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Nitrogen constrains biomass across the Arctic Ocean, with nitrate (NO3) supply to the surface waters fuelling new primary production and net carbon drawdown. In this Review, we explore the physical mechanisms driving NO3 fluxes to the euphotic zone across the Arctic Ocean and how biological processes respond. The volume and inflow depth of Atlantic and Pacific Ocean waters, together with sea ice and halocline dynamics, govern internal physical mixing of NO3. Respectively, these inflows supply ~34 ± 5 kmol NO3 s−1 and 9 ± 1 kmol NO3 s−1, spreading at mid-depth. NO3 from below the euphotic zone is mixed upwards via several mechanisms. Overall, NO3 fluxes associated with diffusive and turbulent mixing, submesoscale fronts and cyclonic mesoscale eddies are relatively low (on the order of ~0.1–0.7 mmol m−2 per day) but cover a large area, with peaks associated with wind events or individual strong eddies. By comparison, upwelling-driven fluxes are much stronger (on the order of ~1 mmol m−2 per day) but are more localized. Near-inertial and tidal mixing over the Arctic Ocean’s complex bathymetry drives perhaps the strongest NO3 fluxes, for example, reaching 4.5 mmol m−2 per day in the Barents Sea. Comparing these fluxes with observed biological NO3 uptake rates indicates that the internal physical supply of NO3 only limits primary productivity in 9 of the 17 cases considered. Thereafter, light limitation and lagged growth responses can result in excess NO3 remaining in the surface waters. Future research should prioritize linking NO3 supply and uptake at corresponding spatiotemporal scales. 

Abstract This study quantifies the overturning circulation in the Arctic Ocean and associated heat transport (HT) and freshwater transport (FWT) from October 2004 to May 2010 based on hydrographic and current observations. Our main data source consists of 1165 moored instrument records in the four Arctic main gateways: Davis Strait, Fram Strait, Bering Strait, and the Barents Sea Opening. We employ a box inverse model to obtain mass and salt balanced velocity fields, which are then used to quantify the overturning circulation as well as HT and FWT. Atlantic Water is transformed into two different water masses in the Arctic Ocean at a rate of 4.3 Sv (1 Sv ≡ 10⁶m³s⁻¹). Combined with 0.7 Sv of Bering Strait inflow and 0.15 Sv of surface freshwater flux, 2.2 Sv flows back to the south through Davis Strait and western Fram Strait as the upper limb of the overturning circulation, and 2.9 Sv returns southward through Fram Strait as the lower limb of the overturning. The Arctic Ocean imports heat of 180 ± 57 TW (long-term mean ± standard deviation of monthly means) with a methodological uncertainty of 20 TW and exports FW of 156 ± 91 mSv with an uncertainty of 61 mSv over the 6 years with a potential offset of ∼30 mSv. The HT and FWT have large seasonalities ranging between 110 and 260 TW (maximum in winter) and between 40 and 260 mSv (maximum in winter), respectively. The obtained overturning circulation and associated HT and FWT presented here are vital information to better understand the northern extent of the Atlantic meridional overturning circulation.