Is Atlantic Ocean circulation collapsing?

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

SCIENTIFIC BACKGROUND:
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 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?

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:
- 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 AMOC exist?
- How stable is current Atlantic Ocean circulation? What is the risk of [near]-future collapse?

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.