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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Freshwater export in an era of Arctic Ocean freshening and sea ice decline

Alternative title: Ferskvannseksport i en æra med minkende havis og et ferskere Polhav

Awarded: NOK 10.0 mill.

The FreshArc project investigates how the export of freshwater and sea ice from the Arctic Ocean to lower latitudes is influenced by ongoing Arctic ocean and climate change. Since the 1990s, the amount of freshwater in the Arctic Ocean has increased substantially while the sea ice has been decreasing. An unusual large input of freshwater and sea ice can have an impact on the North Atlantic Ocean by making surface water less dense since freshwater is lighter than salty seawater. The increase in freshwater could influence the large-scale three-dimensional ocean circulation pattern which, on its turn, may lead to changes in weather and climate in Europe. It is an open question if the sum of ocean freshwater and sea ice that flows southward through Fram Strait, the largest gateway of the Arctic, has changed. The rapid changes in the Arctic may have led to an increased export in the oceanic freshwater. At the same time, the sea ice export may have decreased since there is less sea ice in the Arctic. But since the sea ice is much thinner than before, the motion of the upper ocean and sea ice will react quicker to variations in wind speed and direction. FreshArc analyses a unique data set from the Fram Strait Arctic Outflow Observatory, where ocean velocity, salinity and sea ice data have been collected year-round by ocean moorings since 1997. Both historical data from the early 1990s and newly collected data obtained during the project (until 2020) will make the time series long enough to investigate trends and variations the Arctic freshwater and sea ice export. The following milestones of the project have been reached: all new oceanographic and sea ice thickness data collected from the moorings from the research cruises in September 2019 and August-September 2020 have been processed. The time series of freshwater transport in the East Greenland Current has been updated and analysed up to August 2019 and submitted as a research article. Due to more oceanographic instruments deployed on the moorings in later years than in the earlier 2000s, as well as novel instrumentation closer to the surface, the freshwater transport time series needed special attention in order to construct a consistent record in time, i.e., not biased by additional instrumentation. The research paper describing this elaborate new method and the analysis of the results concludes that: 1) the freshwater transport in Fram Strait has generally decreased since 2015, 2) the reduction is attributed to a decline in velocity and a reduction in freshwater content, 3) the Polar Water depth and eastward extension have declined in the last decade, 4) there was one exception to this trend in winter 2017, when a large freshwater transport persisted for several months associated with a thick freshwater layer as well as increased velocities. The second research article describing the time series of sea ice transport since 1992 to 2018 is currently being revised. The main finding here, was that 2018 showed a record minimum in sea ice export through Fram Strait. This was atmospherically driven, causing slower ice speeds, while sea ice thickness has continued to decline in addition. Finally, we are working with the historical mooring data from 1984-86 and early 1990s, in order to extend the time series back in time, however, we found that there were no salinity records to make accurate freshwater transport estimates. We aim to make some approximations of this, however, based on the current meter data and annual hydrography.

FreshArc quantifies the response of the Arctic Ocean to climate change and delivers a groundbreaking up-to-date 30-year long record of freshwater and sea ice export from the Arctic. The upper ocean freshwater content of the Arctic Ocean has increased significantly since the 1990s while sea ice extent and thickness have declined dramatically. The freshwater accumulation is of particular concern as a release of freshwater and sea ice into the North Atlantic Ocean has the potential to slow down the Atlantic Meridional Overturning Circulation (AMOC), and thereby impact weather and climate in the Arctic, Europe, and Africa. It is at present, an open question whether the total freshwater export (liquid and sea ice) through Fram Strait, the largest gateway of the Arctic, has changed. The rapid changes in the Arctic may already have led to an increase in the ocean's freshwater export while sea ice export may have decreased. However, as sea ice is thinning, the ocean and sea ice circulation become more vulnerable to changes in atmospheric forcing. To assess if the export of total freshwater through Fram Strait to the North Atlantic has increased in the last decades, unique year-round ocean and sea ice data collected by ocean moorings in Fram Strait since 1990 will be integrated and analysed. New, higher resolution, and novel near-surface data collected since 2009 will be assimilated to provide more accurate freshwater and sea ice transports, and will also allow for an improved quantification of exports prior to that time. Model simulations with a Pan-Arctic ocean-sea ice model will demonstrate how observed trends and variability of the total freshwater export during the last decades are related to the large-scale Arctic ocean and atmospheric circulation. Ultimately, FreshArc will provide new state-of-the-art Arctic freshwater exports in Fram Strait and assess its contribution to the AMOC in recent years, essential for the improvement of future climate change projections.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek