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Inflow of Atlantic water into the Nordic Seas measured during ASOF-N from 2003 to 2005

The Atlantic water masses circulating across the Nordic Seas towards the Arctic Ocean, are of prime importance for the climate of the northern hemisphere. An excessive freshening of the Nordic Seas might be a prelude to a slow down of warm and salty Atlantic water masses advected from subtropical regions towards the Arctic Ocean. This reduced input of Atlantic water masses and their transformation in denser Arctic intermediate waters might eventually lead to a shut down of the general thermohaline circulation and overturning in the northern North Atlantic. At the moment the observations taken at various strategic spots in the Nordic Seas and the Arctic Ocean, tend to indicate a temperature increase of Atlantic warm and salty waters all along the continental margin north of Eurasia intruding in large sectors of the central basin of the Arctic Ocean.

ASOF-N allowed studying Atlantic water pathways across the northern Norwegian Sea (Lofoten and Boreas basins) as well as the time and space variability of heat, salt and total transports associated with the Norwegian Atlantic current. The equipment and combined effort of three institutions (LOCEAN-formerly known as LODYC, Paris, France, IMR, Bergen, Norway and IOPAN, Sopot, Poland) was used to install the following main instruments (a) neutrally buoyant floats tracked acoustically underwater, (b) current meters installed on moorings and (c) CTD and LADCP operated from research vessels during field campaigns organised in 2003, 2004 and 2005.

The main objectives consisted in:
(1) studying the Norwegian Atlantic current and eventually confirm the nature of the general circulation of Atlantic water masses in the Lofoten and Boreas basins,
(2) measuring the variability in temperature and salinity of the Atlantic water masses circulating across the northern Norwegian sea,
(3) observing the main pathways of Atlantic water masses entering either in the Barents Sea or heading north towards Fram Strait,
(4) estimating the heat losses to the atmosphere versus the heat transferred to deep Arctic intermediate waters and the freshening via internal mixing of the Atlantic water masses in the Lofoten Boreas basins and the Greenland Sea.

The key scientific results revealed:
(1) The real nature and structure of the so-called Norwegian Atlantic current which looks more like a broad and highly turbulent current extending 100kms offshore from the shelf break of the Lofoten basin, rather than a narrow jet current constrained to the continental slope west of Norway, as often described in the literature.
(2) The two stream nature of the West Spitsbergen Current
(3) The intense and prominent meso-scale eddy variability characterizing the Norwegian Atlantic current, and the West Spitsbergen Current.
(4) A pronounced seasonal and inter-annual variability of temperature and salinity fields showing episodic freshening and cooling events.
(5) A remarkable inter-annual variability of the main transport of Atlantic water masses across the Lofoten basin.
(6) A long-term variability of temperature and salinity fields corresponding to an increase in temperature and salinity of the Atlantic water masses of more than 0.5°C and 0.1psu over the past 25 years.
(7) The pulsating nature of the Atlantic Water transport within the West Spitsbergen Current.
(8) The close relation of the Atlantic Water volume transport with the local forcing in short-term variability.

The strategy used for better documenting the Atlantic water masses pathways in the northern Norwegian Sea was based on a combination of Eulerian techniques (moorings) and Lagrangian techniques (floats) as well as observations taken from research vessels (CTD, LADCP). This strategy provided an unprecedented level of information concerning the nature, structure, time and space variability of the Norwegian Atlantic current, and the main carrier of Atlantic water masses to the Arctic Ocean.

Besides scientists interested in the role of the ocean circulation on climates, our result show important avenues to explore together with biologists concerned with the impacts of the physical environment on biomass accumulation, plankton distribution, over-wintering of fish larvae and all other major aspects characterizing one of the most productive ecosystems on Earth.

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