Climate Model Simulations of Effects of Increased Atmospheric CO2and Loss of Sea Ice on Ocean Salinity and Tracer Uptake

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Abstract
Recent observations show a decrease in the extent of Northern Hemisphere sea ice; this decrease has been attributed to human activities. Climate model simulations are presented that examine how loss of sea ice affects the ocean salinity and density structure, and rates of uptake of an idealized transient tracer. The latter results are indicative of how loss of sea ice might affect the ocean’s rate of uptake of anthropogenic carbon from the atmosphere. In simulations in which there is no freshwater forcing due to sea ice forming or melting, the salinity minimum associated with Antarctic Intermediate Water is much weaker than in simulations of the present-day ocean. This suggests that this salinity minimum is maintained in part by a steady supply of freshwater from melting of Antarctic sea ice. In addition, in simulations with no freshwater forcing due to sea ice, vertical salinity and density gradients in the Southern and Arctic Oceans are weaker than in simulations of the present-day ocean. This supports the notion that these gradients are maintained in part by freshwater forcing due to the seasonal cycle of formation and melting of sea ice. As a result, loss of sea ice due to global warming would tend to decrease the stability in parts of the ocean; this opposes the well-known tendency of global warming to increase ocean stability by warming and freshening the upper ocean. Simulations of ocean uptake of an idealized transient tracer in both constant-CO2 and increasing-CO2 environments are performed to investigate the effects of physical changes in ocean and sea ice on transient tracer uptake. In the Southern Ocean, physical changes to the ocean and sea ice are found that result in slower transient tracer accumulation in most locations. When averaged over the entire Southern Ocean, however, these reductions are small, because changes in convective activity due to increased atmospheric CO2 are relatively small, and because transient tracer uptake is relatively insensitive to changes in convective activity. These results suggest that Southern Ocean uptake of anthropogenic CO2 may decrease less than previously supposed as global warming progresses.

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