Dramatic increase in Arctic water temperature

Three new studies bring the picture of climate processes in the central Arctic into sharper focus. They are based on data collected through the MOSAiC expedition with the German research icebreaker Polarstern from September 2019 to October 2020.

Several hundred researchers from around the world participated in the expedition.

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 “The fundamental question we wanted to answer was why sea ice is melting so fast in the Arctic,” says Céline Heuzé, one of the authors of the three new review articles in the scientific journal Elementa and lead author of one. “The extent of summer ice has nearly halved since the 1980s, and that ice is now only one-third as thick. Decreasing sea ice is a symbol of the ongoing climate crisis.”

Atlantic waters are a major cause

Normally it is difficult to take wintertime measurements in the Arctic. So the Polarstern icebreaker was purposely frozen within the ice pack and floated along with it. This enabled the researchers to take continuous measurements. In addition to measuring temperature, wind strength and salt and carbon dioxide content, they recorded hundreds of environmental parameters for the ocean, air, ice and snow; ecosystems; and chemistry in the central Arctic Ocean over an entire annual cycle.

“The most important thing we have observed so far is the effect of what we call ‘Atlanticification’, the intrusion of warm water from the Atlantic Ocean into the Arctic. The results show that the impact was stronger than expected. The warm water extended further into the Arctic than we previously thought and to greater depths.”

Water temperature has increased by 5 degrees

The warm Atlantic waters are displacing cold Arctic waters, causing ocean temperatures at a depth of 100 metres to increase from about minus 1.8 to plus 3 degrees Celsius.

“We also saw more mixing of warmer and colder water than expected. This may be due to the thinner sea ice being more easily affected by wind, which increases the mixing of waters.

When the waters mix, the surface water under the ice becomes warmer and the ice melts from the bottom up.

 “We also observed that the layer of meltwater created when the sea ice starts to melt disappears very quickly after a storm. That is not good, because we believe that this meltwater temporarily protects sea ice from melting even more. To my knowledge, this has never been observed before. It is unclear what this means for the ecosystem.”

Snow is an important factor

Snow in the Arctic, also an important factor for the climate, complicates current climate models.

Snow protects the sea ice from the Sun. It reflects more light than ice, which has a blue hue and absorbs light more readily.

“The impact of snow is more complicated than accounted for in today’s climate models. And in the Arctic right now, snow is changing rapidly along with temperature and humidity.”

Unexpected weather

The Arctic turned out to be rainier and warmer than expected from models, with stronger winds.

“The combination of atmospheric pressure, wind systems with strong storms in winter and thinner ice affects the climate in the Arctic. The bottom of the sea ice is not flat, but has texture, so when the wind sets the ice in motion, cold water is pushed further and further down through mixing, and the sea water closest to the ice becomes warmer.”

During the expedition, researchers also found that the Polarstern icebreaker drifted with the ice faster than expected. In seven months, it completed the route that explorer Fridtjof Nansen travelled in three years.

Contact: Céline Heuzé, Department of Earth Sciences, University of Gothenburg, Phone: +46 (0)31-786 22 04, Email: celine.heuze@gu.se

Photo: Polar bear in the Arctic and portrait of Céline Heuzé; photographer Céline Heuzé.