Scientific background

The specific aim of the chemical oceano-graphy project during Arctic Ocean 2002 was to assess the mixing of upper and intermediate waters along the path of the East Greenland Current, to estimate the freshwater flux in the current and to determine what water masses contribute to the Greenland-Scotland overflow and thus to the driving of the global thermohaline circulation. These issues are central in the perspective of a possible future climate change and its coupling to the emission of anthro-pogenic carbon dioxide.

The intermediate waters of the Arctic Ocean have been shown to have the general circulation pattern illustrated in figure 1, with waters flowing off the shelves to intermediate depths and several cyclonic gyres that are determined by the topography. Even if this circulation pattern appears fairly stable, its strength and the T-S properties of the water masses have varied during the 1990’s. Parts of the upper water mass and all of the intermediate and deep water masses of the Arctic Ocean exit through Fram Strait. They flow along the Greenland continental slope and shelf and mix with waters of the Greenland and Iceland Seas (also areas of intermediate and deep water formation) to create the overflow waters that cross the Greenland-Scotland Ridge into the north Atlantic Ocean. The intermediate waters sink into the deep north Atlantic, while the upper water eventually enters the Labrador Sea as the West Greenland Current. The flow rate and the properties of both the upper and the intermediate waters display significant natural variability. Some of this variability is the result of changing Arctic Ocean water properties, while some is due to variable mixing with waters of the Greenland and Iceland Seas along the path of the East Greenland Current. The formation of subsurface waters in the Arctic Ocean and Nordic Seas is also a means of carbon sequestration, a part of which includes CO₂ of atmospheric origin.

In order to predict future climate it is of great importance to improve our understanding of the processes contributing to subsurface water formation and their sensitivity to possible climate change. This sensitivity is closely coupled to the carbon cycle, as the increasing atmospheric CO₂ content is the major cause of the potential climate change. To achieve this overall understanding a description

of the present functioning of the carbon cycle is required, but it is also critical to assess how changes in the earth’s climate would alter cycling and partitioning of carbon amongst different reservoirs. Specifically, changes in ocean dynamics and ocean thermodynamics may cause significant changes in air-sea CO₂ cycling.

The changes in properties of the waters of the East Greenland Current between Fram Strait and Denmark Strait, before they exit into the north Atlantic, is caused by mixing, biological activity and ice melt, as well as air-sea fluxes. A significant part of the freshwater transport in the East Greenland Current occurs over the inner parts of the Greenland shelf. The analytical and sampling program performed during Arctic Ocean 2002  was optimized to address these issues. The chemical constituents determined on board Oden were nutrients, oxygen, pH, total dissolved inorganic carbon, total alkalinity, CFCs and sulfur hexafluoride (SF₆). Samples for ³He, ³H and ¹⁸O were collected for later determination back in the laboratory. Sampling was carried out at seven sections crossing the East Greenland Current as well as at eight stations in the Storfjorden. In total 96 stations were occupied and most chemical para-meters were determined at every station, though not always at every depth.

The nutrients, nitrate and phosphate, can be used to evaluate the percentage of Pacific water that flows out in the East Greenland Current from the Arctic Ocean after passing all along the coast of North America (figure 2). The method is based on the fact that Pacific water is depleted in nitrate relative to phosphate as a result of denitrification that occurs in the north Pacific and the Bering-ChukchiSeas, thus giving Atlantic and Pacificwaters different source relationships (figure 2). Other chemical constituents, e.g. total alkalinity, can, together with salinity, be used to evaluate the fraction of river runoff in the East Greenland Current surface water.

In addition to elucidating the source of the waters, it is of interest to evaluate the time elapsed since a water parcel was in contact with the atmosphere. Evaluating the concentration distribution of transient tracers such as CFCs can achieve this. The CFCs have a known surface water concentration that increased with time until a few years ago. Thus the deep water concentration should reflect the time since that water was in contact with the atmosphere, taking into account the mixing history. Figure 3 shows how the bottom water that has passed the Denmark Strait down into the north Atlantic Ocean has elevated CFC-11 and sulfur hexafluoride concentrations. This reflects how the recently ventilated water from the Nordic Seas contributes to the north Atlantic deep water circulation and thus is the northern driver of the global thermohaline circulation.

Sulfur hexafluoride has a similar surface water concentration history, but at about three orders of magnitude lower concentration. In 1996, 320 kg were also released on purpose at about 300 m depth in the center of the Greenland Sea gyre as part of the European Union research project, ESOP-2. This project was followed by another project, TRACTOR (Tracer and Circulation in The Nordic Seas Region), which had the aim of following the horizontal spread of the tracer. The results from figure 3 indicate that some SF₆ from the release is found in the overflow water. A careful analysis comparing the final CFC and SF₆ data is needed in order to clearly state that some SF₆ is from the release experiment.