Response of tropical Atlantic surface and intermediate waters to changes in the Atlantic meridional overturning circulation – RETRO

The objective of the present proposal is to apply for additional financial support in order to be able to carry out a coring cruise originally planned in May 2008 on the research vessel Marion Dufresne II (MD) chartered by the Institut Paul Emile Victor (IPEV). This cruise is a fundamental component of the ESF EuroMARC project RETRO, one of the few EuroMARC projects selected at the European level for funding from July 2007 to July 2010 (http://www.esf.org/activities/eurocores/programmes/euromarc/projects/retro). The RETRO project is funded by the national science agencies of 4 European countries (France, Norway, Germany, and The Netherlands) for a total of 1285 k'. NB: In addition to direct financial contribution, Norway provided R/V SARS for a 15 days long survey cruise on the North-Brazilian margin in December 2007 as an in-kind contribution to the project. Particular context: The RETRO project involves coring in two areas: (1) the Angolan margin, comprising 3 coring sites complementary to existing cores, and (2) the North-Brazilian margin, comprising 15 coring sites which have been identified during the survey cruise on board the Norwegian R/V SARS which took place in December 2007 within the RETRO project. Because of serious logistical problems, IPEV cancelled (on May 3, 2008) the Brazilian portion of the coring cruise which was going to be conducted on board MD from May 14 to June 6, 2008 and encouraged us to carry out the African portion of the planned cruise. A RETRO African leg took thus place from May 24 to June 3 2008. Since May 3, 2008, we have tried to find a solution to carry out the planned coring cruise on the North-Brazilian margin as soon as possible, in order to fulfill the RETRO project's scientific objectives. In August 2008, I was informed by Mr. G. Jugie, IPEV's Director, that the MD will be available to carry out the Brazilian portion of the RETRO cruise from May 15, 2009 on. However, the financial contribution of the European partners to the 2008 MD cruise (275 k' for the Brazilian part) is not sufficient any longer, because, in contrast to 2008, we will have to support the cost of the transit from and to La Réunion in 2009. This brings the total financial contribution asked by IPEV for the RETRO coring cruise on the North-Brazilian margin to 818 k'. Therefore, the objective of the present proposal is to apply for financial help in order to pay for the portion of the new transit costs not covered by the contribution from the European partners (i.e., 543 k'). RETRO's scientific objectives: Paleoceanographic data have demonstrated that the ocean's current mode of ventilation is not unique but can, and has, switched rapidly between dramatically different states with severe and far-reaching climate repercussions. Ocean circulation model experiments, and reconstructions from proxy data indicate that changes in meridional overturning circulation (MOC) are associated with ocean-wide reorganization in heat transport and temperature distribution, notably in conjunction with rapid climate events of the last 60,000 years. Reduced deep-water formation in the North Atlantic results in cooling in the North Atlantic region, but warming in the tropical thermocline and much of the southern hemisphere. Tropical Atlantic thermocline waters may greatly impact on MOC variability by storing heat that becomes available for rapid release, thereby inducing climate shifts. The hypotheses concerning oceanic processes are however still poorly constrained by available observations and core material with sufficient temporal and spatial resolution, thus preventing reliable assessments of the ocean's vulnerability to future changes. Based on geostrophic theory, it is possible to interpret past changes in the dynamics of the Atlantic MOC if 'palaeo-property' gradients can be estimated over depth transects on the eastern and western margins of the Atlantic basin. This forms the basis for contemporary observational programs monitoring the behavior of the MOC, and has guided the design of RETRO. The project requires the retrieval of 18 Calypso-cores from two specified depth transects in the eastern and western tropical Atlantic. Combining expertise and equipment from five leading European institutions in palaeoclimate research, we plan to: 1) reconstruct the vertical temperature and density gradients in the ocean interior and their evolution during climate shifts over the last 60,000 years; and 2) investigate the links between high and low latitude climate change through innovative modeling experiments making use of the generated high quality data. The long piston cores to be retrieved on board MD will allow us to study the feedbacks between climate change and ocean circulation changes, i.e., one of the key remaining unknown with respect to climate change prediction.