Search for a project

Is Atlantic Ocean circulation collapsing?

Dr Ruza Ivanovic (SEE), Dr Lauren Gregoire (SEE)

Contact email: r.ivanovic@leeds.ac.uk

This project uses climate modelling to better understand the stability and variability of Atlantic Ocean circulation and its control on surface climate.

Atlantic Ocean circulation is vital in regulating Earth’s surface climate. By transporting relatively warm, salty, shallow equatorial water north to the high latitudes, where it cools, sinks and returns southwards at depth, this net-circulation (known as the Atlantic Meridional Overturning Circulation, AMOC) drives global-scale ocean currents to redistribute heat around the planet. We know from the geological past that when AMOC is weak, Earth’s surface undergoes strong cooling of several degrees, particularly in the Northern Hemisphere, which experiences especially cold and bitter winters. Furthermore, rapid changes in AMOC are thought to have been responsible for abrupt warming and cooling events known to have taken place since the Last Glacial Maximum (21 thousand years ago).

All of this is important because recent observations suggest that AMOC is getting weaker, but we do not know for sure, because the instrumental record is so short (2004 to present). Could these observations be explained by natural variability in Atlantic circulation? Or do they represent a longer term slowing down? 

Figure 1. The North Atlantic overturning circulation with the location of the RAPID observational array moorings along 26°N. Image credit and more info: http://www.rapid.ac.uk/background.php, modified from Church, 2007.

The only way to answer these questions is to combine climate modelling with records of past AMOC, thus extending the observations backwards in time to (i) assess AMOC’s variability and stability, and (ii) find out whether the recent weakening is part of a longer term trend. However, no direct observations exist for the past. Instead, we must infer the physical ocean circulation from geological records of ocean chemistry, known as geochemical tracers. Including these tracers in climate models enables the direct comparison of simulated circulation to geochemical observations. 

Project Aim

The candidate will incorporate geochemical tracers of AMOC to a fast atmosphere-ocean general circulation model (FAMOUS). Using this tool, the candidate will run climate simulations to evaluate past AMOC and provide context for the short instrumental record.

Examples of research questions to be addressed:

  • How has variability in AMOC changed since the Last Glacial Maximum (21 thousand years ago to present)?
  • What forcings are required for AMOC to collapse and did it ever collapse in the last 21 thousand years?
  • Do multiple stable states of Atlantic Ocean circulation (AMOC) exist?
  • How stable is current Atlantic Ocean circulation? What is the risk of [near]-future collapse?

International Network

The candidate will have the opportunity to benefit from and feed directly into:

Potential for high-impact research

This exciting and novel work presents one of the strictest tests of our understanding of climate-ocean interactions and will directly challenge some of the existing paradigms in Earth System science; for example, the seminal notion that Atlantic Ocean circulation can have multiple stable states. The student will develop a highly sought-after, multidisciplinary skill-set, contributing towards the development of an interdisciplinary field of research that is at the forefront of climate science. By the nature of this work, and due to its timeliness, there is strong potential for the PhD candidate to influence the direction of international research being carried out on this theme, and to thus establish a world-renowned reputation for innovative science.

Training, support and research opportunities

This project affords many exciting opportunities for skills and research development, in particular:

Full-support for all technical and scientific aspects of the project, including the model development work, will be provided in-house (Leeds) and by external collaborators. With this training, the student will be well equipped to pursue their own research interests.

Entry requirements

A good first degree (1 or high 2i), Masters degree or equivalent in a physical or mathematical discipline; such as Physics, Mathematics, Oceanography, Meteorology, Climate Sciences, Earth/Environmental Sciences, Chemistry, Engineering or Computer Sciences. Some experience of computer programming is highly desirable e.g. in Fortran, C++, Python, MATLAB, IDL or R etc...

Further information

Some YouTube videos on: reconstructing climate history, surface ocean circulation and drivers of ocean circulation.

AMOC stability and variability:

Hawkins, Smith, Allison, Gregory, Woollings, Pohlmann and Cuevas. 2011. “Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport”. Geophys. Res. Lett., 38, 6 PP..

Jackson and Vellinga. 2013. “Multidecadal to Centennial Variability of the AMOC: HadCM3 and a Perturbed Physics Ensemble”. J. Clim., 26(7), 2390–2407.

Manabe & Stouffer. 1988. “Two Stable Equilibria of a Coupled Ocean-Atmosphere Model.” J. Climate 1 (9): 841–866.

AMOC observations (recent and past):

RAPID: observational array at 26° N in the Atlantic: http://www.rapid.ac.uk.

Fraija-William. 2015. “Estimating the Atlantic overturning at 26°N using satellite altimetry and cable measurements”. Geophys. Res. Lett., 42(9), 2015GL063220.

Kelly, Drushka, Thompson, Le Bars and McDonagh. 2016. “Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports”. Geophys. Res. Lett., 43(14), 2016GL069789.

Kilbourne, Klockmann, Moreno-Chamarro, Ortega, Romanou, Srokosz, Szuts, Thirumalai, Hall, Heimback, Oppo, Schmittner and Zhang. 2017. “Connecting paleo and modern oceanographic data to understand AMOC over decades to centuries”. A US CLIVAR Workshop Report, Boulder, Colorado. [online].

McManus, Francois, Gherardi, Keigwin, and Brown-Leger. 2004. “Collapse and Rapid Resumption of Atlantic Meridional Circulation Linked to Deglacial Climate Changes.” Nature 428: 834–837.

Roberts, Piotrowski, McManus and Keigwin. 2010. “Synchronous Deglacial Overturning and Water Mass Source Changes.” Science 327 (5961): 75–78.

Srokosz and Bryden. 2015. “Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises”. Science, 348(6241), 1255575.

FAMOUS General Circulation Model:

Jones, Gregory, Thorpe, Cox, Murphy, Sexton, and Valdes. 2005. “Systematic Optimisation and Climate Simulation of FAMOUS, a Fast Version of HadCM3.” Clim. Dyn. 25 (2-3).

Geochemical tracer implementation in models:

Rempfer, Stocker, Joos, Lippold and Jaccard. 2017. “New insights into cycling of 231Pa and 230Th in the Atlantic Ocean. Earth Planet. Sci. Lett., 468, 27–37.

Related undergraduate subjects:

  • Applied mathematics
  • Atmospheric science
  • Chemistry
  • Computer science
  • Computing
  • Earth science
  • Earth system science
  • Environmental science
  • Geochemistry
  • Geography
  • Geological science
  • Geology
  • Geophysical science
  • Geophysics
  • Geoscience
  • Mathematics
  • Meteorology
  • Natural sciences
  • Oceanography
  • Physical geography
  • Physical science
  • Physics