What are the roles of natural and human drivers in historical changes in the Atlantic Meridional Circulation?

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The main aim of our research is to see how human and natural drivers have affected the ocean circulation in the North Atlantic. This circulation transports much of the heat into the North Atlantic and is partly responsible for the relative warmth of Europe and the UK. Some models suggest it might irreversibly stop in a changed climate which could cool Europe by 1-3 degrees centigrade. Based on the analysis of several different climate models the last Intergovernmental Panel on Climate Change assessment reported that the Atlantic circulation would show only a modest decline in the 21st century. NERC is funding two ocean observing arrays. These should produce high quality information on changes in the Atlantic Circulation but have only been operating since about 2005. Consequently, on their own, they cannot infer changes in the Atlantic circulation prior to 2005. These arrays are expensive to operate and maintain and an evaluation of their importance is to be made in 2011. Direct observations of the ocean circulation are sparse but through using ocean models in combination with these observations it is possible to make estimates of the large scale ocean circulation. Different research groups using different approaches have produced different estimates of changes in the circulation from these ocean reanalyses. We will use the existing set of estimates and compare them with key observations and use those that validate against those observations in our subsequent analyses. Using the validated ocean reanalyses and existing analysis of ocean temperatures we will compare 'fingerprints' of change from a large number of climate model simulations. This analysis will allow an estimate of the total human and naturally driven changes in the Atlantic and testing of model simulated changes against our best estimate of what has happened. If the models agree well with what has happened then we have increased confidence in their future predictions; if not then we have evidence of significant model error and thus a need to improve models. We will also separately examine the effect of human and natural drivers on the Atlantic to estimate their contribution to change. Other factors can also affect the climate and circulation of the North Atlantic. The first we plan to consider is a natural multi-decadal mode of variability called the Atlantic Multi-decadal Oscillation. We will explore how much of the difference from human driven changes in the Atlantic can be explained by this mode and if the proposed mechanisms for its existence are supported by observational evidence. Another factor that can affect the ocean circulations is the atmospheric winds; both through a direct effect on the ocean and through changes in the amount of heat and fresh-water moved in and out of the ocean. Changes in the temperature and fresh-water content of the ocean affect ocean density and thus the ocean circulation. Some scientists have suggested that changes in atmospheric circulation are responsible for changes in ocean temperature and circulation. We plan to carry out some novel simulations in which the atmospheric circulation is forced to be close to that observed in order to test this hypothesis. Finally we shall contribute to the evaluation of the RAPID funded ocean observing arrays by asking the question: 'How much would the arrays have told us, on their own, about changes in the Atlantic Circulation?' We will do this by using results from model simulations and the validated ocean reanalysis and simulating what the arrays would have seen. This will allow an estimate of what kind of changes the ocean observing arrays would be able to detect.