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ARCGATE: maximizing the potential of Arctic Ocean Gateway array

Periodic Reporting for period 1 - ARCGATE (ARCGATE: maximizing the potential of Arctic Ocean Gateway array)

Reporting period: 2015-07-01 to 2017-06-30

Arctic Sea ice has been shrinking over many decades, indicating that there is more heat available in the climate system. Future prediction of this development, which is also important for socio-economic aspects like shipping or exploitation of resources, requires better understanding of the relevant processes. Although heat transport to the Arctic through the ocean circulation has been assumed to play an important role on the Arctic Ocean sea ice reduction, we do not know how much oceanic heat has been carried to the Arctic over last decades. By using observations for quantification of this heat transport for the first time, ARCGATE provides important information to assess Arctic change.

The export of fresh water (FW) from the Arctic Ocean to North Atlantic is another important process in the climate system. It has the potential to modulate the Atlantic Meridional Overturning Circulation, and consequently affect European climate. Almost all climate models predict that the Arctic is one of the most vulnerable regions in the world for future climate change. However, we do not know enough about the role of these processes in the present day climate to be able to predict with confidence what the future will look like.

The main Arctic gateways are Davis Strait, Fram Strait, Bering Strait and Barents Sea Opening (BSO). Velocity, temperature and salinity have been observed by six research groups in USA, Germany and Norway over the last decades to monitor the exchanges of mass, heat, FW between the Arctic Ocean and surrounding oceans. (Figure 1). However, no attempts were made to integrate fluxes over all mooring arrays across the Arctic boundary to draw a comprehensive picture of the Arctic heat and FW budget. In this project, data of all mooring arrays were integrated in order to quantify time variability of ocean circulation and associated heat and FW transports between 2004 and 2010 (Figure 2).
To do so, data of around 1,000 individual moored instruments in the Arctic main gateways from six different research groups were assembled. The obtained time series provide a benchmark data set for the validation of numerical general circulation models of the Arctic Ocean and air-sea surface heat and FW fluxes estimates from atmospheric re-analyses products.
The time variability of both horizontal and overturning ocean circulation and associated heat and FW transports during October 2004 to May 2010 were quantified. From the 6 years time series in the Fram Strait and the BSO it is revealed that the transport of Atlantic Water to the Arctic is twice as high in winter (January-March average) as compared to summer (July-September average). Compiling the data from all gateways, it was possible for the first time to estimate the total boundary heat flux, as a sum of oceanic and sea ice contributions, to be 180±57 TW (mean ± standard deviation).

In collaboration with a research group at University of Vienna, 1-year volume and heat transport data were exploited to validate the state-of-art ocean data assimilation product. We found that the data assimilation product reproduces the horizontal ocean circulation reasonably well, in term of mean value and its variability. However, we also found that modelled ocean circulation in Davis Strait and BSO were both stronger than observations by ~30%.

The main outcome of the project was the 6 years of ocean transport of volume, heat and FW transport. The 1-year data (during September 2005 to August 2006) was already deposited in PANGAEA, and the entire 6 years data will be deposited when corresponding scientific paper will be submitted to a peer review journal by end of 2017.
There are on-going EU funded Arctic research projects, such as APPLICATE (2016-2020), Blue-Action (2017-2021), INTAROS (2016-2021). The achievements of ARCGATE contribute to these projects in two ways. First, the 6 years time series will serve as a benchmark dataset to validate the performance of climate models that are used in APPLICATE and Blue-Action, thereby supporting better prediction of future Arctic climate. Second, ARCGATE has demonstrated the scientific impact to integrate existing Arctic data. An assessment of the oceanic heat transport was only possible by compiling the fluxes all the Arctic gateways. This required assembling the large number of mooring data from the six different research groups. The scientific outcome of the 6 years ocean transport time series is a valuable case study for the project INTAROS, which aims to exploit existing Arctic data in different climate systems.
Figure 1: Mooring sites in Arctic boundary (in red) maintained by six research institutes 2004-2010
Figure 2: Monthly gateway volume transports (Sv) and their mean seasonal cycle Oct. 2004 - May 2010