Towards a Theory for the Surface Layer of the Ocean

Due to their huge heat capacity, the oceans are an essential controlling factor in long-term climate variability and climate change. The oceanic boundary layer, corresponding to the upper tens to hundreds of metres of the ocean, plays a key role in air-sea interaction. That layer is responsible for mediating the turbulent fluxes of momentum, heat, pollutants, and dissolved gases such as CO2 (which controls the atmospheric greenhouse effect).

Several studies have highlighted deficiencies in the representation of the oceanic boundary layer in global models, causing an insufficiently deep thermocline (which bounds below the mixed layer, where properties such as temperature and salinity are well-mixed). This causes a positive temperature bias at the ocean surface, which induces errors in the atmospheric response to this forcing.

Recently, Langmuir circulations, which consist of elongated vortices with horizontal axes of rotation roughly perpendicular to the crests of surface waves, have been found to play a key role in the ocean mixed layer. These circulations have very strong vertical velocity perturbations, being as effective as convection for mixing properties vertically, and deepening the thermocline.

These processes occur within the bulk of the mixed layer. The surface layer, which corresponds to the upper metres or tens of metres of the ocean in direct contact with the air-water interface and ultimately controls the fluxes across that interface, provides upper boundary conditions for the mixed layer, but is often not resolved adequately by numerical models.

Observations have shown that the scaling behaviour of the oceanic surface layer is fundamentally different from that of its atmospheric counterpart. There is a consensus that this is due to the effect of surface waves, but whether it results primarily from wave breaking, or from the interaction between non-breaking waves and the turbulence, is under debate.

In this project we intend to clarify whether non-breaking waves may account for the observed differences between the oceanic and atmospheric surface layers, to understand the dynamics of the ocean surface layer for different stability conditions, and to formulate a systematic description of its scaling properties, including heat and momentum fluxes, as was achieved for the atmosphere by Monin-Obukhov theory. This will allow the formulation of more robust boundary conditions at the air-water interface in lower-resolution coupled atmosphere-ocean models, and the improvement of air-sea interaction models.

The student will primarily perform Large-Eddy Simulations of the oceanic boundary layer using the LEM model at high resolution within the surface layer, in conjunction with more idealized calculations for providing physical insight. The project will also make use of recent oceanic turbulence measurements made available in the OSMOSIS project, coordinated by the Department of Meteorology. More specifically, the student will:

• Use existing (Rapid Distortion Theory) results to guide the development of functions describing the variation of mean and turbulent quantities in the oceanic surface layer.
• Use the field data and LEM simulations to test these relations, representing them in dimensionless form for maximum generality.
• Check whether the effect of non-breaking waves (via Langmuir circulations) can explain the profiles of mean and turbulent quantities in the oceanic surface layer.

This project will provide skills in numerical and mathematical modelling and data analysis. It will offer opportunities to attend postgraduate modules and summer schools on fluid dynamics (e.g. the Summer School on Fluid Dynamics of Sustainability and the Environment), as well as visits and placements to the Met Office, in Exeter.