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Author ORCID Identifier

https://orcid.org/0000-0001-5164-0030

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Geosciences

Year Degree Awarded

2019

Month Degree Awarded

September

First Advisor

Robert DeConto

Subject Categories

Climate | Oceanography

Abstract

The late Pliocene (2.6-3.3 Myr) was an epoch of gradual cooling, with expanding Antarctic ice sheets and sea ice preceding a general Northern Hemisphere glaciation. A decline in the strength of the Atlantic Meridional Circulation (AMOC) in the late Pliocene may have decreased Southern Hemisphere oceanic heat transport into the Northern Hemisphere; pre-conditioning it for glaciation. A common explanation for a weakening of the AMOC in paleoclimate is freshwater forcing into the North Atlantic. In this thesis, I posit that a northward shift in the Southern Hemisphere westerlies in the late Pliocene, due to an expanded Antarctic ice sheet, weakens the Agulhas Leakage — a narrow current flowing past the Cape of Good Hope that connects the Indian and Atlantic Ocean basins. Since the Agulhas Leakage transports relatively salty waters from the Indian Ocean into the South Atlantic, it could alter the AMOC by changing the density stratification in North Atlantic deep-water formation sites. To test this hypothesis, I use a high-resolution (1/6°) ocean model, the MITgcm, with passive and Lagrangian particle-based tracers to quantify changes in Agulhas Current leakage. Following a spin-up simulation, I run two experiments, an experiment with a prescribed 6° northward shift in the southern westerlies and a control experiment with unchanged winds, both run for 58 model-years. I found a quick (< 10 year) decline in Agulhas Leakage volume in the northward wind perturbation experiment that, by the experiment end, led to changes in North Atlantic surface densities, including in areas of North Atlantic deep water formation (NADW), with an increase in vertical stratification in the perturbation experiment suggesting a weakening of the NADW. The findings from my research indicate that Agulhas Leakage water has a pathway into the North Atlantic and that the northward wind perturbation decreased the influx of Agulhas water into the North Atlantic, with corresponding implications for salt and heat transport. A northward shift in the southern westerlies is therefore a plausible mechanism for inter-hemisphere heat transport, altering the surface branch of the AMOC and inter-hemisphere heat transport and as such likely played a role in altering climate during the late Pliocene.

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