Rapid changes in ocean circulation and climate have been observed in marine sediments and polar ice cores, demonstrating that the ocean's current mode of circulation is not unique but can rapidly switch between dramatically different states, in conjunction with climate changes. These observations highlight the non-linear character of the climate system.

Numerical modeling indicates that changes in the Atlantic meridional overturning circulation (AMOC) induce ocean-wide reorganization in heat transport and temperature distribution. In turn, changes in climatic conditions impact ocean circulation (e.g. through the amount and location of freshwater input to the ocean). Therefore, a reliable assessment of future climate changes can only be achieved through the quantification and understanding of the interactions between ocean circulation changes and climate changes. However, instrumental records are too short to distinguish natural variability from anthropogenic change. Moreover, they only cover a small portion of the total range of climate variability. For these reasons, long-term data extracted from natural climate archives such as deep-sea cores, are required to understand the complex dynamics of the climate system.

To date, observed past rapid changes in climate and ocean circulation are still not fully explained and none of the models used for future climate projection has yet been able to simulate them. The most commonly invoked theories to explain AMOC changes focus on the importance of northern hemisphere ice sheet-ocean interactions, due to the ability of ice sheets to slowly build large stores of freshwater which can then be catastrophically discharged. However, climate modeling studies have so far not identified tipping points (i.e. critical thresholds at which a small change in forcing triggers a strongly nonlinear response in the internal dynamics of part of the climate system) for these rapid climate events, as fast responses are usually obtained with fast forcing. Only some simplified models show spontaneous abrupt millennial events or stochastic resonance behavior in the presence of a small periodic forcing.

Therefore, a number of key questions remain unanswered: What role did the ocean circulation play in past natural climatic variability? What are the mechanisms explaining past rapid climate changes? Under which conditions would new climate shifts occur?

The objectives of the ACCLIMATE (Elucidating the Causes and Effects of Atlantic Circulation Changes through Model-Data Integration) project are to address the above key questions. To do so, a 4-D database is being built, assembling existing and new well-dated Atlantic deep-sea time series over the last 40 ky, and full climatic information from these records will be extracted through a new model-data integration scheme using a proxy forward modeling approach. Such a modeling approach provides computed isotopic ratios that can be directly compared with deep-sea core measurements.

ACCLIMATE 4-D Atlantic database will primarily be composed of planktonic and benthic foraminifer d¹⁸O and d¹³C time series versus calendar age over the last 40 ky, with time resolutions ranging from 200 y to 50 y over specific climatic events. We will start from published and unpublished foraminifer?isotopic records obtained at LSCE on cores from Images coring cruises, make additional measurements in order to increase the time resolution, and generate new records in complementary locations. Before 2016, only 82 Atlantic cores stored in Gif-sur-Yvette were adequate for this purpose. However, most of these cores were collected in the North Atlantic. Only 14 of them were collected in the tropical South Atlantic during the RETRO 2008 and 2009 cruises, and only 1 was collected at higher latitudes in the South Atlantic. This set of cores is thus not sufficient to constrain circulation changes originating at high southern latitudes, whereas the Southern Ocean has been shown to play a major role in millennial climate variability. Therefore, the objective of the ACCLIMATE cruises in the South Atlantic is to fill this critical gap in data coverage.