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Abstract. While there is agreement that global warming over the 21st century is likely to influence the biological pump, Earth system models (ESMs) display significant divergence in their projections of future new production. This paper quantifies and interprets the sensitivity of projected changes in new production in an idealized global ocean biogeochemistry model. The model includes two tracers that explicitly represent nutrient transport, light- and nutrient-limited nutrient uptake by the ecosystem (new production), and export via sinking organic particles. Globally, new production declines with warming due to reduced surface nutrient availability, as expected. However, the magnitude, seasonality, and underlying dynamics of the nutrient uptake are sensitive to the light and nutrient dependencies of uptake, which we summarize in terms of a single biological timescale that is a linear combination of the partial derivatives of production with respect to light and nutrients. Although the relationships are nonlinear, this biological timescale is correlated with several measures of biogeochemical function: shorter timescales are associated with greater global annual new production and higher nutrient utilization. Shorter timescales are also associated with greater declines in global new production in a warmer climate and greater sensitivity to changes in nutrients than light. Future work is needed to characterize more complex ocean biogeochemical models in terms of similar timescale generalities to examine their climate change implications. 

Earth system models (ESMs) project that global warming suppresses biological productivity in the Subarctic Atlantic Ocean as increasing ocean surface buoyancy suppresses two physical drivers of nutrient supply: vertical mixing and meridional circulation. However, the quantitative sensitivity of productivity to surface buoyancy is uncertain and the relative importance of the physical drivers is unknown. Here, we present a simple predictive theory of how mixing, circulation, and productivity respond to increasing surface buoyancy in 21st-century global warming scenarios. With parameters constrained by observations, the theory suggests that the reduced northward nutrient transport, owing to a slower ocean circulation, explains the majority of the reduced productivity in a warmer climate. The theory also informs present-day biases in a set of ESM simulations as well as the physical underpinnings of their 21st-century projections. Hence, this theoretical understanding can facilitate the development of improved 21st-century projections of marine biogeochemistry and ecosystems. 

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. 

Abstract We examine the effects of the submesoscale in mediating the response to projected warming of phytoplankton new production and export using idealized biogeochemical tracers in a high‐resolution regional model of the Porcupine Abyssal Plain region of the North Atlantic. We quantify submesoscale effects by comparing our control run to an integration in which submesoscale motions have been suppressed using increased viscosity. Annual new production is slightly reduced by submesoscale motions in a climate representative of the early 21st‐century and slightly increased by submesoscale motions in a climate representative of the late 21st‐century. The warmer climate at the end of the 21st century reduces resolved submesoscale activity by a factor of 2–3. Resolving the submesoscale, however, does not strongly impact the projected reduction in annual production under representative warming. Organic carbon export from the surface ocean includes both direct sinking of detritus (the biological gravitational pump) and advective transport mediated pathways; the sinking component is larger than advectively mediated vertical transport by up to an order of magnitude across a wide range of imposed sinking rates. The submesoscales are responsible for most of the advective carbon export, however, which is thus largely reduced in a warmer climate. In summary, our results demonstrate that resolving more of the submesoscale has a modest effect on present‐day new production, a small effect on simulated reductions in new production under global warming, and a large effect on advectively mediated export fluxes.