IAP2-23-051

The Ocean’s Evolution: Deciphering Circulation Changes Since the Last Glacial Maximum

The ocean is thought to have played a central role in changing earth’s climate during the ice ages. Responding to slight changes in Earth’s orbit, changes in ocean circulation, biology, and chemistry resulted in substantial shifts in the exchange of carbon between the atmosphere and ocean. Through its impact as a greenhouse gas, this carbon exchange acted to amplify the warming or cooling that initiated from the orbital changes. To date, exactly how and why the ocean circulation changed during these periods remains a mystery (Ferrari et al., 2014; Marzocchi and Jansen, 2019). What is clear, however, is that the properties of the ocean were considerably different during ice ages to what they are today. Recorded in the shells of marine fossils, signals in paleo-oceanographic data show that, as well as a colder surface ocean, the deep ocean was also filled with much colder, saltier water (Adkins, 2013; Tierney et al., 2020; Annan et al., 2022).

What changes took place in the ocean circulation between then and now, and what role did those changes play in creating the comparatively warm and stable climate that we have today?

Previous attempts to understand this have suffered from shortcomings in data, and from a reliance on models that do not represent crucial process well. Applying novel theory and analysis used to understand changes in the contemporary ocean (Groeskamp et al., 2021; Zika et al., 2021; Sohail et al., 2023), this project will take a fresh perspective on the ocean during the last ice age. Specifically, we will reconstruct changes in the volumetric temperature distribution of the ocean – i.e., how much of the ocean is cold, how much is warm – to constrain the circulation changes that must have taken place over the last several thousand years. This direct approach avoids the caveats and pitfalls of classical numerical modelling approaches and will provide a complementary perspective on paleo-oceanographic changes. Insights gleaned from this work will inform the processes that drive major changes in CO2, long-term and rapid reorganisations of ocean circulation, and the vulnerability of major ice sheets to changes in ocean heat content.

The project will make use of an exciting array of data, from paleo-oceanographic datasets, contemporary observations, gridded observational products, and numerical circulation models. The initial challenge will be to accurately reconstruct the ocean temperature distribution during the last ice age. This will involve careful analysis of paleo-oceanographic data, including the application of advanced data science techniques (e.g. machine learning), to derive an approximate volumetric temperature distribution for the last ice age. Numerical simulations will be used to assess the accuracy of the approach, and to quantify the errors. The results will also be checked and evaluated against other reconstructions of the ice age ocean. Subsequently, the volumetric temperature distribution of the contemporary ocean will be calculated from modern observations, and the change in this distribution will be assessed. Using additional paleo-ocean constraints (e.g. the change in total ocean heat content; Pöppelmeier et al., 2023) , the volumetric temperature distribution changes will be attributed to specific processes, such as ocean mixing and ocean heat uptake. Further advances will extend the approach to understand changes in other components of the ocean, such as its biology and chemistry (Rafter et al., 2022).

Year 1

Training in the analysis of large datasets, paleo-oceanographic data, and numerical models. Literature review of ocean circulation during the last ice age, and approaches for evaluating it. Learning about novel theory and approaches for assessing change in the contemporary ocean. Begin the application of advanced data analysis techniques for reconstructing the volumetric temperature distribution from sparse data. Attend a national conference (e.g. Challenger Society) to get to know fellow students and researchers. Attend a relevant summer school or workshop.

Year 2

Deriving an estimate of the volumetric temperature distribution during the last ice age and assessing this estimate against other reconstructions. Prepare a publication on this reconstruction. Present this work at a national conference. Attend a relevant summer school or workshop.

Year 3

Develop and apply a methodology to use the change in volumetric temperature distribution between the last ice age and the contemporary ocean (together with additional paleo-oceanographic constraints, including changes in the carbon cycle) to understand the circulation changes that must have taken place, and their impact on CO2 rise. Assess this understanding against previous perspectives on these changes. Prepare this work for publication.

Year 3.5

Write up and submit thesis and present research at an international conference.

Geophysical fluid dynamics and ocean circulation theory.
Large-scale climate dynamics and the global carbon cycle.
Paleo-oceanographic proxies and isotope geochemistry.
Advanced data analysis techniques (e.g. machine learning).
Coding and data analysis in Python.
Big data analysis.
Contemporary approaches to climate data analysis.