The role of the Southern Ocean in abrupt transitions and hysteresis in glacial ocean circulation

• Published in: Paleoceanography and Paleoclimatology
• Main Authors: Hines, Sophia K. V., Thompson, Andrew F., Adkins, Jess F.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union 2019
• Subjects: Abrupt climate change; Glacial ocean circulation; Ocean circulation; Hysteresis; Greenland; Greenland ice cores; NADW; North Atlantic Deep Water; North Atlantic; Southern Ocean
• Online Access: https://doi.org/10.1029/2018pa003415

Highâ€latitude Northern Hemisphere climate during the last glacial period was characterized by a series of abrupt climate changes, known as Dansgaardâ€Oeschger (DO) events, which were recorded in Greenland ice cores as shifts in the oxygen isotopic composition of the ice. These shifts in inferred Northern Hemisphere highâ€latitude temperature have been linked to changes in Atlantic meridional overturning strength. The response of ocean overturning circulation to forcing is nonâ€linear and a hierarchy of models have suggested that it may exist in multiple steady state configurations. Here, we use a timeâ€dependent coarseâ€resolution isopycnal model with four density classes and two basins, linked by a Southern Ocean to explore overturning states and their stability to changes in external parameters. The model exhibits hysteresis in both the steadyâ€state stratification and overturning strength as a function of the magnitude of North Atlantic Deep Water (NADW) formation. Hysteresis occurs as a result of two nonâ€linearities in the modelâ€â€â€the surface buoyancy distribution in the Southern Ocean and the vertical diffusivity profile in the Atlantic and Indoâ€Pacific basins. We construct a metric to assess circulation configuration in the model, motivated by observations from the Last Glacial Maximum, which show a different circulation structure from the modern. We find that circulation configuration is primarily determined by NADW density. The model results are used to suggest how ocean conditions may have influenced the pattern of DO events across the last glacial cycle. © 2019 American Geophysical Union. Received 6 JUN 2018; Accepted 4MAR 2019; Accepted article online 15MAR 2019; Published online 5 APR 2019. We would like to thank Raffaele Ferrari, Emily Newsom, Andrew Stewart, David Marshall, James Rae, and Andrea Burke for helpful discussions, and two anonymous reviewers, whose comments improved the manuscript. S. K. V. H. received support from NSF grants OCEâ€1503129 and OCEâ€1204211 and the ...