IAP-24-079
Westerly winds overturning oceans
The westerly winds that blow over the Southern Ocean are some of the most energetic on Earth. Colloquially known as “the roaring forties” (warning seafarers of their fierce nature and the latitude band in which to expect them), the winds are strong enough to drive the global ocean circulation – bringing cold, dense, carbon-rich water from the ocean’s dark abyss all the way to its surface – with far-reaching implications for global climate. Changes in the strength and location of these winds, and associated impacts on the ocean circulation and carbon cycle, have together been hypothesized as a major driver of past climate transitions, such as the ice ages, as well as a crucial uncertainty in projections of future climate change. Nevertheless, the fundamental dynamics of these westerly winds, and their relationship to past and future climate changes, remain highly uncertain.
This ambitious PhD will focus on understanding the underlying dynamics of atmosphere-ocean interactions in the Southern Ocean. Specifically, the project will address three crucial unanswered questions:
1. What governs the strength of the westerly winds, and the empirical relationship between wind strength and latitude? How does this relationship vary across ocean conditions, and in response to global warming?
2. What determines the position of the major ocean currents in the Southern Ocean, and how does this change in response to changes in westerly wind strength and location?
3. How does the coupled atmosphere-ocean system in southern mid-latitudes change in response to global warming, and how do these changes accelerate or decelerate climate change via their impact on the carbon cycle?
Success in addressing these questions will advance fundamental understanding of the coupled atmosphere-ocean system and will provide new insights into the pace of global warming, the extent of sea level rise, and changes in marine ecosystems. Specifically, the models used in climate change projections that inform the Intergovernmental Panel on Climate Change (IPCC) will be validated against this newly developed dynamical understanding, allowing their veracity to be scrutinized.
Methodology
The student will address these questions using a host of tools, including theoretical frameworks and conceptual models, gridded observational datasets, idealized numerical simulations, and complex climate models. They will have freedom to explore the above questions in an order guided by their curiosity, but the following timeline is suggested:
Project Timeline
Year 1
Develop a conceptual model of the relationship between westerly wind strength and ocean surface temperatures, based on the atmospheric momentum budget. Simultaneously, using an idealized numerical model configuration of the atmosphere overlying a prescribed ocean state, vary the ocean conditions and examine the induced changes in the atmosphere to test and refine the conceptual model.
Year 2
Explore and progress existing conceptual models of the ocean currents’ position and strength, and their sensitivity to the overlying winds, the ocean temperature profile, and the ocean bathymetry. Test this conceptual model using an idealized, regional ocean model configuration with prescribed overlying atmospheric conditions. Vary the atmospheric conditions to explore the ocean response in a vorticity framework.
Year 3
Run global warming scenarios with a complex climate model, resolving the interacting dynamics between changing ocean and atmosphere conditions. Use this model to interrogate the previously derived conceptual models and develop further theory on how a dynamic relationship between the atmosphere and ocean alters the expected behaviour. Assess Southern Ocean atmosphere-ocean interactions in IPCC-grade climate model simulations.
Year 3.5
Write up and submit thesis.
Training
& Skills
Throughout the PhD, the student will write up and publish their results in academic journals as well as attend conferences to present their work, integrating them within the research community and alerting the community to their work. Additionally, they will attend summer schools on two or three occasions to continually enhance and broaden their learning and to develop relationships with their peers.
The student will be trained in several aspects of physical climate science including atmospheric dynamics, ocean dynamics and climate change. The student will also be trained in highly sought-after technical skills in computational modelling, high-performance computing, and quantitative ‘Big Data’ analysis.
References & further reading
Gray, William R., Casimir de Lavergne, Robert C. Jnglin Wills, Laurie Menviel, Paul Spence, Mark Holzer, Masa Kageyama, and Elisabeth Michel. 2023. ‘Poleward Shift in the Southern Hemisphere Westerly Winds Synchronous With the Deglacial Rise in CO2’. Paleoceanography and Paleoclimatology 38 (7): e2023PA004666. https://doi.org/10.1029/2023PA004666.
Rintoul, Stephen R. 2018. ‘The Global Influence of Localized Dynamics in the Southern Ocean.’ Nature 558 (7709): 209–18. https://doi.org/10.1038/s41586-018-0182-3.
Deng, Kaiqiang, Cesar Azorin-Molina, Song Yang, Chundi Hu, Gangfeng Zhang, Lorenzo Minola, and Deliang Chen. 2022. ‘Changes of Southern Hemisphere Westerlies in the Future Warming Climate’. Atmospheric Research 270 (June):106040. https://doi.org/10.1016/j.atmosres.2022.106040.
