Late winter deepwater formation in the Nordic seas north of Iceland and in the Labrador Sea between the coasts of Greenland and Canada produces cold and well-ventilated deep water. Together with local winds, this supply of newly formed deep water fuels a global circulation of great importance for Earth’s climate.
As warmer waters in the eastern part of the North Atlantic get pushed north, colder waters move to great depths, where they form one branch of the global conveyor belt known as the Atlantic Meridional Overturning Circulation (AMOC). This circulation transports heat from the tropics to the North Atlantic and constantly warms the atmosphere above. The circulation can profoundly alter climate patterns in the North Atlantic but also has an influence on the African monsoons and North American hurricanes. Deepwater formation draws down large amounts of carbon dioxide and thus counteracts part of global warming.
For about 17 years, scientists from GEOMAR have been measuring the strength of the Deep Western Boundary Current (DWBC), which flows at great depths off the coasts of Greenland and Labrador. Previously, few oceanic monitoring systems consistently measured the transport of water extending all the way from the surface to the seafloor.
Using the GEOMAR data, Zantopp et al. assessed the transports, water temperature, and density at roughly 53°N off the Labrador coast. The data come from an ocean observatory that typically consisted of five moored stations set up between 1997 and 2014. Combining these data with observations from 13 hydrographic surveys aboard their research vessels, the team characterized the currents extending down to within 50 meters of the bottom—a much deeper and better resolved picture than previous studies have achieved.
Their analysis revealed several surprises. Although the deepest, near-bottom outflow of the DWBC tended to be steadier than the midlevel currents, for example, the team found that this deep, cold, and dense outflow varies with a roughly 10-year period, in phase with the North Atlantic Oscillation (NAO), a climate phenomenon that strongly affects weather in western Europe. This Deep Western Boundary Current variability could be an important link between the NAO and AMOC, they said.
Understanding how such currents interact with other climate-relevant processes is vital to predicting future climate change. The evidence presented by the authors suggests that these long-term fluctuations are caused by quasi-decadal changes in the wind field rather than by buoyancy-driven convection events. Even centennial AMOC changes of up to 30%, suggested by some climate models, may be difficult to detect in light of the reported large and prolonged transport fluctuations. (Journal of Geophysical Research:Oceans, https://doi.org/10.1002/2016JC012271, 2017)
—Emily Underwood, Freelance Writer