| Summary: | The Gulf Stream transports macronutrients poleward as a part of the Atlantic meridional overturning circulation (AMOC). Scaling shows that this advective transport is greater than diapycnal transport from deep convection in the North Atlantic and is therefore crucial for sustaining the nutrient supply to the subpolar North Atlantic on interannual timescales. Simulations of the RCP8.5 emissions scenario with the Community Earth System Model (CESM) reveal 25% declines in the Gulf Stream volume transport above the potential density surface σθ = 27.5 kg/m3 and 35% declines in the associated nitrate transport between 2006 and 2080. The declining Gulf Stream transport largely explains contemporaneous 40% declines in zonally‐integrated volume and nitrate transports in the subtropical part of the AMOC. In addition, scaling suggests that the declining Gulf Stream nitrate transport (2.4 kmol/s per year) is the dominant driver of the declining export of particulate organic nitrogen across σθ = 27.5 kg/m3 in the subpolar North Atlantic (0.57 kmol/s per year), because the declining nitrate entrainment from water with σθ > 27.5 kg/m3 is only 0.44 kmol/s per year. A review of various small‐scale ocean physical processes suggests that the projected decline in the Gulf Stream nutrient flux is qualitatively robust to uncertainties associated with ocean physics.
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