北極海西部の氷の縁で測定値を収集。 クレジット: Peigen Lin/ウッズホール海洋研究所
最近の研究では、カナダ盆地の支配的な循環であり、北極海の最大の淡水貯留である高気圧性ビューフォート環流の安定化に関する最初の観測証拠が示された。
この研究では、2011 年から 2019 年までの「動的な海洋地形」衛星情報の最近長期記録が利用されており、この情報は 2 人の共著者によって提供され、2003 年から 2019 年までの包括的な水路データセットと組み合わせて、海洋の進化を測定しています。近年の環流の海面高さ。
2014年までの初期の動的な海洋地形データに依存したこれまでの観測とモデリングでは、環流が強化され、1970年代の気候と比較して淡水含有量が40％増加したことが文書化されている。 環流の安定化は、大量の淡水放出の前兆となる可能性があり、地球規模の気候の重要な要素である大西洋子午線逆転循環（AMOC）への影響など、重大な影響を与える可能性があります。
ビューフォート環流は「準安定状態に移行しており、環流の海面高さの増加が鈍化し、淡水含有量が頭打ちになっている。 さらに、太平洋とチュクチ海棚から生じる冷水と塩水の流入が減少したことと、大西洋からのより軽い水の同伴が増加したことにより、温水や塩分を含んだ大西洋水を深部で隔離している冷たい塩分層が大幅に薄くなっています。ビューフォート海東部。 に発表された最近の研究によれば、この最近のビューフォート環流の変化は、地域的な風力の変動の結果として、その位置が南東方向に移動したことに関連している。 
Sunrise at the ice horizon in the western Arctic Ocean. Credit: Peigen Lin/Woods Hole Oceanographic Institution
“Our results imply that continued thinning of the cold halocline layer could modulate the present stable state, allowing for a freshwater release,” the article states. “This in turn could freshen the subpolar North Atlantic, impacting the AMOC.”
Because there could be many potential local and remote impacts of the changing gyre on the hydrographic structure, physical processes, and ecosystem of the Arctic, “it is of high interest to better understand the factors associated with such changes—including the underlying causes,” the article notes.
“People should be aware that changes in the circulation of the Arctic Ocean could threaten the climate. It’s not only the melting ice and animals losing their habitat that should be a concern,” said Peigen Lin, lead author of the paper. Lin, who is an associate professor at the Shanghai Jiao Tong University’s School of Oceanography in China, conducted his research as a postdoctoral investigator at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts.
With the gyre being the Arctic Ocean’s largest freshwater reservoir, “if that freshwater gets released and ends up spreading into the North Atlantic, it could impact the overturning circulation, and, in an extreme case, disrupt it,” said co-author Robert Pickart, a senior scientist in WHOI’s Department of Physical Oceanography.
Collecting measurements in the ice, the western Arctic Ocean. Credit: Peigen Lin/Woods Hole Oceanographic Institution
The study, which includes an examination of long-term trends of the Beaufort Gyre and the causes of the thinning of the cold halocline layer, quantifies the evolution of the gyre in terms of its sea surface height as well as its freshwater content. “Both of these indicate that the Beaufort Gyre has stabilized in the second decade of this century,” Lin said.
The study also addresses the causes of the halocline thinning considerably as the gyre has evolved. The study notes that the recent decrease in the amount of Pacific origin winter water exiting the Chukchi Sea explains some of the thinning, and that the enhanced influence from the eastern Beaufort Sea – due to the gyre’s southeastward shift– likely also contributes to the thinning.
The recent documented state of the gyre “does not represent a return to the initial condition of 2003 when the gyre was weak and located partially in the southeastern basin. Instead, under the strengthened wind stress curl, the gyre has continuously intensified even though it has contracted, and it has maintained its excess freshwater storage,” the article notes.
Lin said a goal regarding the gyre is to understand the mechanisms behind its changes, which ultimately could allow scientists to predict what the gyre might do in the future.
“The community has been confounded by the fact that this gyre has kept growing and growing, and everyone is expecting it to release,” Pickart said. “Wouldn’t it be something if the gyre system and its freshwater accumulation and release could become somewhat predictable? Then, perhaps, we could also shed light on what a warming climate is going to do to this system.”
Reference: “Recent state transition of the Arctic Ocean’s Beaufort Gyre” by Peigen Lin, Robert S. Pickart, Harry Heorton, Michel Tsamados, Motoyo Itoh, and Takashi Kikuchi, 8 May 2023, Nature Geoscience.
DOI: 10.1038/s41561-023-01184-5
The study was funded by the National Science Foundation; the National Oceanic and Atmospheric Administration; the Shanghai Pujiang Program and Shanghai Frontiers Science Center of Polar Science; the European Space Agency Project and Natural Environment Research Council; the Arctic Challenge for Sustainability projects of the Ministry of Education, Culture, Sports, Science and Technology, Japan; and the Arctic Challenge for Sustainability II (ArCS II).
Co-authors Harry Heorton and Michel Tsamados of University College London provided the updated dynamic ocean topography data from 2011-2019 for the study. Co-authors Motoyo Itoh and Takashi Kikuchi of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) provided the mooring data regarding the Beaufort Gyre source water.