Regime Shift in Annual Nitrate Concentration in the Upper Southern Chukchi Borderland Responded to the Westward Shift of the Beaufort Gyre. Issue 10 (7th October 2022)

• Record Type: Journal Article
• Authors: Dong, Chunming; Luo, Xiaofan; Zheng, Zijia; Zhao, Wei; Wei, Hao
• Is Part Of: Journal of geophysical research. Volume 127:Issue 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-07
• Subjects: nitrate concentration -- Beaufort Gyre -- upper layer -- Chukchi Borderland -- western Arctic Ocean; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC018846
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Recent strengthening and westward movement of the Beaufort Gyre (BG), along with the rapid retreat of Arctic sea ice creates a new hotspot for investigating ecosystem responses to environmental changes over the Chukchi Borderland. As an important basis for primary production, nutrient variation and its controls in this hotspot region require to be preliminarily clarified. Using a hindcast simulation of a coupled ocean‐sea ice‐biogeochemical model (NAPA‐BGC) covering 1998–2015, this study suggests a regime shift in annual nitrate concentration in the upper southern Chukchi Borderland with the lower concentration since 2009. The relative contributions of biological and physical processes to annual nitrate variation are quantified via budget estimates. The results show a less temporally variable contribution of biological processes to nitrate variation, while the role of advection shifting from a nitrate source to a nitrate sink since 2009. This suggests physical processes mainly determine the nitrate transition in the upper southern Chukchi Borderland. Driven by the westward shift of the BG, the westward and southward expansion of the upper oligotrophic Canada Basin water narrows the pathway of nitrate‐rich shelf water flowing over the Chukchi Borderland. Meanwhile, the enhanced anticyclonic velocity at the marginal BG leads to an increased westward nitrate flux across the Chukchi Borderland and decreased nitrate supply to the north. Consequently, higher nitrate concentration exists in a narrow region to the south of 75°N, whereas the remaining areas have lower nitrate concentration, contributing to the overall reduction in annual averaged nitrate concentration since 2009 in the southern Chukchi Borderland. Plain Language Summary: With the intensified and westward movement of the Beaufort Gyre (BG) along with the rapid retreat of sea ice in the western Arctic Ocean, the Chukchi Borderland has become a frontier of research on the physical oceanography, biogeochemical cycles, and marine ecosystems. The westward shift of the BG has induced a regime shift in the nitrate concentration state from high to low in the upper southern Chukchi Borderland since 2009. Nitrate budget results show that biological processes contributed to nitrate variation with narrow temporal variability. Advection has shifted from a nitrate source to a nitrate sink since 2009, thus determining the nitrate transition in the upper southern Chukchi Borderland. Under the variation of the BG, upper layer oligotrophic water in the Canada Basin narrows the pathway range of nitrate‐rich shelf water flowing to the Chukchi Borderland. Changes in the anticyclonic circulation velocity structure around 2009 led to an increased westward nitrate flux through the Chukchi Borderland and decreased nitrate supply to the north, resulting in an overall decrease in annual averaged nitrate concentration. As the material basis for phytoplankton growth, the distribution pattern of nitrate will further affect the primary production and biological carbon pump. Key Points: The westward movement of the Beaufort Gyre drives the enhanced westward component of ocean currents in the southern Chukchi Borderland With the changed current, westward nitrate flux in the southern Chukchi Borderland increases, while northward flux decreases Nitrate concentration in the upper southern Chukchi Borderland has been shifting from a high level to a low level since 2009