El-Niño : lessons from the past using simulations and observations – ELPASO

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 occurs every 2 to 7 years. Its monitoring, understanding and prediction has received lots of attention in the last decades because it strongly affects the economy of tropical regions (McPhaden et al., 2006). There are however major uncertainties regarding the future evolution of ENSO. The last IPCC report concluded that the different climate simulations suggest that ENSO type variability will continue in the future, but there is no consistent indication of discernible changes in projected ENSO amplitude or frequency in the 21st century.
Improved projections of ENSO and a better evaluation of their fidelity is one of the fundamental issues of the coming years. However, a good representation of the present-day climate and of the characteristics of the present day variability doesn’t guaranty a good simulation of future ENSO characteristics. The ELPASO project proposes to constrain the sensitivity of ENSO to climate change by studying the past history of ENSO using proxy-based climate reconstructions as well as numerical models. Such approach has proved to be successful as part of the Paleoclimate Modeling Intercomparison Project (PMIP, pmip2.lsce.ipsl.fr/). Compared to previous work the proposed study is going one step further and will provide a strong linkage between paleoENSO study, understanding of the modern ENSO dynamics, and the projected future changes of ENSO and ENSO-monsoon teleconnections. This is now possible given the increase number of paleo paleoclimate proxies with seasonal to decadal timescale that are required to analyse past ENSO variability. These proxy records can be completed by records of lower variability provided indications on the background climate state. The complementary objectives of ELPASO are :
1) To better understand the relationship between ENSO and the mean 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 periods of interest are 4 climatic periods (pre-industiral, future, mid-Holocene and Last Glacial Maximum) for which multi-model simulations will be performed in preparation of the next IPCC assessment (Taylor et al. 2009). These periods are completed by the analyses of the early Holocene, the late Holocene, and a period with strong monsoon in the last glacial (32 ka) to better understand the relationship between ENSO and the mean state, in periods where the insolation forcing favours or damped the development of the boreal summer monsoon. The 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 asses the background state. The focus will be primarily put on the tropical Pacific and Indian Oceans, and over land in Africa, India and South American in regions affected by monsoon. The proposed strategy will provide a suite of indicators ranging from simple diagnostics to more sophisticated tests to evaluate the fidelity of model to reproduce the observed ENSO and hence help the expert assessment of future climate projections based on our knowledge of past climate variations.