Regime Shift in Annual Nitrate Concentration in the Upper Southern Chukchi Borderland Responded to the Westward Shift of the Beaufort Gyre

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 degrees 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.

Automated summary

The recent strengthening and westward movement of the Beaufort Gyre in the Arctic, along with the retreat of Arctic sea ice, creates a new hotspot for investigating ecosystem responses to environmental changes over the Chukchi Borderland. This study suggests a regime shift in the nitrate concentration in the southern Chukchi Borderland since 2009, with the primary influence being physical processes rather than biological processes. The westward movement of the Beaufort Gyre narrows the nitrate-rich shelf water flowing over the Chukchi Borderland, leading to lower nitrate concentration in the region since 2009.