ALBERT

Description: Waters of the Atlantic and Pacific tropical oxygen minimum zones (OMZs), located in the poorly ventilated shadow zones of their respective ocean basins, reach the sea surface mostly in the eastern boundary and equatorial upwelling regions, thereby providing nutrients sustaining elevated biological productivity. Associated export of sinking organic matter leads to oxygen consumption at depth, and thereby helps to maintain the tropical OMZs. Biogeochemical feedback processes between nutrient-rich OMZ waters and biological production in the upwelling regions and their net impact on the evolution of the OMZs depend on the strengths of the flow pathways connecting OMZs and the upper ocean, because even though water has to be isolated below the mixed layer for some time in order for OMZs to develop, it has to be brought up to the surface mixed layer eventually in order to exchange properties with the atmosphere. Here, we investigate the connections between OMZs and the surface mixed layer, and their sensitivity to global warming with a coupled ocean–atmosphere general circulation model by analyzing the fate of simulated floats released in the OMZs. We find that under present-day climate conditions, on decadal time scales a much larger portion of the model's OMZ waters reaches the surface ocean in the Pacific than in the Atlantic Ocean: within 20 years, 75% in the Pacific and 38% in the Atlantic. When atmospheric CO2 is doubled, the fraction of modeled OMZ waters reaching the upwelling in the same time decreases by about 25% in both oceans. As a consequence, feedback between biogeochemical processes in OMZs and in the surface ocean is likely to be weakened in the future.