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ASOF-W - Arctic Sub-Arctic Ocean Flux array for European Climate: West

Summary information

Funding:FP5 - Research project
Total cost:1987679
Ec contribution:1333081
Start date:2003-02-01
End date:2005-08-01
Duration:30 months
Coordinator:Jens Meincke (
Organisation:University of Hamburg, Institute of Oceanography – Germany
Themes:Ocean current changes; deep circulation changes; freshwater inflow
Regio:Arctic; North Atlantic
Project name:ASOF-W - Arctic Sub-Arctic Ocean Flux array for European Climate: West
Project summary:Abstract
This project centred on the fact that the climate of NW Europe, the Nordic Seas and Scandinavia is already abnormally warm for its latitude, and is correspondingly sensitive to change in the factors that are responsible for it. We expect that the changes of climate in this sector may be both rapid and radical as we move from the most extreme development of the atmosphere driving in the 1990s to what must surely be the most extreme anthropogenic contribution to climate change in the next Century. Learning how to mitigate the socio-economic effects of these changes requires the development of believable predictive models of the main processes at work. A central requirement for model improvement is the availability of time series measurements of the oceanic heat and freshwater exchanges between the Arctic and the North Atlantic.

The spread of warmth to high latitudes in the Atlantic sector is due to a complex ocean-atmosphere interaction, which includes a vast amount of heat (about 1015 W) carried northward by the ocean’s Thermohaline Circulation (THC). There is growing concern that “global warming” will be accompanied in our sector by strong regional cooling across Northwest Europe and Scandinavia due to slowing of this THC loop. This disruption is likely to take effect through the processes that control the deep overflows of dense water, which drive the THC, and the surface freshwater fluxes from the high Arctic that are supposed to shut it down. The North Atlantic Oscillation (NAO) as the main mode of recurrent atmospheric behaviour in our sector is heavily implicated in these changes and is predicted to amplify with future greenhouse gas forcing. For this reason, the ASOF-W Project has been designed to meet the following overall objective:

To measure the variability of the dense water and freshwater fluxes between the Arctic Ocean and the North Atlantic in the critical location off Southeast Greenland with a view to understanding and predicting their response to climatic forcing, especially to the NAO.

Clustering of ASOF-W with the two parallel EU FP 5-projects ASOF-N and ASOF-E has assured, that the time-series measurements cover all of the fluxes through the Nordic Seas in a synoptic manner. This effort on the European side has prompted parallel field activities in North America for covering fluxes from the Arctic through the Canadian Archipelago into the North Atlantic.

The approach:
Time series measurements have been carried out on the shelf and the slope off Southeastern Greenland by means of two arrays of sensors for currents, temperature and salt to obtain fluxes and flux-variability of the dense Denmark Strait overflow and the freshwater export from the Nordic Seas to the North Atlantic. The overflow measurement array over the slope has been successfully installed during the EUVEINS project and has been extended to a decadal record by this project. The measurement of the freshwater flux on the shelf in the liquid phase has never been achieved directly before the start of this project. Novel instrumentation has been developed and was deployed in a shelf array to measure the profiles of water speed, temperature and salinity from the drift ice covered surface layer to the bottom.
Project outputs:Major results:
The need to resolve the problem of fluxes and flux-variability arises from the fact, that it is now that the atmospheric driving of the northern North Atlantic has seen the strongest signals ever recorded in the time scale range from interannual to decadal. The response to these signals is presently propagating through the high-latitude oceanic-system and has just begun to affect the headwaters of the deep limit of the Atlantic THC. If we had not instrumented the key locations along the paths of these changes, we would have lost the rare opportunity of measuring the oceans response to atmospheric forcing in a phase of an elevated level of signal to noise ratio. This project and its European and North American partners have therefore provided benchmark observations needed to improve coupled climate models. Also provided was the necessary experience for longer-term measurements in high latitude ocean areas in the context of a future ocean observing system. Major results from ASOF-W are listed as follows:

Despite losses of equipment imposed by the hazardous nature of the East Greenland shelf, a prototype ASOF-W freshwater flux array has been established and maintained beneath a highly variable ice cover. A first liquid freshwater flux estimate yielded a mean value of 2.000 km³/yr with a standard deviation of the daily mean estimates of 1.200 km³/yr. This number fitted well into a newly compiled estimate of freshwater fluxes in the Arctic and Subarctic Seas, based on the most recent data available to the project. The freshwater array is continued beyond the duration of the actual project to provide this much needed time series to the upcoming International Polar Year 2007-9 with its activities on the role of the Arctic in the hemispheric water cycle.

The project continued the time series of direct transport measurements of the dense Denmark Strait overflow with a "picket fence" array of 6 to 8 current meter moorings over the East Greenland slope at 64° N. This array was paired with a bottom-mounted acoustic profiling current meter maintained at the Denmark Strait sill some 500 km upstream. In addition, coupled ocean circulation models with realistic atmospheric forcing were run. The mean transport value from altogether 9 years of array data was calculated to be 4.0 ± 0.4 x 106 m³/s with no trend on the decadal time scale and with no correlation to the parallel transport fluctuations of the overflow branches east of Iceland. Significant temperature and salinity signals were recorded with the moored sensors, notably in 1999 and 2004. They could be traced from upstream in the Nordic Seas to downstream to the Labrador Sea, providing information on how atmospherically induced surface anomalies in the high latitudes are advected into ocean depths exceeding 2.500 m in the Subpolar seas to the southwest of the Greenland-Scotland Ridge, i.e. the deep headwaters of the THC. As with the freshwater array, the dense overflow array will be continued through the IPY.

From a total of 9 years of hydrographic sections worked between the Denmark Strait and the southern tip of Greenland we could provide time series of the downstream development of geostrophic transports of the dense overflow and changes of its water mass characteristics. The transport estimates agree well with the results from the moored array. From the water mass analysis the persistence of stratification along the overflow plume indicates lesser entrainment rates with the ambient waters than assumed so far.

The project included a component for the final development, the testing and the field use of a profiling temperature-salinity probe, operated from a bottom mounted winch. Despite considerable engineering progress with the system and ample field campaigns, no scientifically usable data could be obtained. The prototype continues to be improved. Once operational this instrument is considered ideal for under-ice profiling of shelf waters.

In a synthesis of the project data the observed variability set the local data into the context of the large-scale climatic signals passing between the Nordic Seas and the deep Atlantic. One of the principal results from our array over a decade of continuous observations is the finding that although the transport time series show distinct interannual variability, there is no obvious evidence for any long term trend in the deep overflow west of Iceland. Models with realistic forcing confirm this. Likewise the recent analysis by the ASOF-E group of the deep overflow east of Iceland over the full record from 1995 to 2005 shows neither evidence for any long term downturn in transport nor an inverse relation to the overflow transports west of Iceland, which is in contrast to earlier reports. Thus we find no evidence yet that deep overflow-transport is directly implicated in the recent reported slowdown of the deep limb of the Atlantic Thermohaline Circulation. Our observations of long term changes in the temperature and salinity properties of the overflows are a different matter and may well be involved in the circulation changes as reported for the lower latitudes. They are certainly involved in the observation of Atlantic-wide changes in salinity. Whereas for our Subarctic region we have analysed a decadal freshening of the ocean fluxes from the Arctic to the Atlantic, there is a corresponding salinification of the upper ocean in the subtropical and tropical regions. This indicates a significant role of the coupled ocean/atmosphere freshwater cycle in linking the high latitude ocean basin with the global ocean. Since this is one of the priority topics during the upcoming International Polar Year 2007-9 the ASOF measurement arrays and data sets provide an important platform to build on for the climate research projects of the next decade.

Project Outreach:
The overall aim of ASOF was to implement a longer term system of critical measurements needed to understand the high latitude ocean's steering role in decadal climate variability. This cannot be achieved on a regional basis but can only be met by studying the complete system of oceanic exchanges through the subarctic seas. This need was acknowledged from the outset and it was an extra workpackage added to the ASOF-W project with three main successes: (i) The three components of the European ASOF cluster (ASOF-W, ASOF-N and ASOF-E) were closely integrated by instituting task-groups for planning across all of the Nordic Seas and for establishing a system-wide Numerical Experimentation Group. (ii) An international ASOF programme was developed between the EU-groups and relevant North American groups financed through NSF and NOAA, enabling a true pan-Arctic approach to the ASOF-aim. International Steering Committee Meetings, a regular Newsletter and a website have brought together a scientific programme, which has made ASOF the largest ocean-observing network in the hemisphere for the time being. (iii) In anticipation of the upcoming International Polar Year 2007-9, a larger scale context for the ASOF effort itself was achieved by compiling a Science Plan for a pan-Arctic integrated Arctic Ocean Observing System (iAOOS), which was endorsed by the relevant international scientific organizations and accorded lead-status by the International Council of Scientific Unions Joint Committee for the International Polar Year Partners.