A solid understanding of the mechanisms behind the presently observed, rapid warming of the northwest North Atlantic Continental Shelf is lacking. We hypothesize that a weakening of the Labrador Current System (LCS), especially the shelfbreak jet along the Scotian Shelf, is contributing to these changes and that the future evolution of the LCS will be key to accurate projections. Here we analyze the response of a transient simulation of the high-resolution GFDL Climate Model 2.6 (CM2.6) which realistically simulates the regional circulation but includes only a highly simplified representation of ocean biogeochemistry. Then, we dynamically downscale CM2.6 using a medium-complexity regional biogeochemical ocean model to obtain projections of several ecosystem-relevant variables. In the simulation, the shelfbreak jet weakens throughout the century because of a reduction of the along-shelf pressure gradient caused by a buoyancy gain of the upper water column along the shelf edge. This buoyancy gain is the result of an increased presence of subtropical waters in the continental slope. Importantly, we find that the weakening of the shelfbreak jet is not in response to a northward shift of the Gulf Steam, as has been hypothesized by others, and that previous reports of a northward shift of the Gulf Stream North Wall (GSNW) are an artifact of the temperature-based GSNW criterion in common use. The projected weakening of the shelfbreak jet is likely to lead to a reduction in nutrient availability and a subsequent decline in productivity on the Scotian Shelf, Gulf of St. Lawrence, and Grand Banks.

Global ocean oxygen loss - deoxygenation - is projected to persist in the future. Previous generations of Earth system models (ESMs) have, however, failed to provide a consistent picture of how deoxygenation will influence oxygen minimum zones (OMZs; O2<= 80 μmol/kg), in particular the largest OMZ in the tropical Pacific Ocean. The expansion of the Pacific OMZ would threaten marine ecosystems and ecosystem services such as fisheries and could amplify climate change by emitting greenhouse gases. Here, we use the latest generation of ESMs (CMIP6) and a density framework that isolates oxygen changes in the thermocline and intermediate waters. We show that the Pacific OMZ expands by the end of the century in response to high anthropogenic emissions (multi-ESM median expansion of 2.4 * 10^15 m^3m, about 4% of the observed OMZ volume). The expansion is driven by a reduction of the shallow overturning circulation in the thermocline and a robust weakening of the oxygen supply to the upper OMZ in all ESMs. The magnitude of this expansion is, however, uncertain due to the less constrained balance between physical and biological changes in the lower OMZ. Despite uncertainties in the biological response, our results suggest that models with more complex biogeochemistry project weaker changes in the lower OMZ, and therefore stronger overall OMZ expansion. The fact that the OMZ largely expands in the upper ocean maximizes its ecological, economic, and climatic impacts (release of greenhouse gases).

We describe the baseline model configuration and simulation characteristics of GFDL’s Atmosphere Model version 4.1 (AM4.1), which builds on developments at GFDL over 2013–2018 for coupled carbon-chemistry-climate simulation as part of the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL’s AM4.0 development effort, which focused on physical and aerosol interactions and which is used as the atmospheric component of CM4.0, AM4.1 focuses on comprehensiveness of Earth system interactions. Key features of this model include doubled horizontal resolution of the atmosphere (~200 km to ~100 km) with revised dynamics and physics from GFDL’s previous-generation AM3 atmospheric chemistry-climate model. AM4.1 features improved representation of atmospheric chemical composition, including aerosol and aerosol precursor emissions, key land-atmosphere interactions, comprehensive land-atmosphere-ocean cycling of dust and iron, and interactive ocean-atmosphere cycling of reactive nitrogen. AM4.1 provides vast improvements in fidelity over AM3, captures most of AM4.0’s baseline simulations characteristics and notably improves on AM4.0 in the representation of aerosols over the Southern Ocean, India, and China—even with its interactive chemistry representation—and in its manifestation of sudden stratospheric warmings in the coldest months. Distributions of reactive nitrogen and sulfur species, carbon monoxide, and ozone are all substantially improved over AM3. Fidelity concerns include degradation of upper atmosphere equatorial winds and of aerosols in some regions.