Temporal trends and causes of deoxygenation: a comparison of the Northwest Atlantic Shelf and Atlantic Basin

Oxygen concentrations are of fundamental importance for organisms as well as geochemical cycling in oceans. Since the middle of the 20th century, oxygen concentrations have been declining in the open ocean and the coastal ocean. Located near the intersection of subtropical and subpolar circulation, the northwest Atlantic (NWA) Shelf is sensitive to climate variability. Recent work has been done on regional NWA trends in sea-surface temperature, salinity, and chlorophyll, but the trends and drivers of oxygen in the region have not yet been established. Here, we use World Ocean Database oxygen observations to determine the temporal trend of subsurface oxygen concentrations between 50-100m on the NWA Shelf from 1988 to 2019. We also use a regional NWA ROMS and MOM6 configuration to simulate the historical decadal trends and spatial patterns in dissolved oxygen concentrations over the shelf. Our results indicate a significant decrease of oxygen by 1.542±0.308 µmol/kg/year, which surpasses the established Atlantic basin-wide trend. The greatest subregional oxygen loss occurs on the Scotian Shelf and in the Gulf of St. Lawrence. A detailed analysis revealed that the oxygen trends on the NWA shelf are driven by changes in Apparent Oxygen Utilization (AOU), consistent with the decreased influence of Labrador Current in the region and associated water mass properties. Our model identifies the location of minimum oxygen concentrations occurring both at the bottom but also at midwater column depths in the Mid-Atlantic Bight and Gulf of Maine. Under SSP5-8.5, our dynamically downscaled projection (2014-2098) projects that the bottom oxygen in the NWA Shelf will accelerate relative to the historical period (1980-2014). Diagnosis of the mechanisms behind the future acceleration as well as the mid-water column minimum oxygen pattern using various tools will be presented.