What did we do?     We did two things. First, we examined many climate model simulations of modern climate and the Last Glacial Maximum (LGM). Our goal was to find the relationship between the stratification and the circulation of the Southern Ocean. Secondly, we reconstructed the stratification of the Southern Ocean from the seafloor sediment taken from the south of Australia.

What did we find?     While the models show diverging predictions for the circulation strengths of the LGM Southern Ocean, we find a consistent relationship between the stratification and the circulation intensity. The stratified the oceans is, the more intense the circulation is. This is expected from the basic principle of physical oceanography called “thermal wind”. Based on the analysis of the sediment data, we did not find significant change in the stratification of the Southern Ocean between modern and LGM oceans. Even with the relatively large uncertainty in the proxy-based estimates, extreme scenarios exhibited by some climate models with ACC transports of greater than 250 Sv and highly saline Antarctic Bottom Water are highly unlikely.

Why is it important?   Understanding past climate of the Earth helps us to understand the present and future.  At the moment, climate model simulations show widely different representation of the Southern Ocean circulation during the LGM. Our analysis help constrain the possible scenarios for the stratification of the Southern Ocean during LGM. The principles of the physical oceanography can then be applied to relate the stratification change to the circulation change.

Reference: Lynch-Stieglitz, J., T. Ito, and E. Michel (2016), Antarctic density stratification and the strength of the circumpolar current during the Last Glacial Maximum, Paleoceanography, 31, doi:10.1002/2015PA002915.