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

The FreshArc project investigated 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 (freshwater is lighter than salty seawater). Hence, an increase in freshwater input 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 has been an open question if the sum of oceanic freshwater and sea ice that flows southward through the Fram Strait, the largest gateway of the Arctic, has changed. Recent changes in the Arctic may have led to an increased export in the oceanic freshwater while sea ice export may have decreased since there is less sea ice in the Arctic. FreshArc has analysed a unique data set from the Fram Strait Arctic Outflow Observatory at 78°50'N and west of the Prime Meridian where ocean velocity, salinity and sea ice data have been collected cby ocean moorings since 1997 and 1990 respectively. The objectives of the project were to quantify the variability and trends in the ocean liquid freshwater transport and in the sea ice volume transport and to relate those to upstream drivers and variability and changes. New oceanographic data collected from the moorings during the research cruises in September 2019 and August 2020 have been included to update and analyse the time series of freshwater transport in the East Greenland Current. In addition, more and new types of instrumentation closer to the surface during the last 10 years , allowed to construct an improved record in time. The freshwater transport in Fram Strait was reduced between 2015-2019 when compared to the period 2010-2014. This reduction was attributed to a decline in ocean velocity and a reduction in the freshwater content. Besides, it the thickness of the Polar Water layer decreased in the last decade and the front between Polar and Atlantic Water moved westward in 2015. A study of the freshwater transport on the broad East Greenland Shelf showed that there is a considerable contribution of the flow on the shelf in particular in winter when it may double the estimates from the mooring array on the shelf slope. Finally, the variability of the freshwater transport was investigated together with ERA5 reanalysis and dynamic ocean topography of the whole Arctic Ocean. This showed that the freshwater variations are driven 1) locally by monthly northerly winds, and on time scales of several months up to over a year by 2) North Pole convergence freshening the region north of Fram Strait, and 3) Beaufort Gyre weakening allowing spreading of fresh water to the margins of the Arctic Basin and to Fram Strait. Newly collected data of sea ice after 2014 and including all available Upward Looking Sonar data from the moorings allowed to construct a consistent time series of sea ice volume transport between 1990-2020. A main finding was that 2018 showed a record low in sea ice export through Fram Strait. This was atmospherically driven, causing slower ice speeds, while sea ice thickness was record thin. More importantly, the sea ice thickness in the Fram Strait has been subject to a regime shift in 2007from thicker, deformed ice to thinner and more uniform ice. A simple model demonstrated how the shift can be explained by shorter residence time of ice floes in the Arctic since 2005-2007. In addition, the sea ice velocity data obtained from moored instrumentation from 1996-2004 vs 2015-2019 show that satellite derived ice velocities typically underestimate the ice drift. These in-situ ice velocity data contribute to evaluating biases in remote sensing products. Decadal scale ocean model simulations including tracer experiments were used to quantify pathways of FW components from the Arctic to Fram Strait. Using nutrient measurements based reconstruction of Pacific Water abundance in Fram Strait and the modelling results of the pan-Arctic region, the dominant EOF modes of Pacific Water distribution in the Arctic were found. Joint analysis of data and model output from 1980-2020 made it possible - based on the sea level pressure gradient between the central Arctic and the North Pacific - to predict the Pacific Water abundance in Fram Strait on decadal timescales with a time lag of 11 years. Finally, synthesis of the oceanic freshwater and sea ice volume transport through Fram Strait shows that the total fresh water through Fram Strait (sum of oceanic and sea ice) has reduced with approximately 25% in 2016-2020 compared to 1997-2001. This is reduction is primarily determined by the reduction of sea ice volume exported through the strait in the last decade.

The updated and consistent time series of liquid freshwater and sea ice volume export through Fram Strait have delivered important findings: Ocean freshwater export was smaller during the period 2015-19. Sea ice volume has been significantly reduced in the last decade and shows important changes in it composition. The latter one is the longest of its kind and is unique data representing the effects of climate change on the Arctic. The time series of oceanic freshwater and sea ice volume transport through Fram Strait will be public available and form important benchmark datasets for the modeling community. Observed variations and trends can be compared with those found in forced models. The identified mechanisms can be used to validate coupled climate models, which are subsequently used to obtain projections of future changes in the hydrological cycle in the Arctic and future freshwater exports such as in Jahn and Laiho (GRL 2020), Haine (GRL 2020) and Wang et al. (OLA Res. 2023). The finding that satellite derived ice velocities generally underestimate the in-situ observed sea-ice velocities will have a large impact on the earlier estimates of sea ice volume transport through Fram Strait. A paper describing this significant discrepancy and providing a correction factor is currently in preparation. That information provides valuable validation data for satellite products and will deliver further improved sea ice volume estimates.

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.