How the complexity of continental breakup controls Southern Ocean circulation

Around 30 million years ago, the globe underwent a climate transition from “greenhouse” to “icehouse” conditions, usually termed the Eocene-Oligocene transition (EOT). Co?evally, Southern Ocean horizontal circulation changed from a two-gyre pattern with a weak proto-Antarctic Circumpolar Current (ACC) to a more modern regime of ACC dominance. In addition, the meridional overturning circulation (MOC) shifted from a bipolar mode of the early MOC in the Pacific to the modern Atlantic MOC, associated with the tectonically driven opening of Southern Ocean gateways. In an Earth system, ACC and MOC are coupled together through their dependencies on the stratification. But they are not driven by each other.
The mechanisms driving the changes in Southern Ocean horizontal circulation through the EOT have been investigated for decades. Two main factors are emphasised in the paleoceanography community; the opening of ocean gateways and wind stress adjust?ments. However, previous studies show that neither deepening of ocean gateways nor wind stress modification alone obtain a strong proto-ACC with transport comparable to the modern ACC. Most past studies have focused on the effect of the gateway deepening and wind stress and/or have been run with a coarse-resolution model, with the parameterization of turbulent processes, such as mesoscale eddies. In addition, surface buoyancy (heat or salt) forcing, a key driver of deep-water formation, also likely controls the modern ACC, but has remained understudied in the context of EOT circulation changes.
In chapters 2 and 3, to understand the dynamics of the EOT ocean circulation changes, especially turbulent processes in the proto-ACC and their ability to transport heat, I use a high-resolution ocean model with realistic late Eocene bathymetry. These models reveal that when the maximum westerly winds align with both the deep Tasmania Gateway (TG) and Drake Passage (DP), the circumpolar proto-ACC initiates, with a maximum DP transport of 38.3 Sv, ∼27% of the modern strength. Proto-ACC transport still fails to reach the value of modern ACC even under a doubled wind stress. However, when applying modern surface buoyancy (heat/salt) forcing, a vigorous proto-ACC with a DP transport of 139 Sv, closely aligning with modern ACC strength, is formed. I, therefore, hypothesise that deep convection due to change in surface buoyancy forcing is the primary mechanism driving the transition from a weak proto-ACC to the modern ACC provided that deep Southern Ocean gateways allow the weakening of subpolar gyres and onset of proto-ACC.
In chapter 4, I demonstrate, using an idealised eddying ocean model, that the opening of a Southern Ocean gateway leads to the abrupt onset of vigorous, deep-reaching, meridional overturning circulation. The strength of the meridional overturning circulation is greatest with a shallow gateway and decreases with further deepening of the gateway. The abrupt change in the meridional overturning circulation when the gateway initiates can be explained by mesoscale processes that induce deep vertical heat transport at high latitudes where bottom waters are produced.