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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities

Lead Supervisor: Andrew Meijers, British Antarcitic Survey  Co-Lead Supervisor: Peter Haynes, DAMTP Cambridge University

Importance of the area of research concerned: 
The global ocean transports and stores vast quantities of heat and is a critical component of the global climate. This circulation is modulated by both the temperature of water and its salinity, producing what is known as the global thermohaline circulation. Once the temperature and salinity properties of a parcel of water is set at the surface by interaction with the atmosphere or cryosphere and it is moved into the ocean interior, it can only be changed by subsequent mixing with other water. Climate models struggle to represent many of these transformation processes, particularly mixing in the interior of the ocean. Consequently they do not have consistent thermohaline circulations and this can strongly influence their global heat distribution and future climate projections of global surface temperature, increasing uncertainty in all aspects of the global climate. There is considerable effort being placed on climate model improvement and new metrics based on the thermohaline circulation are a promising new tool to look at the global impact and effectiveness of such improvements.
Project summary : 
This project will utilise new numerical projections of ocean model output to characterise the range of global thermohaline overturning circulations in the Intergovernmental Panel on Climate Change (IPCC) suite of climate models. These projections, alongside data on model mixing and surface fluxes of heat and freshwater, will then be used to identify the causes of differences across the ensemble, identify the most realistic representations and produce an improved climate projection on this basis. It will identify and test how transformative processes (mixing and surface fluxes) act to set the structure of these circulations and how they may change under future climate forcing. By identifying models with more realistic circulations and responses to forcing we will be able to more accurately weight projections for the future and reduce uncertainty in climate forecasts.
What will the student do?: 
This work is facilitated by recent advances that have produced ‘Temperature-Salinity (TS) streamfunctions’ (Zika et al., 2012;Döö et al., 2012); unique projections of the global thermohaline circulation. These have the useful property that their structure is set solely by ocean mixing or surface forcing. The student will apply the established T-S streamfunction technique to existing ocean models, primarily the IPCC CMIP5 ensemble. They will then quantify the differences in the T-S streamfunctions of these models and use the known surface heat and freshwater forcing to estimate effective mixing values. These may be compared against observed T-S streamfunctions and mixing estimates (Groeskamp et al., 2017) to rate the effective ‘realism’ of the models. They will then quantify the impact that the effective mixing and surface forcing has on how the TS-streamfunction is set, and how this changes in the presence of changes in surface forcing, such as under climate change. This may be done using both CMIP5 climate change scenarios, and by direct experimentation running a similar ocean model. Based on these experiments they will reevaluate future climate change in the CMIP5 ensemble.
Zika, J. D., England, M. H., & Sijp, W. P. 2012. The ocean circulation in thermohaline coordinates. Journal of Physical Oceanography, vol. 42(5), pp. 708-724.
Döös, K., Nilsson, J., Nycander, J., Brodeau, L., & Ballarotta, M. 2012. The world ocean thermohaline circulation. Journal of Physical Oceanography, vol. 42(9), pp. 1445-1460.
Groeskamp, S., Sloyan, B. M., Zika, J. D., & McDougall, T. J. 2017. Mixing inferred from an ocean climatology and surface fluxes. Journal of Physical Oceanography, vol. 47(3), pp. 667-687.
You can find out about applying for this project on the British Antarctic Survey (BAS) page.