School of Earth and Environment

Climate and Atmospheric Science (ICAS) PhD Projects

The role of sea-surface temperatures and vegatation in greening the sahara

Supervisors: Dr Alan Haywood, Dr Peter Knippertz, Dr Chris Taylor (Centre for Ecology and Hydrology, Wallingford, UK) & Dr Jean Maley (Université des Sciences et Techniques du Languedoc, Montpellier, France)
Collaborators: Matthew Collins (University of Exeter), John Hughes (Met Office), Tamsin Edwards (University of Bristol), Gil Mahé (Université Mohamed V-Agdal, Rabat, Maroc - and IRD, Montpellier, France)

Background and Rationale
Many climate archives across the Sahara point to significantly wetter conditions than today mainly during the first part of the Holocene, between ca. 10000 and 5000 years before present, and also during shorter periods in late Holocene. The wet conditions were reflected mainly by expansions of lakes, fluviatile deposits and a reduction in desert area in favour of xerophytic shrubland/woodlands and savanna. Work in the 1970s and 1980s led to the hypothesis that this wet phase was due to a northward progression of the West African monsoon, but more recent geological data suggest two Saharan rainy seasons per year. It is unclear to what degree changes in monsoon dynamics or mid-latitude influences contributed to this seasonality.

Attempts to model this African humid period have centered on the response of the West African monsoon to seasonal variations in the amount of incoming rain.

Reconstructed and modelled climate 6,000 years ago.  Left: Change in lake level status compared to the current situation. All lakes in Africa (in particular North Africa) indicate enhanced precipitation. Right: Failure of climate models to increase precipitation far enough north to enable the steppe vegetation climate archives indicate.

Aim and objectives of the project
The overarching aim of the project is an improved understanding of interactions within the Earth system that can support the assessment of potential future climate change. The main objectives are:

  1. Conduct climate simulations with a state-of-the-art climate model (HadCM3/HadGEM2) for the African Humid period about 6,500 years ago.
  2. Couple the model with a dynamic land surface vegetation model (JULES) to investigate interactions between the large-scale atmospheric circulation, rainfall and vegetation.
  3. Vary sea-surface temperatures (SSTs) within the range of uncertainties based on climate archives and explore the response in the climate system over Africa.
  4. Run the climate model with perturbed physics to assess potential consequences of model errors.
  5. Analyze the seasonality of rainfall and try to separate contributions from changes in the West African monsoon and in the extratropical circulation.

Potential for high-impact outcome
The conundrum of the green Sahara has fascinated scientists and the public for many years. The fact that climate models still struggle to reproduce the precipitation patterns suggested by climate archives is worrying and challenges our confidence in projections for the future. Any progress in our understanding and modelling of the African humid period is likely to lead to high-impact publications. The usage of the latest generation of climate models coupled to a land-surface-vegetation model together with sensitivity studies with respect to SSTs and model physics is a promising approach to make progress on this important scientific question. The issue of extratropical influences and seasonality has been disregarded by the research community so far and has therefore potential for a scientific breakthrough.

Training
Based within the Sellwood Group for Palaeo-Climatology (http://homepages.see.leeds.ac.uk/~sgpc) the PhD studentship will provide a board spectrum of training in analytical methods used in climate modelling and palaeoceanographic reconstruction. This will equip the student with the skills necessary to become the next generation of global change scientist, capable of contributing to understanding and predicting past and future climate change. Specifically, the student will be trained in coupled ocean-atmosphere modelling gaining considerable IT and programming skills. The student will attend the NERC Earth System Science and Urbino Summer School (in Italy) to broaden his/her understanding of Earth Systems Science and palaeoclimatology, and will have the opportunity to undertake a variety of postgraduate training workshops at the University of Leeds.  The student will also benefit from a close link with the NERC Centre for Ecology and Hydrology (CEH) and will visit CEH each summer to facilitate the integration of their modelling with the research priorities of this world-renowned institute. The co-supervision of Jean Maley will bring profound expertise in Saharan climate archives to the project.  

Further Reading
Joussaume, S., et al. (1999). Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP). Geophysical Research Letters 26 (7), 859-862.

Renssen, H., Brovkin, V., Fichefet, T., Goosse, H. (2006). Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period. Quaternary International 150, 95-102.

Coe, M. and Harrison, S. (2002). The water balance of northern Africa during the mid-Holocene: an evaluation of the 6 ka BP PMIP simulations. Climate Dynamics 19, 2, 155-166, DOI: 10.1007/s00382-001-0219-3.

Knippertz, P, Fink, A.H. (2009). Prediction of dry-season precipitation in tropical West Africa and its relation to forcing from the extratropics. Weather Forecast, 24, 1064-1084. doi:10.1175/2009WAF2222221.1.