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Rezultaty
The long term consolidated data sets necessary to determine the variability of the freshwater and heat fluxes between the Arctic Ocean and the North Atlantic require an innovative design of the cost effective and well-calibrated measurement arrays. The northward flow of the Atlantic water carrying heat into the Arctic Ocean has to be monitored in two main gateways, the eastern Fram Strait and Barents Sea Opening. The southward freshwater flux, which enters the Nordic Seas both as liquid water and sea ice needs to be measured across the western Fram Strait, including the wide shelf east of Greenland. The latter requires current measurements and salinity stratification in shallow depths as well as ice thickness and velocity. To monitor fluxes not only the large cross-sections of the gateways have to be covered by measurements but also intensive variations on a wide range of scales, revealed by earlier observations have to be resolved.
Thus a larger number of moorings and instruments is needed or integral methods, measuring the whole water column have to be used. During ASOF-N observational arrays in Fram Strait and Barents Sea Opening were augmented and optimised in accordance with the observed time-space variability of measured parameters. Newly developed instruments were installed in the moorings, the observational arrays were redesigned for optimal combination of various properties, and transport estimates were proposed based on empirical relations to especially selected instruments. To secure data collected under harsh environmental conditions the possibilities of the near real-time data transfer via satellite link with pop-up buoys was tested. To complement the observing system of mooring arrays a grid of hydrographic stations was designed and repeated every summer to get the spatial variability of the Atlantic water pathways.
Key innovations
The Fram Strait mooring array was optimised to achieve better performance in measurements of temperature, salinity, currents and ice thickness - a prerequisite to derive heat and freshwater fluxes. In the eastern and deep part of the strait additional instruments were added at the depth of ca 700m to resolve the lower boundary of the Atlantic water layer. Two new moorings placed in the deep part of the strait to resolve the recirculation patterns of the Atlantic water and thus to reduce the error in volume transport estimates.
In addition to the moorings, integral measurements were performed with the use of bottom pressure recorders (BPRs) and inverted echo sounders with pressure sensors (PIESs) to estimate the barotropic currents and heat content of the water column. The performance of different current meters and TS sensors during the long-term deployments was evaluated and unreliable instruments were replaced to achieve the highest data recovery rate and the best data quality. The freshwater part of the mooring array in the western Fram Strait was equipped with near surface salinity sensors. Tube moorings in combination with Acoustic Doppler Current Profilers (ADCPs) were successfully deployed on the shelf, surviving the extensive ice cover and drifting icebergs. The moorings in the Barents Sea Opening were combined with a high-resolution hydrographic section repeated 6 times per year. Additionally this moored array was augmented with two bottom-mounted ADCPs in shallow parts. A new strategy was used for tracking the Atlantic water pathways based on combination of moorings, floats and hydrographic sections. The hydrographic sections included not only standard CTD casts but also currents profiles, which were measured by the lowered ADCP and quasi-continuously by the vessel-mounted ADCP. The grid of stations was adjusted to optimise the coverage of spatial structures.
Potential users
Scientists needing information on the physical conditions in the Arctic Ocean. Companies working on development of the novel oceanographic instrumentation. Environmental Protection Agencies and fishery management (recommendation for sustainable observing systems). Climate or ocean observing programmes such as GOOS/GCOS could be interested in recommendations for the Arctic Ocean Observing System regarding methods of measurements, instrumentation, efficiency of the observing system, data transfer, etc.
Expected benefits.
Improvement of the existing observing system to assess Arctic change including additional observed properties, better coverage of the key areas, higher data recovery rate. Improved estimates of the oceanic fluxes due to more reliable and accurate time series, measured in representative locations with higher resolution. This allows for a better model validation and improvements in the potential of the models to predict environmental conditions. The experience gained in the design of a sustainable and cost effective observational system under harsh Arctic conditions.
The Barents Sea influences the Arctic Ocean both by providing a pathway for Atlantic Water (AW), but also as a shallow shelf sea producing dense water through cooling and brine release. The Barents Sea provides intermediate water down to 1200 m depth in the Arctic Ocean, and, together with the Kara Sea, is the only source area for shelf waters ventilating the Nansen Basin below the halocline. Thus, knowledge of the variability of the Atlantic inflow to the Barents Sea is important for the understanding of the climatic state of the Arctic Ocean, and for evaluations of climate change.
ASOF-N continued a time series started in 1997 of volume and heat flux through the Western Barents Slope (WBS) using moored instruments. These cover the cross section where the Atlantic inflow takes place -with the exception of heavily fished waters. Temperature and velocity are monitored, allowing to integrate heat fluxes and to distinguish between eastward, westward and net fluxes. The time series is now sufficiently long to determine the variability of the oceanic fluxes through the WBS on inter-annual time scales - and to approach the declared objective of the ASOF cluster to capture variability on decadal time scales.
The long-term mean heat flux is 40 Terrawatt (TW) into the Barents Sea. Considering the inter-annual variability there was a relatively high heat flux into the Barents Sea in the winter of 2002/2003. Thereafter there was a pronounced decrease and 2003/2004 had the lowest heat flux observed during winter. In addition to the moorings, hydrographic measurements with high spatial resolution were used to derive flow field and heat flux six times a year.
Key innovative features and findings:
1) The combination of these two observational methods is one innovation made during the ASOF-N in obtaining an estimate of heat transport as accurate as possible:
2) The splitting of the flow of AW across the WBS was observed to take place in one wide branch but also to be split into several narrower branches, depending on the wind field. Between the branches there might be a weaker inflow or a return flow. At times the flow across the section is dominated by outflow (westward flow) and AW is flowing into the Barents Sea only in the southernmost part of the section.
3) There is no correlation between the fluxes and the temperature of the inflowing water. In fact, in certain periods temperature increases while the volume flux decreases. This shows that the WBS temperature is independent of the volume flux. The reason is that while the temperature of the inflowing water depends on the temperatures upstream in the Norwegian Sea, the volume flux depends mainly on the local wind field. This shows the importance of measuring both volume transport and temperature, since they not always are varying in the same manner. The short-time variations in the heat flux closely resemble the short-time variations in the volume flux, while the temperature variations influence the longer-term variations in the heat flux.
4) A realistic model representation of the Barents Sea region was made possible by another innovation, the coupling of a dynamic-thermodynamic sea ice model to a three-dimensional ocean general circulation model for the purpose of conducting climate dynamical downscaling experiments for the Barents Sea region. The Regional Ocean Model System (ROMS, http://marine.rutgers.edu/po/index.php?model=roms) was chosen as most appropriate since its model architecture is suitable for shelf seas as the Barents Sea. The improvement of the ROMS model is the sea-ice model extension, which performs well also on the high-resolution grid used and is absolutely necessary for realistic Barents Sea simulations. The model will be used to conduct a hind-cast for the period 1990-2005 including 3D fields of velocity and hydrography as well as water level and sea ice thickness and concentration. The horizontal grid resolution is ~10km. The vertical is resolved by 32 terrain following levels. Previous model validation with results from a similar model implementation (i.e. using slightly different and less accurate forcing) against observations show that seasonal and inter-annual variability in the ocean are tracked successfully. Furthermore the model results are used to examine details in space not covered by observations (incl. estimates of the through-flow of AW in the Barents Sea to the Arctic Ocean).
Users:
- Scientists in climate research - the oceanic heat flux through the Barents Sea is an important part in the North Atlantic and Arctic heat balance
- Scientists in Arctic Ocean research - influence of ocean temperature on sea ice, atmosphere, chemical & biological processes
- Offshore technology and shipping in the Barents Sea and the Arctic - oceanic heat is expected to affect the Arctic ice cover
- Commercial fishery - the temperature effect on the distribution of marine organisms
- Advisory panels for national and international policies
Summary:
The region covered by the ASOF-N includes some of the most productive fishing grounds of the world, where environmental changes have direct effects on the growth, recruitment, distribution, migration and food consumption of commercial fish stocks, and where a sustainable fishery is of central importance for the social and economic conditions of nations. The need is for a reliable system of environmental change monitoring to use in developing a predictive capability, which will reliably anticipate changes in fish-stocks.
The Barents Sea for example is a high latitude ecosystem that is heavily depending on the inflow of Atlantic water from the south. Recent current measurements show a great variability of heat flux to the Barents Sea, which has consequences for the marine ecosystem. The heat flux has impact on species composition, distribution and migration of commercially important fish species. In addition the heat flux also determines the possibility of a Northern Sea route and the exploitation of natural resources (fossil fuel).
It is therefore important to continue scientific activities to monitor this flow in order to investigate how it is related to climate variability and change. At the same time it is of prime importance to disseminate the scientific results and their implications to politicians and stake holders like fisheries organisations and the general public.
Current status:
ASOF-N data were compared with models and used for the preparation of information of ASOF-N results in a layman language. ASOF-N is an excellent example where observations and model results support each other in a very positive way: observations are used to validate model results and on the other hand, models are important tools for explaining variability in the observations. The results of ASOF-N therefore support the requirements for data derived both from mathematical models and observational data.
In addition information on ASOF-N results were supplemented by results from related projects (ECOBE; ProClim and NESSAS) with financial support from the Research Council of Norway. These results were presented in layman language to fisheries organisation as talks, newspaper articles and in form of a leaflet that was distributed during an Aquaculture exhibition through IMR, Norway. Scientific results on the effect of the changing climate on the marine ecosystem were also presented to the Norwegian Ministry of Fisheries during an oral presentation. In addition results on climate variability and its link to ecosystem development have been conveyed to the general public especially students and high school teachers during lectures given at universities and schools.
Results from ASOF-N and the EU-funded preceding projects VEINS and MAIA also gave background information to the scientific report from the Arctic Climate Impact Assessment (ACIA) published in 2005. ACIA is an international project of the Arctic Council and the International Arctic Science Committee (IASC), to evaluate and synthesize knowledge on climate variability, climate change, and increased ultraviolet radiation and their consequences. The Arctic Council is a high level intergovernmental forum of the following member states: Canada, Denmark, Finland, Iceland, Norway, the Russian Federation, Sweden, and the United States of America.
The ASOF-N results are included into reports to ICES (the International Council for the Exploration of the Sea), which gives advice to the member countries and helps them manage the North Atlantic Ocean and adjacent seas. The results are included in a few reports presented at different ICES working groups like Working Group on Oceanic Hydrography (WGOH) and Arctic Fishery Working Group (AFWG ), and during the last two years also included in ICES assessment reports.
Benefits:
The communication of scientific results based on measurements in the Arctic as well as on predictive models to decision takers is a central requirement for anticipating and mitigating the regional effects of global warming.
Fram Strait is the only deep connection between the Arctic Ocean and Nordic Seas and represents the major gateway for the flux of warm water from mid latitudes to the Arctic Ocean. The oceanic heat imported from the North Atlantic has the potential to affect the ice cover in the Eurasian Arctic and to be released to the Arctic atmosphere. Thus, it is an important component for understanding Arctic climate, which is strongly linked with European climate, necessitating long-term measurements and simulations in a regional model.
Since 1997 a continuous time series of volume and heat flux through Fram Strait was derived from measurements with moored instruments. The moorings cover the cross section over the entire deep part of Fram Strait. Temperature and velocity are monitored, allowing to integrate heat fluxes and to distinguish between northward, southward and net fluxes. ASOF-N allowed continuing the time series, which is now sufficiently long to determine the variability of the oceanic fluxes through Fram Strait on inter-annual time scales - and also to approach the declared objective of the ASOF cluster to capture variability on decadal time scales.
The yearly averaged northward heat flow through Fram Strait increased dramatically, from about 38 to 60 Terrawatt (TW), during 1997-2000. In the following years the heat flow decreased slightly although summer temperatures of the inflow measured during ship surveys showed record high values in 2004 and 2005. While moorings record year round data and provide high temporal resolution, they still have a limited spatial resolution. Therefore, since 2001 they are complemented by ADCP (Acoustic Doppler Current Profiler) recordings during ship cruises that deliver high spatial resolution temperature and velocity data sets (typically two or three per year in the summer season). Yearly hydrographic measurements with high spatial resolution were also used to derive the flow field and heat flux with a third independent method.
Key innovative features:
1) The combination of these three observational methods, which to the best of our knowledge has never been published before, is one innovation made during the ASOF-N project in obtaining an as accurate estimate as possible of the heat transport.
2) The complicated topographic structure of Fram Strait leads to a splitting of the warm West Spitsbergen Current into various branches transporting water northward and eastward or recirculating immediately in Fram Strait. The size and strength of the different branches largely determine the input of oceanic heat to the inner Arctic Ocean and have to be distinguished. Therefore one key progress made during ASOF-N was recognizing the strong impact of areas of high recirculation on calculated heat and volume flow. By deploying additional moorings in the central part of Fram Strait the error of calculated heat and volume flow was considerably reduced.
3) A realistic model representation of these different branches was made possible by another innovation, the improved spatial resolution in the North Atlantic-Arctic Ocean-Sea Ice Model (NAOSIM) of AWI.
The NAOSIM models use meteorological data and simulate their influence on sea ice, currents, temperature and salinity in the ocean north of approximately 50°N. The improved model has a horizontal resolution of approximately 9km and a vertical resolution of down to 10m in upper ocean layers (with 50 depths layers in total). The modelling was done for the period from 1990 (using initial conditions in 1990 based on the coarser resolution version of the model) to 2005 using NCEP (National Centres for Environmental Predictions) reanalysis data for the integration.
This model version resulted in a number of improvements, for instance the far better reproduction of the recirculation of Atlantic waters in Fram Strait. The averaged northward volume transport increased to around 10Sverdrup (Sv) between 1995 and 2003. This fits observations much better than the around 3Sv of the previous model. The good agreement between the model and the observationally based estimates in Fram Strait makes it possible to use the model to relate changes in Fram Strait to large scale oceanic developments.
Potential users:
- The scientific community working in climate research (because the oceanic heat flux through Fram Strait is an important part in the North Atlantic and Arctic heat balance)
- Scientists working in Arctic Ocean research (influence of ocean temperature on sea ice, atmosphere, chemical & biological processes)
- Offshore technology and shipping in the Arctic (since the oceanic heat is expected to affect the Arctic ice cover)
- Commercial fishery (because of the temperature effect on the distribution of marine organisms)
- Advisory panels for national and international policies
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.
Users:
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.
Result description CTD Data:
CTD (Conductivity, Temperature, Depth) profiles were measured during 10 cruises in 2003, 10 in 2004, 11 in 2005 and 1 cruise in 2006 by AWI, IFMH, IMR, IOPAS, and NPI across Fram Strait, in the East Greenland Current between 74° and 79° N and in the eastern part of the Greenland Sea. A detailed information on the CTD stations can be found at the ASOF-N website.
The current CTD dataset contains 2514 profiles, which can be retrieved via http://www.awi-bremerhaven.de/OZE. A summary of CTD datasets is presented in Tables provided in the documentation.
Instruments:
All institutions used a Sea Bird SBE911plus CTD profiler with a single CT-Sensor package (double CT-Sensor for AWI profilers). The CT sensors were frequently calibrated at Seabird Electronics. In addition salinity samples were taken to correct for sensor drift. The final data have been processed using Seabirds post-processing software which includes all necessary operations. The raw data have a vertical resolution of 0.04dbar but still including noise e.g. due to ship motion. To reduce this noise, data were averaged to a vertical resolution of 1dbar. The number of data cycles averaged in each 1dbar record was stored together with the individual data points.
ASOF-N continued a CTD time series started in 1997, which is now sufficiently long to determine the variability on inter-annual time scales - and also to approach the declared objective of the ASOF cluster to capture variability on decadal time scales.
Result description Mooring Data:
Moorings were deployed and recovered during 15 cruises from 2003 to 2005 by AWI, IFMH, IMR, and NPI across Fram Strait and in the East Greenland Current at 74°N. A detailed map with mooring location is provided in the documentation. The current dataset contains 900 time series, which can be retrieved via http://www.awi-bremerhaven.de/OZE. A summary of mooring records is presented in the documentation.
Instruments:
Most instruments being used in the mooring were current meters. Reliable current measurements were maintained by frequent services (usually before and after deployment). The common current meter type is a rotor current meter from Aanderaa Instruments. In addition to current speed and direction these instruments also measure temperature. CT recorders from Seabird (SBE16, SBE37) were used to measure precise temperature and salinity based on calibrations before deployment and after recovery. The sample rate depends on the different instrument types (and their power consumption and memory size) and ranges between 10 minutes and 2 hours. Data processing was done by the institution that provided the instrument. The standard procedures included converting binary data to engineering units, apply magnetic deviation correction, removing of spikes (small gaps were filled by interpolation and long gaps were filled with a dummy data value, e.g. NaN) and correcting for sensor drifts for example by applying post calibration.
Potential users:
Scientists working in Arctic Ocean research
Archiving and providing the data:
As important as collecting new data about the temporal variability of oceanographic parameters, is the availability of collected data. The ASOF-N partner send their processed and calibrated CTD and/or mooring datasets to the AWI where they are stored together with information about additional dataset available for this region. The data are transferred into a uniform format and archived in AWI's database with general cruise and instrument information. ASOF-N partners have password protected direct access via the Internet or can order complete datasets on CD.
This process has the following key advantages:
Project partners can access all collected data from all over the world via the Internet without delay. This is of great importance especially in the oceanographic community where scientists are often on cruises and answering requests for data are therefore delayed.
Data are provided centrally in a uniform data format. The time consuming process of converting data is performed centrally using a fixed routine. The user accesses a uniform data format and can compare profiles directly without tedious and time-consuming conversion of data formats. The data format supports importing of data using Matlab if desired by the user and can be visualised using the software ODV (Ocean Data View). ODV can be retrieved from http://odv.awi-bremerhaven.de.
In contrast to archives, this working database can be easily edited after the upload. This easy modification of the database allows for a fast correction if comparisons of data sets reveal faulty data points. Furthermore it enables the subsequent expansion of the database. CTD data are processed fast and can made available immediately and data sets that need more time to process e.g. oxygen profiles can be easily added at a later stage without affecting the accessibility of CTD data.
Background and result description:
Fram Strait is the main source of freshwater for the Greenland and Iceland Seas. Along with Davis Strait it is also the main provider of freshwater for the Labrador Sea and North Atlantic. This freshwater export from the Arctic to subarctic seas has the potential to influence the northbound current systems by modifying the stratification of the receiving basins. This would alter the oceanic heat transport, which again would influence the climate of North Western Europe. The export of fresh water occurs in liquid (polar water) and solid (sea ice) phase. With the advent of ASOF-N we were able to continue and extend existing time series (solid phase freshwater flux since 1990 and liquid freshwater flux since 1997), and hence determine the seasonal and inter-annual variability of freshwater fluxes through Fram Strait.
The annual cycle of the liquid freshwater transport in the East Greenland Current (EGC) has a minimum in spring (~600km3/yr) and a maximum in late summer (~2000km3/yr). The mean of the transport time series is 900km3/yr. The anomalies with respect to the mean seasonal cycle have a magnitude of typically 500km3/yr. There is no general trend in the freshwater transport over the period 1997 to 2005. From the first ASOF-N winter time CTD section and the first mooring on the shelf it is clear that additional moorings are needed, as there is a potential for considerable freshwater transport across the wide shelf. During the May 2005 cruise low salinity water was found also east of the EGC, meaning that an additional freshwater transport could occur east of the existing mooring array.
The comparison between fresh water observations at 79 N, 74 N and 63 N so far shows no significant signal propagation of water mass characteristics along the EGC path, in contrast to what is observed along the Atlantic Water path in the east.
Key innovations:
Time series have been maintained using more basic observational arrays since 1997 but ASOF-N allowed using a full-scale version of the observational array facilitating key innovations to be made:
1) With the advent of ASOF-N we were able to tailor make the observational set-up to study fresh water transport, as far as the physical conditions allow. This includes near surface salinity sensors and tube moorings with ADCPs (Acoustic Doppler Current Profiler).
2) The wide shelf at this latitude was an open issue as due to extensive ice cover and icebergs drifting through the region, access is difficult and moorings are not likely to survive. Few observations have therefore been made in this area. Introducing tube moorings has provided our first direct measurements of salinity on the shelf at 79N.
3) The moorings provide year round point measurements in the vertical and horizontal. Annual cruises during summer provide high-resolution hydrography, which aids in the interpretation of the point measurements.
However, for the observation of freshwater fluxes we also need high-resolution spatial information on the seasonal cycle of the stratification. An extensive ice cover with heavy multiyear ice has prevented access to the region during winter. In 2005 a coastguard icebreaker was used to penetrate into the pack, serving as a base for helicopter CTD transects with portable equipment. This allowed ASOF-N to do the first high-resolution wintertime hydrographic transect.
4) For some years freshwater observations have been performed also at the 74 and 63 North latitudes. With ASOF-N we were able to do the first comparisons of freshwater observed at these different latitudes. The aim of this ongoing work is to draw conclusions on the fate and pathways of freshwater exiting the Arctic at 79N.
5) Model results from the high-resolution version of AWI's NAOSIM have been used to fill spatial and temporal gaps in the observations and to link the freshwater fluxes in the East Greenland Current with the large-scale oceanic circulation as well as with the meteorological forcing fields. The large liquid fresh water export event of the mid-1990s could thus be linked to changes in the Arctic Ocean freshwater distribution during previous years. These redistributions were forced by the strong positive NAO wind forcing during the early 1990s.
Current status:
The time series are now approaching a length, which will enable us to quantify variability on decadal time scales, a stated objective of the ASOF cluster.
Users:
Climate scientists, since the freshwater output from the Arctic is thought to influence the net densification at high latitudes, and hence the current systems governing the oceanic heat transport to northern regions. Oceanographers working in the Labrador and Nordic Seas, since the freshwater output will modify the stratification and hence the processes occurring here. Advisory panels for national and international policies, particularly when the link between Arctic freshwater output and the climate system is better established.
Neutrally buoyant floats are devices that drift with the currents and thereby track the water pathways. During ASOF-N standard RAFOS floats were used. They were equipped with acoustic receivers tuned to the same frequency as the SOFAR sound sources installed on 13 moorings. The floats' acoustic receivers are detecting the time of arrivals (TOA) of signals transmitted regularly by the SOFAR sources. With these TOAs the position of the floats is tracked underwater acoustically. Thirty-eight floats were ballasted to drift at depths of about 300m and an additional four floats were ballasted to drift at 1000m. The floats were deployed west of the Lofoten Islands, across the Norwegian Atlantic Current and close to the continental slope. After drifting for approximately 6 months, the floats were released to pop up at the surface, where they transmitted via satellite the data recorded during the previous 6 months. During this transmission the floats also recorded in situ temperature and pressure.
Of the 38 floats ballasted for 300m, 25 transmitted data. While in general the data transmission rate was very good (of 24 floats deployed between spring 2003 and 2004 only 3 were lost) all ten floats deployed in November 2004 were lost. The best explanation to account for this strange loss is that they got transported northward by a current surge and got stuck under the ice cover. This would then indicate that in winter 2004 none of floats got transported into the Barents Sea, where they would have been safe but all ten floats got transported into Fram Strait.
Innovations:
During ASOF-N the balance depth was increased to 300m as previous experience from the MAIA project showed that this is the depth were the core of Atlantic water is found. The 5 float deployments made during ASOF-N means that for the first time variability between the years can be traced. During the predecessor project MAIA only 1 deployment of 5 floats took place. The measurements with floats made during ASOF-N showed that the Atlantic current is not restricted to a swift boundary current (like a jet) but more like a broad current dominated by mesoscale eddies.
Users interested in the result:
Oceanographer, especially scientists interested in Arctic ocean research, Scientists investigating ocean circulation, Scientist studying the interactions of water circulation and climate.
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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