CR2025_32 Resolving the ocean energetics conundrum: does buoyancy forcing drive the ocean circulation?

Lead Supervisor: Rêmi Tailleux, Department of Meteorology, University of Reading

Email: r.g.j.tailleux@reading.ac.uk

Co-supervisors: Alessandro Silvano, University of Southampton; Tillys Petit, National Oceanography Centre; Bablu Sinha, National Oceanography centre

Large scale (> 1000 km) ocean circulation controls Earth’s climate and human life. Particularly relevant is its role in redistributing heat and nutrients across the planet essential, among others, to make some areas inhabitable (e.g. Northern Europe) and to sustain fisheries.

Over the past century several theories and modelling work have been developed to understand what drive large scale currents and how they can change in a warming climate. Despite several decades of scientific research, fundamental aspects of ocean circulation remain unclear limiting our ability to understand and predict Earth’s climate. Particularly challenging is quantifying the relative role of wind and buoyancy (i.e. heat and freshwater exchange between ocean and atmosphere/cryosphere) forcing. While the importance of the wind in powering the ocean circulation is well established, the importance of the buoyancy forcing has remained controversial, with opinions ranging from the latter having no role to being of similar importance to that of the wind. Pioneering theories indicate that wind forcing is essential in driving large-scale ocean circulation, including ocean gyres, the Antarctic Circumpolar Current, upwelling (i.e. vertical flow) in the Southern Ocean and the Gulf Stream (Vallis, 2006). More recently, modelling studies propose that also buoyancy forcing plays a key role in driving ocean currents (e.g. Hogg and Gayen, 2020), potentially dominant in some cases.

This PhD project aims to help reconcile these views using new theoretical approaches and numerical models. Theory will be based on recent developments of ocean thermodynamics (e.g. Tailleux, 2010,2013) which allows to study the ocean system in terms of energy balance and its relationship with external forcing such as winds and buoyancy input. Specific ocean currents will be studied including the Antarctic Circumpolar Current and sub-polar/tropical gyres. Numerical simulations capturing the key ingredients of large-scale ocean currents will be also implemented to study their sensitivity to wind and buoyancy forcing. By combining theory and numerical models, the project will provide a unique perspective on the drivers of ocean currents and thus on their sensitivity to global warming, helping resolve the energetics conundrum about the role of buoyancy forcing.

Training opportunities:

The student will learn about climate modeling, using the UK supercomputer ARCHER2, JASMIN, and the NOC cluster Anemone. Training includes running the NEMO ocean model. Opportunities for 1-2 month expeditions to the North Atlantic or Southern Ocean are available, with training in ocean observation techniques and a Sea Survival Course. Students can present results at conferences and participate in workshops and summer schools on climate variability or geophysical fluid dynamics. Access to lecture courses at the Universities of Reading and Southampton is provided. 

Student profile:

This project would be suitable for students with a degree physics/mathematics or a closely related environmental or physical science.

References:

• Hogg, A. M. C. & Gayen, B. Ocean gyres driven by surface buoyancy forcing. Geophys. Res.
• Lett. 47, e2020GL088539 (2020).
• Tailleux, R. Entropy versus APE production: on the buoyancy power input in the oceans energy cycle. Geophys. Res. Lett. 37, L22603 (2010).
• Tailleux, R. Available potential energy density for a multicomponent fluid with arbitrary nonlinear equation of state. J. Fluid Mech., 735, 449-518 (2013).
• Vallis, G. K. Atmospheric and Oceanic Fluid Dynamics Fundamentals and Large-scale Circulation (Cambridge Univ. Press, 2006).