Forecasting the Near-term Evolution of the Southern Ocean Aragonite Saturation Horizon

The Southern Ocean is particularly vulnerable to acidification due to cold seawater, upwelling, and a large influx of man-made carbon from the atmosphere. Despite this looming threat, our understanding of Southern Ocean acidification remains incomplete due to sparse chemical observations from infrequent scientific cruises. Over the past four years, the Southern Ocean Carbon and Climate Observations and Modeling project deployed over 100 floats that autonomously collected chemical observations from depths of 2000 m to the surface. We use this new data stream to calculate the depth of the present-day aragonite saturation horizon. We find generally deeper saturation horizons north of the Antarctic Polar Front and shallower horizons south of the Antarctic Polar Front, consistent with available ship-board observations. South of the Antarctic Polar Front, our analysis reveals multiple locations where a new, shallow saturation horizon will soon emerge, driven by the accumulation of anthropogenic carbon in the surface and thermocline. We use an ensemble of forecasts of Southern Ocean circulation and biogeochemistry from the Community Earth System Model Decadal Prediction Large Ensemble to pinpoint when and where this new horizon will emerge in the coming decade.