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Supplementary material to “Variability in El Niño–Southern Oscillation Patterns and Potential Climate Effects”

Henry F. Diaz, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder

Yves M. Tourre, MEDIAS-France, Toulouse, and Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York

Citation:

Diaz, H. F., and Y. M. Tourre (2008), Variability in El Niño–Southern Oscillation Patterns and Potential Climate Effects, Eos Trans. AGU, 89(24), 220.

[Full Article (pdf)]

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Climate researchers discuss ENSO: its variability during the past millennium and potential impacts from climate change

In the last few decades there have been various efforts made to extend, by means of proxy climate records, the relatively short instrumental record of the El Niño-Southern Oscillation (ENSO) phenomenon. Most of these reconstructions are available at annual resolution (some with sub-decadal time-scales). Spatial and temporal changes in ENSO events have been documented and discussed in the literature, including changes in its secular and tropical variance structures [Quinn and Neal, 1992; Stahle et al., 1998; Allan, 2000; Mann et al., 2000; Ortlieb, 2000; Evans et al., 2002; Cobb et al., 2003; D´Arrigo et al., 2005; Tourre and White, 2006; Garcia-Herrera et al., 2008]. These changes may affect the ability of proxy records to reproduce ENSO variability in the equatorial Pacific with high accuracy. Long-term ENSO proxies calibrated against modern data exhibit significant power at decadal time-scales (from quasi-decadal to multi-decadal). While some of this low-frequency variability may be internal in nature, some it may be driven by external forcing, both natural (e.g. volcanoes and solar irradiance variations) and anthropogenic. Regardless of the source, such variability is found to not be homogeneous over time and interactions between different timescales of variability may occur or not during different periods, leading for example to so-called protracted ENSO events (Allan and D´Arrigo, 1999).

Given the importance of ENSO in generating global climate variability with well-documented regional effects and societal impacts, the implementation of an accurate portrait of ENSO becomes a high priority particularly in a climate change context. To examine the above issues, a workshop was held at the Richard B. Gump, South Pacific Research Station administered by the University of California-Berkeley, in Moorea French Polynesia, April 2–3, 2008. Instrumental records for monitoring ENSO are available from the 1860s, and reconstructions based of various proxy records (i.e., tree rings, corals) are available for about a millennium. The workshop evolved around two major themes: 1. Modern observations and proxy reconstructions of ENSO, and 2. Changes in ENSO and its signatures based on diagnostic analyses and modeling activities. Participants began by evaluating several known indices, such as the MEI [Wolter and Timlin, 1998], the SOI [Ropelewski and Jones, 1987; Allan et al., 1991; Können et al., 1998], and SST-based indices. They then evaluated the methodology and temporal characteristics of a suite of ENSO proxy records and how they matched in the time and frequency domains (cross validation).

A goal of the workshop was to better understand the temporal structure of past natural variability of the climate system through a better determination of the ENSO chronology for about the past 500 years. An improved understanding of ENSO variability should allow assessments of climate model simulations of past climate, as well as better evaluations of future ENSO variability in the context of global climate change. For example, interpretation of model projections of possible future changes of ENSO are complicated by considerable inherent multi-decadal ENSO variability evident in long climate model control runs. Additionally, there is new evidence that solar forcing can affect ENSO variability on at least the decadal timescale in observations, and some climate models show this response as well. Thus, interpretation of recent observations and future climate change projections requires a better quantification of both forced and inherent ENSO variability.

With that motivation, participants also considered the following question: Is it feasible to construct a consolidated or "best available" set of ENSO indices derived from the different proxies and reconstruction methodologies, and for how far back in time? The workshop also explored the possibility of generating new and more precise long-term ENSO reconstructions based on new proxy records and calibration techniques. Comparisons were made with long-term simulations of both forced (e.g. volcanic and solar-driven) and internal variability from a variety of coupled climate models that reproduce at least certain aspects of ENSO variability relatively well. There exist a number of methodologies that could be used to analyze these different ENSO indices, such as a Bayesian approach, and modeling of ENSO temporal variability using nonlinear interactions.

Although significant efforts have been made to collect relevant ENSO information in the early instrumental and historical documentary records, there remain considerable uncertainties regarding ENSO variability at lower frequencies. Evidence presented at the workshop indicated that there is still a lack of agreement between many of the climate proxies regarding past occurrences of ENSO. There are at least three major impediments to continued progress in assessing long-term ENSO variability. One involves the difficulty in finding and recovering suitable proxy climate records (including archival material) with suitable time resolution and continuity. This problem leads directly to the second—and perhaps more critical impediment to progress, which is related to a dearth of trained researchers throughout the world that could carry-out such time consuming projects. The third barrier to progress deals with the difficulty in securing adequate long-term support from funding agencies all over the world.

At a time when increasing efforts and funding support are being harnessed to improve coupled climate models in order to produce realistic simulations of ENSO's spatiotemporal patterns and variability, funding support for research and training to work on extending the ENSO record, lags behind considerably. Participants agreed to submit one or more review article to cover all the scientific aspects of ENSO variability discussed at the workshop.

References

—Henry F. Diaz, University of Colorado, CIRES, Boulder, CO; Yves M. Tourre, MEDIAS-France, Toulouse, and LDEO of Columbia University, Palisades, NY.

AGU is a worldwide scientific community that advances, through unselfish cooperation in research, the understanding of Earth and space for the benefit of humanity.

© 2008 American Geophysical Union

published 10 June 2008