also having impacts on the food supply for deep-sea ecosystems. • The subpolar gyre recorded its freshest values on record in the 2010s. Ongoing freshwater build-up in the rapidly changing Arctic Ocean may exacerbate this freshening. What could happen • Projections from climate models consistently project a weakening of the Atlantic Meridional Overturning Circulation due to anthropogenic climate change. • Warming of Atlantic waters is expected to reduce the depth of mixed layers and limit nutrient supply to surface layers