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C436: The dynamics of the changing Brewer-Dobson Circulation (Lead Supervisor: Peter Haynes, DAMTP

Supervisor: Peter Haynes (DAMTP)  

Importance of the area of research:

The Brewer-Dobson Circulation (BDC) is a large-scale circulation in the stratosphere which transports air upward at low latitudes and downward at high latitudes.  The BDC affects the distribution of important chemical species including anthropogenic species such as halocarbons that have an important greenhouse effect and also lead to ozone depletion. Future changes in the BDC, as part of the overall changes in the climate due to increased greenhouse gases are of great interest. The strength of the BDC is controlled by atmospheric waves on scales of 100s to 10000s km.  Therefore the ability of models to predict future changes in the BDC depends on their ability to predict future changes in the generation and the propagation of the waves and the resulting wave force on the mean flow. As with any aspect of long-term climate change, the model predictions cannot be checked directly and confidence in predictions relies on detailed examination of the processes being simulated by the models and comparison with the observed behaviour of the climate system in the recent past.

Project summary:

The project will examine different models of wave forces and the implications of the different models for predicted variations in the strength of the BDC, including on decadal (i.e. observable) time scales, and long-term change. The project will focus on low latitudes, where there are subtle interactions between the waves, and the resulting wave forces, and the background state. Different physical hypotheses have been proposed to explain the changes in wave forces and in the BDC that have been simulated in climate models. Assessing the relevance of these hypotheses (and whether they are of value in interpreting climate model predictions and in improving models) requires conversion of each into a concrete mathematical model and assessment of the behaviour that results.

What the student will do:

The student will survey relevant aspects of the theory of wave propagation and dissipation and of interaction between waves and mean flow and how they relate to the different physical hypotheses.  Simple models will be formulated that capture the effect of mean flow changes on the waves and hence the wave forces. (Different models might on the one hand emphasis changes in wave propagation and on the other might emphasis changes in the mixing by the waves as they break and dissipate.) These models will then be tested, using both analytic and numerical approaches, by considering the variation in the large-scale circulation that results when perturbations on different time scales (e.g. annual, interannual, centennial) are applied. This will be done first with the simplest possible representations  of large-scale processes and will then be extended to more realistic representations, e.g. of the effect of greenhouse gases, and potentially to representations that allow interaction between the circulation and important radiative chemical species such as ozone and water vapour. The behaviour generated by the various models will be assessed against that of state-of-the-art climate models.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Butchart, N., 2014: The Brewer-Dobson circulation. Rev. Geophys.,;52, 157-184, doi:10.1002/2013RG000448.

Ming, A., Hitchcock, P., Haynes, P.H., 2016: The response of the lower stratosphere to zonally symmetric thermal and mechanical forcing. J. Atmos. Sci., 73, 1903-1922.

Haynes, P.H., 2005: Stratospheric dynamics. Ann. Rev. Fluid Mech., 37:263-293.

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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