° Quantifying THC (*) variability on time scales up to centennial and identification of the key processes and feed-back mechanisms responsible for this variability.
Concept and objectives
The climate of Europe is strongly influenced by the North Atlantic ocean circulation. Variations of the strength of the Thermohaline Circulation (THC) or the Meridional Overturning Circulation (MOC) are in several studies implicated as a main driver for decadal and longer time-scale changes for European and Northern hemisphere climate. Likewise, variations in THC is a commonly attributed mechanism for non-linear and abrupt (i.e., decadal scale) climate changes. Yet the observational and model underpinning of these hypotheses are at best sketchy making it very difficult to come to firm conclusions. Reliable quantification of the variability and stability of the THC and its atmospheric implications in todays and a warmer climate are therefore a major challenge in climate research. Whilst global models have been developed to produce long-term climate change projections, and short-term weather forecasts are carried out on a routine basis, there is a significant need for medium term regional climate forecasts, not only for the purpose of assessing the likelyhood for and eventually detecting rapid climate changes, but also to assist planning in both public and private sectors.
THOR will establish an operational system that will monitor and forecast the development of the North Atlantic THC on decadal time scales and assess its stability and the risk of a breakdown in a changing climate. Through the assimilation of systematic oceanic observations at key locations into ocean circulation models it will provide a set of geo-observational products that will be used to forecast the development of the system using global coupled ocean-atmosphere models.
• THOR will identify induced climate impacts of changes in the THC and the probability of extreme climate events with special emphasis on the European/North Atlantic region. Assimilation of observational data into ocean models will provide comprehensive long-term data sets making it possible to quantify the impact of THC variability on climate parameters, both on the regional and the larger scales. Millennium time scale experiments with coupled climate models and analysis of Palaeo data will identify the relevant key processes and feedback mechanisms between ocean, atmosphere and cryosphere.
• THOR will develop and operate an optimal ocean observing system for the North Atlantic component of the THC. This observation system, consisting of arrays of self contained instruments as well as ship- and space-borne measurements, will provide accurate time series of mass, heat and salt fluxes at key locations, allowing for the first time to assess the strength of the Atlantic THC.
• THOR will forecast the Atlantic THC and its variability until 2025. Coupled model simulations will provide forecasts of the ocean state on decadal time scales and at the same time quantify the significance of these predictions. Parameters predicted are strength of the THC, fluxes of mass, heat and salt, sea surface temperature and salinity and interior ocean fields, and associated climate variables for Europe and more widely.
• THOR will assess the stability of the THC to increased fresh water run-off from the Greenland ice sheet for various global warming scenarios. Increasing rates of fresh water from the Greenland ice sheet may reduce the strength of the THC. In THOR, the combined effect of global warming scenarios and melting of the Greenland ice sheet will be thouroughly assessed in a coupled climate model.
THOR builds upon techniques, methods and models developed during several projects funded within FP5 and FP6 as well as many nationally funded projects. The project will contribute to Global Monitoring for Environment and Security (GMES), to Global Observing Systems such as to the Global Ocean Observing system (GOOS), and to the International Polar Year (IPY).
(*) The Atlantic THC describes the meridional circulation of water, heat and salt, associated with the northward volume flux in the upper part of the ocean and the southward flux at depth.