Global ocean circulation

The meridional overturning circulation (MOC; also known as the Thermohaline Circulation) plays an important role in the climate system by transporting heat from low to high latitudes, and hence keeping some regions (such as the UK) warmer than they would otherwise be. The source water of the Atlantic MOC comes from cooling of the surface in high latitudes, producing dense water which sinks to several kilometres below the surface. Climate models suggest that as the climate warms, the water in high latitudes becomes warmer and less salty, leading to less sinking and a weakening of the MOC. Some studies have suggested that this slowdown could happen abruptly, with serious impacts on regional climate.

The group aims to increase our understanding of processes involved in the MOC through studying the circulation in current, future and paleo (historical) conditions. This is done by utilising the Met Office Hadley Centre Met Office seasonal and climate models, including an ensemble based on Met Office climate prediction model: HadCM3  but with variations in the representation of physical processes. This is a useful tool in understanding uncertainties in the processes. We are also involved in comparisons with observations and climate models from other climate centres.

Key aims

• Understand the processes affecting the MOC and how robust these are between different models.

• Assess the probability and impacts of rapid or irreversible shutdown of the MOC.

• Assess the effects of potential Greenland ice sheet melt on the MOC.

• Comparing computer simulations with paleoclimate data to understand the past climate.

Current projects

• The Risk assessment and probabilities of impact team (RAPIT) aims to produce a risk assessment of rapid or irreversible changes in the MOC by conducting a large number of experiments using climateprediction.net. This is part of the Natural Environment Research Council programme RAPID-WATCH to monitor and understand the MOC.

• Thermohaline Overturning - at Risk? (THOR) is an EU project whose goals are to establish an operational system that will monitor and forecast the development of the North Atlantic MOC on decadal time scales and assess its stability and the risk of a breakdown in a changing climate.

• There are also strong links with the Decadal prediction who are investigating predictions of the MOC in conjunction with the THOR and ENSEMBLES projects.