March 13, 2008: "Ventilation, Pathways, and Overflows of the Nordic Seas"

Jan Even Nilsen, Nansen Environmental and Remote Sensing Center / Bjerknes Centre for Climate Research



The overflows from the Nordic Seas contribute with 2/3 of the Atlantic Meridional Overturning Circulation (AMOC).   These overflows result from ventilation of the Atlantic Water entering the Arctic Mediterranean, of which 2/3 is transformed and returns as Overflow Water feeding the deep branch of the AMOC.
In this work we describe and quantify the relative contributions from the different processes that modify water masses, densify surface waters, and contribute to the ventilation of the Arctic Mediterranean, through a combination of review and use of observational data and model simulations.
Important processes are eddy exchanges between the Atlantic, Arctic, and coastal waters; freshwater input; heat loss from the Norwegian Atlantic Current; convection in the Greenland and Iceland Seas; and ice formation producing brine enriched cold shelf water that cascades into the deeper ocean. Assessing the relative contributions from these processes is of key importance for understanding the northernmost limb of the AMOC.
A main result is that production of potential overflow water by brine enriched cold shelf water and by the gradual densification of the Norwegian Atlantic Current are both substantially higher than that of the open ocean convection (i.e. the Greenland Sea Gyre). Furthermore, the Atlantic inflow loses 80-90% of its heat before it reaches the Arctic Ocean through the Fram Strait. Of this loss, lateral eddy mixing to the interior regions is found to be of the same magnitude as the advection to the Barents Sea. The two combined is found to be of the same magnitude as the heat loss to the atmosphere.