The global meridional overturning circulation of the ocean is a critical component of the earth’s climate system.  Surface water warmed by insolation at low latitudes flows to high latitudes where it becomes denser in winter by cooling and the injection of salt to surrounding water when sea ice forms.  This dense water sinks and flows back towards low latitudes and crosses the equator entering the opposite hemisphere, slowly upwelling into the surface ocean as it spreads.  The densest water that fills the oceans below about 1500 m forms at only a few locations: the northernmost North Atlantic Ocean and several locations around Antarctica. Line W focused on investigating the North Atlantic component of the global meridional overturning circulation.  Water properties, including temperature, salinity, oxygen, and the anthropogenic tracers: CFC-11, CFC-12, CFC-113, (chlorofluorocarbons-11, -12, -113), SF6 (sulfur hexafluoride) and I-129 (iodine-129), were measured on research cruises along a line extending from the continental shelf south of Cape Cod to Bermuda (Fig 1).  This line crosses the Gulf Stream, a major pathway for the transport of warm water from low latitudes northward to high latitudes, and the Deep Western Boundary Current (DWBC), which transports cold dense water that has sunk from the surface in the northern North Atlantic and flows southward. Cruises were conducted 1 - 2 times per year to measure water properties for a 10-year period, providing information on the decadal mean water mass structure and strength of the overturning circulation.

CFC-11, CFC-12, CFC-113 and SF6 are manmade gases that have been used extensively in industrial processes since the mid-20^th century and have entered the atmosphere and surface ocean.  The atmospheric concentrations have been carefully monitored and are well known functions of time (Fig 2b).  The concentration in the surface ocean can be calculated from the atmospheric record.  When surface water sinks, it carries CFCs and SF6 with it and concentrations in subsurface water can be compared to the atmospheric concentration record to estimate the time since the water sank.  I-129 is produced by nuclear fission and is released when nuclear fuels are reprocessed.  Nuclear fuel reprocessing plants in Sellafield, England and La Hague, France, release I-129 laden effluent into the North Sea, which becomes incorporated in the North Atlantic Current and enters the Norwegian and Greenland seas and the Artic Ocean (Fig 2a).  This water circulates through these seas becoming denser and then flows southward across the Greenland-Iceland-Scotland Ridge into the deep North Atlantic.  The concentration history of I-129 in the overflow waters has been determined from measurements and the release history from the reprocessing plants (Fig 2c) and can be used in a similar manner to CFCs to estimate deep water ages. 

A similar program to Line W was carried out along Line AR7W extending across the Labrador Sea (Fig 1) by Canadian scientists.  This line sampled the deep flow upstream of Line W and the CFC-11 and I-129 measurements along this line were used as a boundary condition for a numerical model calculation of the flow between there and Line W.  The transit time for the DWBC was calculated to be 5.5 years.  In 2010 Line W was extended southwest of Bermuda and revealed an interior branch of deep-water flow.  The model applied to these data yielded a transit time of 9 years indicating a slower spreading rate by this pathway.

The water mass properties and anthropogenic tracers were also used to determine the surface source regions for deep water and the fraction of water from each region, along the GA03 section (which includes Line W) extending southeastward across the subtropical North Atlantic Ocean and northward along the African coast (Fig 3).  The source water locations and the percent of water from each region that comprise the water between 1000 m and the bottom along the section are displayed in Figure 3.  The composition of deep water along Line W is very similar to the average composition for the entire GA03 section, although Line W has a slightly higher percent of water from the North Atlantic.  The two primary sources are inflow of dense water from the Arctic Ocean/Norwegian Sea/Greenland Sea (Fig. 2a), and the Labrador Sea where convection can reach depths of 2000 m during winter.  This accounts for about 55% for the dense water. The Mediterranean Sea is an important source providing about 14% of the water.  High salinity dense water forms there due to extensive evaporation and it overflows a shallow sill, sinking to deeper depths in the Atlantic.  Dense water formed around Antarctica flows northward and crosses the equator spreading to the subpolar regions of the North Atlantic where is mixes with the northern source waters and provides about 25% of the deep water along the section.  North Atlantic subpolar and subtropical surface water also provide a small fraction of the water.

Last Modified: 06/13/2018
Modified by: William M Smethie Jr.