Taking the pulse of change in the Arctic Ocean for more than 30 years

Since 1990, the Fram Strait Arctic Outflow Observatory has delivered oceanographic and sea-ice data from the East Greenland Current. With continuous refinement of its observational capacities, it also tracks marine mammals and ocean acidification. Such data are vital to monitor the changing Arctic.

The upper layer of the Arctic Ocean hosts a large reservoir of oceanic freshwater including river discharge from Siberian and North American rivers, low salinity water from the Pacific Ocean, and sea-ice meltwater. Large variations in the export of freshwater and sea ice from the Arctic are expected to play a role in modulating the large-scale overturning circulation in the Atlantic Ocean, which brings warm and saline Atlantic Water northward and returns cold and fresh Arctic Water and sea ice southward. Arctic freshwater circulates mostly in the clockwise Beaufort Gyre while the mobile sea ice travels across the Arctic Basin with the Transpolar Drift Stream. Since approximately 90% of the sea ice and 50% of the liquid freshwater exported from the Arctic Ocean passes through western Fram Strait, this gateway is an ideal location to monitor changes in pan-Arctic sea ice and upper ocean properties.

History

In 1981, the Norwegian Polar Institute (NPI) acquired the research vessel Lance, originally an ice-strengthened sealing ship which was converted into a research platform. Dr Torgny Vinje used RV Lance for annual expeditions to the Fram Strait, which focussed on drilling campaigns to study ice thickness and “freeboard of the sea ice” (ie, how high above the surface the ice reached into the air), with the goal of calculating the volume flux of sea ice out of the Arctic Ocean. To do this task, continuous measurements were necessary, and Vinje initiated the development of the moored Upward Looking Sonar, an instrument anchored in the ocean to obtain sea-ice thickness measurements year-round.

Sustained observations of the East Greenland Current began in 1990, when NPI, the University of Bergen and the University of Washington, deployed ocean moorings at 79°N with upward looking sonars and sensors for ocean velocity, salinity, and temperature to obtain estimates of how much freshwater and sea ice was being transported southward from the Arctic. In 1997, a full array of oceanographic moorings was established at intervals across the strait by NPI and the Alfred Wegener Institute. NPI maintains and collects data from the moorings in the western part with the Fram Strait Arctic Outflow Observatory. The observatory was expanded to include freshwater tracers in 2008, with passive acoustic monitoring, and more sensors to improve the freshwater transport estimates, in 2012. In 2021 sensors for ocean acidification were added on the moorings. Since 2018, the observatory has been serviced by the icebreaking ship RV Kronprins Haakon.

Ocean climate indicators

Here we highlight some key indications of changes in ocean climate derived from the continuous measurements from the observatory:

1. A distinct regime shift in sea-ice thickness occurred around 2007, when thick and deformed (uneven) ice was replaced with thinner and more uniform (flat) sea ice. After the shift, the fraction of thick “deformed” ice dropped by half and has not recovered.
2. Between 2015 and 2019, freshwater transport in Fram Strait was low because of a weaker flow and a larger amount of Atlantic Water in the central strait.
3. Declining summer sea ice has resulted in increasing temperatures of the Polar Water at ~55 m depth in summer, which, in turn, reduces sea ice locally and downstream along the coast of East Greenland.
4. The Atlantic Water in the Norwegian Atlantic Current in eastern Fram Strait has shown a warming trend over recent decades (from 2001-2017).

Biogeochemistry

Changes in the chemical composition of the Arctic outflow give us clues as to why freshwater transport is changing. For example, changes in oxygen isotope ratios in the upper 25 m observed in summers since the late 1990s reveal the contribution of sea-ice meltwater to the ocean as the sea ice retreats. A shift in the relative proportions of nitrate and phosphate observed in 2011 pointed to an increased contribution of water from the Pacific Ocean, transported to Fram Strait via the Transpolar Drift due to an anomalous surface circulation at that time.

Arctic rivers are typically stained brown with high concentrations of dissolved organic matter and these signatures of the rivers are preserved during transport across the central basins to the Fram Strait, allowing riverine sources to be detected downstream. Measurements of the carbonate system over many years indicate that the outflow is undergoing acidification. This is caused by ocean uptake of carbon dioxide from rising atmospheric CO₂, enhanced by freshwater dilution, which further decreases the availability of carbonate to calciferous organisms.

In situ sea-ice work

Since 2003, the annual Fram Strait cruises have included systematic collection of in situ sea-ice and snow thickness measurements, and other physical property data; samples have also been collected to assess sea-ice conditions on the local scale. The purpose of these studies is to obtain data for validation of the upward-looking sonar monitoring and satellite remote sensing. The investigations have revealed declining spatial ice thickness trends across Fram Strait for winter/spring conditions, but not during summer. A general thinning of sea ice from the early 2000s to the 2010s was confirmed, providing an important spatial perspective to the point observations from the upward looking sonars. Older ice types, usually abundant in the 2000s, became rare in the 2010s and 2020s. This is consistent with pan-Arctic findings for the Arctic Ocean.

Whale research in Fram Strait

Since 2008 a passive acoustic listening device has been mounted on an oceanographic mooring in the observatory – near the Greenland shelf – recording underwater sounds from biological and physical sources. Surprisingly, a very high presence of narwhals and bowhead whales has been detected at this drift-ice covered site. This discovery led to a suite of scientific investigations that has markedly changed our understanding of the ecology of these two species regionally. Satellite tracking, genetic studies, acoustic monitoring, and a survey have shown that we have: 1) much higher numbers of these animals in the region than previously thought; 2) populations that constitute unique entities; and 3) populations that are more highly ice-affiliated than conspecifics in neighbouring populations, residing in heavily ice-covered waters from the western Fram Strait eastward into Russian waters.

The future

The Arctic is warming about four times faster than the global mean and significant changes in the ocean and sea ice have been identified by the Fram Strait Arctic Outflow Observatory in recent decades. The observatory is an important gateway component of the international pan-Arctic observing system, which is essential to quantify ongoing changes in the Arctic environment and assess impacts in the Arctic and downstream. The continuation of these efforts will contribute to the development of the Atlantic–Arctic Distributed Biological Observatory and will further enhance their relevance for understanding and predicting Arctic Ocean changes.