Simulated reduction in upwelling of tropical oxygen minimum waters in a warmerclimate

Abstract

Waters of the Atlantic and Pacific tropical oxygen minimum zones (OMZs), located in thepoorly ventilated shadow zones of their respective ocean basins, reach the sea surfacemostly in the eastern boundary and equatorial upwelling regions, thereby providingnutrients sustaining elevated biological productivity. Associated export of sinking organicmatter leads to oxygen consumption at depth, and thereby helps to maintain the tropicalOMZs. Biogeochemical feedback processes between nutrient-rich OMZ waters andbiological production in the upwelling regions and their net impact on the evolution ofthe OMZs depend on the strengths of the flow pathways connecting OMZs andthe upper ocean, because even though water has to be isolated below the mixedlayer for some time in order for OMZs to develop, it has to be brought up to thesurface mixed layer eventually in order to exchange properties with the atmosphere.Here, we investigate the connections between OMZs and the surface mixed layer, and theirsensitivity to global warming with a coupled ocean–atmosphere general circulation modelby analyzing the fate of simulated floats released in the OMZs. We find that underpresent-day climate conditions, on decadal time scales a much larger portion of the model’sOMZ 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 atmosphericCO2 is doubled, the fraction of modeled OMZ waters reaching the upwelling in the same timedecreases by about 25% in both oceans. As a consequence, feedback betweenbiogeochemical processes in OMZs and in the surface ocean is likely to be weakened in thefuture.