'Modulation of the North Atlantic Deoxygenation by The Slowdown of the Nutrient Stream'

We investigate the projected centennial changes in the heat and dissolved oxygen content of the upper water column (0-700m) simulated by a subset of Earth System Models included in the CMIP5 catalogue and a eddy-permitting regional ocean climate and biogeochemistry model configured for the subpolar North Atlantic basin. Atlantic Meridional Overturning Circulation (AMOC) and the associated western boundary currents act as the key transport pathways for nutrient-rich waters from low to high latitudes, termed as “nutrient stream”, which is responsible for maintaining biological productivity in the North Atlantic and North Pacific basins. Slowdown of the AMOC can weaken the supply of nutrients and potentially reduce the biological productivity of the North Atlantic. In all models examined, the Atlantic warms faster than the Pacific Ocean, resulting in a larger basin-scale solubility decrease. However, this thermodynamic tendency is compensated by changes in the biologically-driven O2 consumption which dominates the overall O2 budget. While an increase in biological O2 consumption reinforces and accelerates the O2 loss in the North Pacific, the reduction in biological O2 consumption moderates the deoxygenation of the North Atlantic. This is explained by the prominent reduction in the biological productivity, nutrient loading and northward nutrient transport in the North Atlantic, linked to the slowdown of the nutrient stream and the AMOC weakening. As a result, the North Atlantic resists the warming-induced deoxygenation due to the weakened biological carbon export and remineralization, leading to relatively higher O2 levels. On the contrary, the projected nutrient stream and macro-nutrient inventory in the North Pacific remain nearly unchanged. Our results have direct implications to the inter-basin differences in the oxygen-to-heat ratios and different trajectories of the oxygen contents and marine habitats between the two basins.