By Elisabeth Halvorsen, UiT/The Nansen Legacy
The global ocean conveyor belt
To answer this question, you need to understand how the Barents Sea is part of a global ocean circulation, the Atlantic Meridional Overturning Circulation (AMOC). The AMOC consists of two “limbs” of ocean currents: one northward flowing, warm and salty in the surface, and one southward flowing, colder current in the deep. The AMOC is an important component of the Earth’s climate system.
The Barents Sea. Photo: Christian Morel/christianmorel.net
Small and shallow not necessarily a bad thing
The two “limbs” of the AMOC are linked near the poles, where warm surface water is cooled and plunges into the deep, feeding the deep flowing cold current. Most of the overturning on the Arctic side of the conveyor belt, takes place in the Nordic and Labrador Seas. Despite its small size, also the Barents Sea plays a relatively big role in this process. This is where the refrigerator comes into the story.
The Barents Sea cooling machine
Warm Atlantic Water (AW) flows into the Barents Sea between Norway and Spitsbergen. On its way through the shallow Barents Sea low air temperatures cause a cooling of the AW, which transforms AW into Barents Sea Water (BSW). As such, it reaches the sea ice. The sea ice closes a lid over the water and stops it from further cooling by the air. Since cold and salty water is denser than fresh and cold water, the AW slips beneath the fresher surface water under the sea ice (fresh due to ice melt), plunges into the deep and exits the Barents Sea to the north, into the Arctic Ocean. Here, it circulates and finally joins the overflow waters from the Nordic Seas that feed into the AMOC. A particularity of the Barents Sea cooling machine is that it has a positive feedback loop. That is, if more or warmer AW enters the Barents Sea, sea ice is reduced. This leads to more ocean heat lost to the atmosphere, resulting in more BSW exiting the Barents Sea into the Arctic Ocean. This again strengthens the deep cold water “limb” of the AMOC and the Earth climate system.
Ocean feedback loop about to change?
In a recent study, Nansen Legacy researcher Øystein Skagseth (IMR) and co-authors, compared hydrographic observations from a cold period (1985-1999) to a warm period (2004-2018) in the Barents Sea. The results indicate that less cooling took place in the Barents Sea in the warmer period, contradicting the mechanism described above. However, the observed reduced cooling is attributed to the domination of unusual southerly winds, leading to warmer air temperatures and thus less cooling. Hence, the water exiting the Barents Sea has become warmer but is still salty and dense enough to plunge as BSW into the Arctic Ocean. However, that may change in the near future, as a fresh anomaly has been observed upstream in the AW in 2017/2018 and is expected to propagate into the Barents Sea in the years to come. Then the export water might no longer be dense enough to plunge.
“Under the assumption that the recent reduced cooling of the Barents Sea continues, the resulting BSW feeding the deep waters of the Arctic Ocean and the Norwegian Sea, and subsequently the Atlantic Meridional Overturning Circulation, will by far be the least dense throughout the instrumental period.” Øystein Skagseth
Skagseth Ø, Eldevik T, Årthun M, Asbjørnsen H, Lien VS, Smedsrud LH (2020) Reduced efficiency of the Barents Sea cooling machine. Nature Climate Change 10: 661-666. doi.org/10.1038/s41558-020-0772-6