IAP-26-057

Ocean ventilation in a warming world

The ocean is playing a fundamental role in our changing climate. As the atmosphere warms due to rising greenhouse gases, the ocean is absorbing a large fraction of the excess carbon dioxide and almost all the excess heat. The rate at which this absorption happens is largely set by the transport of water from the ocean surface to its interior – a process known as “ventilation”. Ocean ventilation, and its potential to change in a warming world, is thus a key control on the overall rate of planetary warming. However, ventilation – and the associated absorption of excess heat and carbon – is a complex function of atmosphere-ocean interaction, ocean mixing and circulation, and biogeochemical dynamics, making it challenging to understand and predict. In this wide-ranging PhD, you will explore and test our basic understanding of how ocean ventilation operates, and its sensitivity to change in a warming world. You will ask probing questions such as “what sets the overall rate of ocean ventilation?”, “how is it related to features of the ocean like stratification and overturning?”, “is there evidence of change during the industrial era?”, and “how does it impact the warming rate of the climate?” Ultimately, the PhD will seek to provide a novel framework and theory for ocean ventilation that can be applied to constrain uncertainty in global warming projections.

The PhD will adopt a broad suite of methodologies including theory, numerical modelling, and observations (including both contemporary and paleo-ocean observations). The student will thus develop skills across a suite of scientific fields and analytical techniques to apply to the problem.

Using both simple conceptual models and coupled climate models, the student will explore the broad controls on ocean ventilation. Initially, the investigation will be diagnostic, using Lagrangian analysis to calculate timescales and pathways of ventilation in climate models. Subsequently, prognostic controls on these timescales and pathways will be analysed in simple conceptual models and verified against novel simulations in climate models. At all stages, results and understanding will be assessed against observational constraints, including proxy records extending back over the last thousand years.

Year 1

– Develop an intuition for existing frameworks and theories for understanding ocean ventilation, particularly transit time distributions and reduced-complexity models.
– Develop skills in relevant, commonly applied analytical approaches, including tracer inverse methods and Lagrangian analyses, and an understanding of observational constraints on ventilation.
– Apply Lagrangian analysis on a well-constrained problem such as thermocline ventilation in coupled climate models.

Year 2

– Write up work from Year 1.
– Work toward developing a theory for controls on ocean ventilation via the application and expansion of existing conceptual models.
– Refine theory to a testable hypothesis related to prior analysis.

Year 3

– Write up work from Year 2.
– Expand theory and testable hypothesis to apply across observational products and coupled climate models.

Year 3.5

– Write up work from Year 3.
– Compile and submit thesis.

– Deep scientific understanding in geophysical fluid dynamics, climate dynamics, and ocean biogeochemical dynamics.
– Technical and computational skills in model development, data analysis, and numerical simulation, including co-development with LLMs.
– Extensive expertise in scientific writing and oral and visual communication, for specialist and non-specialist audiences.