The Atlantic Meridional Overturning Circulation (AMOC) involves a northward movement of warm surface waters balanced by a southward movement of cold deep waters. The net effect is to transport ~1PW of heat northwards. This heat is released to the atmosphere in mid-high latitudes, where it acts to warm the climate, notably in northern Europe. The future behaviour of the AMOC is an issue of major importance in climate prediction. Forecasts presented by the Intergovernmental Panel on Climate Change (IPCC) suggest that, in response to greenhouse gas forcing, the AMOC may slow down, reducing the northward transport of heat by the Atlantic Ocean, and leading to a cooling of northern Europe that could offset anthropogenic warming. Moreover, there is evidence from palaeoclimate records that the AMOC can undergo very rapid transitions such as a total shutdown within little more than a decade.

It is possible that increasing levels of greenhouse gases could trigger such a rapid change with potentially serious consequences for societies in Europe and other regions surrounding the Atlantic basin. In the face of such risks, there is an obvious need for better, more quantitative, forecasts of the future behaviour of the AMOC. Such forecasts could provide early warning of possible rapid changes in the AMOC in future The RAPID array is a measurement system for observing the current state of the AMOC.

The overarching goal of the VALOR project was to assess the value of these observations for predicting the future behaviour of the AMOC, and its impacts on climate. The project explored a range of issues concerning the design of a potential AMOC prediction system. To achieve its goals VALOR exploited the RAPID observations in a variety of ways. First the observations were used as independent data to assess the quality of current ocean "analyses" (An "analysis" provides a quantitative description of the state of the ocean at a given time.). Next, they were used to improve the analyses. Finally, they were used to provide the starting conditions for a large set of "hindcasts". Hindcasts are predictions made from a date in the past, which only make use of information that would have been available at that date. These predictions can then be compared to what actually happened to assess prediction skill.

VALOR carried out a suite of hindcast experiments to quantify the extent to which the RAPID observations can improve the skill of predictions of the AMOC and its impacts on climate. An important dimension of the project is that it involves agencies who are directly involved in operational climate forecasting: the Met Office Hadley Centre (MOHC) and the European Centre for Medium Range Weather Forecasting. By involving these partners from the start the project will benefit from their experience and expertise, and the scientific advances achieved through the research will feed directly into better climate predictions.