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Exchanges of volume, heat and freshwater between the Atlantic and the Arctic Ocean

Motivation: One key question in Arctic climate research is, whether the sensible heat carried by the oceanic inflow can influence the formation of sea ice and whether e.g. a large increase in oceanic heat input could considerably diminish the ice cover. The inflow of water and heat from the Atlantic Ocean to the Arctic Ocean was studied during the ASOF-N programme. The Atlantic water (AW) reaches the Arctic Ocean through two passages, the deep (2600m) Fram Strait and across the broad shelf of the Barents Sea, where it enters through the Bear Island Channel and passes through the Barents Sea into the Kara Sea and most of the AW continues into the Arctic Ocean via the St. Anna Trough. The programme also attempted to quantify the outflow of sea ice and low salinity surface water (the export of liquid freshwater) from the Arctic Ocean to the Nordic Sea through Fram Strait and, since Fram Strait is the only deep passage connecting the Arctic Ocean to the world ocean, to estimate the exchanges of intermediate and deep waters between the Arctic Ocean and the Nordic Seas.

The AW was followed on its way through the Norwegian Sea from the Greenland - Scotland Ridge to the two inflow passages and the strength of the transport and the changes in water mass characteristics were studied.

Background information: The AW loses a considerable amount of heat in the open area north of Svalbard, the Whalers' Bay, to the atmosphere and to the melting of ice. As the AW then passes eastward along the continental slope, it is covered by a less saline surface layer comprising AW diluted by ice melt, and its still large heat content is isolated from the sea ice and the atmosphere. The transformations of the Barents Sea inflow are much larger. Denser water masses are created that enter the deeper layers of the Arctic Ocean, as well as less dense waters that eventually enter the central Arctic Ocean and supply the low salinity surface water, the polar mixed layer, of the Arctic Ocean. The strongest transformation affecting the AW after it has entered the Arctic Ocean occurs north of the Kara Sea, where it meets and mixes with the colder, less saline Barents Sea branch entering the Arctic Ocean via the St. Anna Trough. The mixing between the two branches leads to a cooling of the AW in the Fram Strait branch. The heat still remains in the Atlantic layer, but it is now distributed over a larger volume.

The largest changes in AW characteristics occur, when it penetrates from one basin into another basin and mixes with the water column present there. Another process changing the water mass properties in the deep Arctic Ocean basins is the injection of cold, dense water, formed by ice formation and brine rejection on the shelves, which sinks down the slope as dense, entraining boundary plumes. Some plumes enter and cool the Atlantic layer; some sink deeper, entrain AW and bring it into the deep, warming the deeper layers.

Key findings: Time series from the ASOF-N moorings in Fram Strait showed that several pulses of warmer AW combined with a stronger inflow, passed through the strait, adding heat to the interior of the Arctic Ocean. The net flow through Fram Strait is generally southward, ranging between 1 and 2Sv. However, over long periods, extending over more than a year a monthly mean net transport into the Arctic Ocean was measured. The strength of this transport was close to 1Sv.

The AW supplies the main inflow of volume, salt and heat to the Arctic Ocean. The inflow through Fram Strait is, according to the recent ASOF-N results, an order of the magnitude larger than the inflow of Pacific water through Bering Strait, 10Sv as compared to 1Sv from the Pacific. In addition the inflow over the Barents Sea contributes about 1.5Sv of AW. However, not all 10Sv entering through Fram Strait are AW but also include intermediate and deep waters and a large part may be involved in a recirculation in, or just north of, the strait.

The circulation loops (described in more detail in "potential offered for further dissemination and use") in the different basins have different residence times, and the heat that enters the Arctic Ocean as a warmer, and perhaps stronger, pulse becomes spread out spatially and temporally. Its return to Fram Strait extends over a period from perhaps less than 1 year for the re-circulation in (or just north of) the strait to 20-30 years for the farthest loops passing through the remote Canada Basin. This redistribution of the heat added to the Atlantic layer in the Arctic Ocean makes it difficult to determine how much of the oceanic sensible heat is lost to ice melt and released to the atmosphere, and how much is stored in the layer to eventually return to Fram Strait and the Nordic Seas.

Potential users:
-Oceanographers working in the Arctic.
-Scientists studying climate and climate change.
-Fisheries, shipping, oil drilling companies (open water is a condition, which benefits these activities)

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