The El‐Niño/Southern Oscillation (ENSO) is the major mode of climate variability in the tropical regions (Philander, 1990). It is a manifestation of the strong ocean‐atmosphere coupling in the tropical Pacific, and corresponds to a large scale east‐west sea‐saw across the Pacific basin that occurs every 2 to 7 years. During El‐Niño events warm waters are found in the east Pacific basin, along the Peru coast and the equatorial upwelling. The atmospheric deep convection and heavy precipitation are also displaced from the west Pacific warm pool to the middle of the basin. Its counterpart, La‐Niña, has the reverse pattern with enhanced cooling in the east and westward shift of the major structures of the atmospheric circulation. ENSO variability strongly affects the hydrological cycle in several tropical regions, such as monsoon areas in India (Webster and Yang, 1992), Africa (Janicot et al., 1996: Rowell, 2001) and South America (Grimm et al., 2000: Ropelewski and Halpert, 1987). Its monitoring, understanding and prediction have received lots of attention in the last decades because it strongly affects the economy of tropical regions (McPhaden et al., 2006). Progresses have been made in its representation in climate models, and in its prediction 3 to 4 months in advance (Wang, B. et al., 2009).

In this context the objectives of ELPASO are :

1. To better understand the relationship between ENSO and the mean climate state by exploring ENSO variability and ENSO/ monsoon teleconnections in different climatic contexts.

2. To provide a set of metrics based on mechanisms and on paleodata to test the ability of climate models to reproduce ENSO variability and ENSO/ monsoon teleconnections in a climate different from the modern one.

To achieve these goals the ELPASO consortium gathers specialists of present, past and future climate modelling, and specialists of different proxy records, combining high resolution archives such as corals, molluscs, speleothems and laminated sediments to assess climate interannual to multi‐decadal variability and lower resolution archives such as marine sediments, moraines, tropical ice cores or lake sediments to assess the background state. This project is closely connected to the effort made as part of the international CMIP (http://cmip‐pcmdi.llnl.gov/cmip5/) and PMIP (http://pmip3.lsce.ipsl.fr/) intercomparison projects to improve our understanding of climate change. Results from this research project will be rapidly integrated in similar efforts that will emerge at the international level.