Abstract
Abstract. Western boundary currents act as transport pathways for nutrient-rich waters from low to high latitudes (nutrient streams) and are responsible for maintaining midlatitude and high-latitude productivity in the North Atlantic and North Pacific. This study investigates the centennial oxygen (O2) and nutrient changes over the Northern Hemisphere in the context of the projected warming and general weakening of the Atlantic Meridional Overturning Circulation (AMOC) in a subset of Earth system models included in the CMIP5 catalogue. In all models examined, the Atlantic warms faster than the Pacific Ocean, resulting in a greater basin-scale solubility decrease. However, this thermodynamic tendency is compensated by changes in the biologically driven O2 consumption which dominates the overall O2 budget. These changes are linked to the slowdown of the nutrient stream in this basin, in response to the AMOC weakening. The North Atlantic resists the warming-induced deoxygenation due to the weakened biological carbon export and remineralization, leading to higher O2 levels. Conversely, the projected nutrient stream and macronutrient inventory in the North Pacific remain nearly unchanged.
Highlights
Deoxygenation of the oceans is potentially one of the most severe ecosystem stressors resulting from global warming given the high sensitivity of dissolved oxygen to ocean temperatures
The Earth system models (EaSMs) are broadly in agreement over the North Pacific regarding the PO4 spatial gradients, with the exception of CESM1-BGC, which underestimates the latitudinal differences with higher nutrient levels overall
There is a slight underestimation of PO4 in the subpolar region that is reflected in the multi-model mean (MMM) where values are ∼ 0.3 μM smaller than in the WOA09
Summary
Deoxygenation of the oceans is potentially one of the most severe ecosystem stressors resulting from global warming given the high sensitivity of dissolved oxygen to ocean temperatures. The dissolved oxygen is controlled by air–sea exchange, circulation, and biology, and the dissolved oxygen concentrations in the interior ocean reflect a balance between ventilation, circulation, and biological consumption. The solubility of dissolved oxygen is inversely proportional to seawater temperature, and air–sea O2 exchange is a relatively fast process in the ice-free open ocean, of the order of O(20 d) (Broecker and Peng, 1974; Wanninkhof, 1992). Changes in ocean stratification, ventilation, and biological productivity can further change dissolved oxygen. During the transient trajectory of the climate system as it adjusts to anthropogenic forcing, near-surface waters warm faster than deeper waters, leading to an increase in ocean stratification. Increased stratification is expected to weaken the Atlantic Meridional Overturning Circulation (AMOC) and the ventilation of Published by Copernicus Publications on behalf of the European Geosciences Union
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