The Atlantic Meridional Overturning Circulation, or AMOC for short, is a fundamental component of the global ocean circulation. Warm surface waters are transported northward from the subtropical North Atlantic to the Nordic Seas and Arctic Ocean, where they become dense, subsequently sink, and are then transported southward as part of a system of deep currents. The redistribution of heat and fresh water via this overturning cell greatly impacts the Earth's climate. In an effort to measure the strength and variability of the AMOC at northern latitudes, an international observing system was begun in 2014 known as the "Overturning of the Subpolar North Atlantic Program" (OSNAP). The projects described below represent two components of OSNAP. The first consists of an array of moorings deployed from 2014-present spanning the shelf and continental slope of southwest Greenland to measure the boundary current there, and the second is a series of subsurface float deployments to document the pathways of the dense water returning southward.  

OSNAP southwest Greenland mooring array

Using the data from the moorings we quantified the different components of the west Greenland boundary current system, consisting of near-surface polar-origin water, middepth water stemming from the subtropical North Atlantic, locally formed water in the Labrador Sea, and two types of deep, dense water emanating from the Nordic Seas. We documented how the properties and volume transport of these components vary on a seasonal basis and identified the factors driving this, including wind and air-sea exchange of heat. The part of the boundary current system located near the edge of the shelf was shown to be highly variable due to a time-dependent dynamical process which leads to significant shelf-basin exchange of heat. Throughout the year there are strong wind events that modulate the boundary current. Winds from the north weaken the flow and cause the near-surface fresh water on the continental shelf to be transferred offshore. Winds from the south do the opposite by strengthening the boundary current and acting to trap the fresh water on the shelf. We demonstrated that such variations in the offshore flux of fresh water impacts the ability for water in the interior Labrador Sea to sink during the cold winter months - with consequences for the AMOC. 

OSNAP Floats

The equatorward flow of dense water as part of the AMOC has previously been measured in just a small number of locations, precluding the construction of a comprehensive map of their pathways over a large area. More than 130 acoustically tracked floats were released during 2014-2017 at various locations between Greenland and Scotland at depths below 1800 meters and tracked for two years. Over time, they spread out over the entire northern North Atlantic.  We used these new measurements to describe the deep-water current pathways  throughout the northern North Atlantic. Previous studies indicated that there was essentially a single pathway for these currents, encircling the ocean basins of the northern North Atlantic. The float data revealed three new features of the deep-ocean circulation that challenge the original paradigm. First, there are multiple pathways (not just one) - some along the basin boundaries, and others in the ocean interior. Second, there are numerous eddies - swirling quasi-circular current features 10-25 kilometers in diameter - throughout the region that act to transport different water types and mix the deep ocean like eggbeaters. Third, a current usually thought to exist mainly in the upper ocean - the North Atlantic Current - has an unexpectedly significant impact on the pathways of the deep currents in some regions. All of these features influence how climate-change signals are transferred from the atmosphere to the deep ocean, where they will be stored for hundreds to thousands of years.

Last Modified: 06/24/2024
Modified by: Robert S Pickart