Our Common Future Under Climate Change

International Scientific Conference 7-10 JULY 2015 Paris, France

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ENSO IN A CHANGING CLIMATE

Overview


Organizers : CLIVAR
Date : July 8th, from - to be precised
Location : IPSL, Paris, France
Expected number of participants : 1-50
Nature of participants : ENSO experts, by invitation
Keywords : ENSO, Tropical climate variability
Keynote speakers :
  • E. Guilyardi, LOCEAN/IPSL, UPMC, Paris, France
  • A. Wittenberg, GFDL, Princeton, United States of America



Summary

The El Niño – Southern Oscillation (ENSO) is the dominant mode of interannual climate variability on Earth, with worldwide impacts. Because ENSO involves a complex interplay of ocean and atmospheric processes, accurately modelling this climate phenomenon with coupled General Circulation Models (CGCMs), and understanding and anticipating its behaviour in a warming climate, still pose major challenges. Over the past three decades, new theoretical insights, together with longer and more comprehensive observations, increased computer power, and improved physical parameterizations of subgrid-scale processes, have resulted in better understanding of ENSO dynamics and much improved simulations of ENSO in CGCMs.

While many of the basic properties of ENSO are now well understood and simulated, the community is still faced with the problem of addressing its detailed mechanisms and impacts (e.g. extremes, inter-event diversity, physical feedbacks, asymmetries between El Niño and La Niña), their statistical robustness in light of short observational records, and how these evolve in a decadally- to centennially-varying background climate. Metrics to evaluate ENSO and the tropical Pacific in models need to be designed. In addition, a review of new developments – including recent research on extreme El Niño events, ENSO predictability, and ENSO non-linearity – is needed to consolidate existing understanding and plan for future research, including the design of a future Tropical Pacific Observation System (TPOS).

The World Climate Research Program (WCRP) has long recognised the central importance of understanding and predicting ENSO and this workshop is the first to be organised under the new CLIVAR ENSO Research Focus. Beyond addressing CLIVAR goals, this workshop will contribute to several of WCRP’s Grand Challenges and discuss research priorities as well as topics for future workshops.

The specific goals of the workshop are to review, discuss and report on 1) The current mechanistic understanding of ENSO, particularly in the context of a warming climate, 2) Observations of the tropical Pacific, and metrics to evaluate and intercompare models, 3) The role of the tropical Pacific in global-scale decadal variability including the recent hiatus in global warming and 4) Anticipated changes in ENSO properties over the coming decades.

Key outcomes


Recommendations



  • Understanding ENSO underlying mechanisms is a very active field and should continue to be encouraged.

  • The role of intraseasonal variations should be explored further – e.g. how the character of intraseasonal wind events may change in a warmer climate, how these events are modulated by ENSO itself, as well as the degree of their predictability, if any.

  • Coordinated simulations (ENSOMIPs) could be performed to further explore several themes discussed during the workshop. The trend vs. decadal variability issue should be discussed with the CLIVAR and CMIP decadal groups, as they are topics of common interest. Assessments of ENSO predictability, evaluating the roles of both large-scale precursors and stochastic wind forcing, would benefit from coordinated simulations. To better understand the teleconnections and impacts of ENSO in a warmer climate, SST-forced atmosphere-only (“AMIP” style) runs could be devised using present-day and projected future SST anomaly patterns, added to a projected future SST climatology.

  • Climate model evaluation is a specific task of the CLIVAR Research Focus group, and should involve both the definition of the metrics, and identification of the observations needed (see TPOS recommendations below).

  • Work is also needed to improve our ability to interpret changes in oxygen isotope records in paleo observations, in terms of changes in different physical variables, such as water temperature and salinity.Engagement from the atmospheric and oceanographic scientific community in this undertaking is encouraged.

  • The role of CLIVAR ENSO research scientists in contributing to ENSO alerts was discussed, and the conclusion was that such alerts are best left to the operational centers.


  TPOS recommendations



  • Beyond direct measurements of the tropical Pacific ocean temperatures, surface fluxes, and currents, improved models, assimilation systems, and reanalysis efforts are needed to make optimal use of these observations. In addition,new measurement types, such as salinity and seawater oxygen isotope ratios, would help to constrain coral proxy reconstructions of ENSO’s past, towards understanding ENSO’s future. And in addition to TPOS, it is key to encourage and support the recovery of past observations that have not yet been digitized (e.g. from ships’ logs), as proposed by several on-going community efforts.

  • The surface wind stress on the ocean is critical to constraining ENSO air-sea feedbacks and capturing the impacts of equatorial wind bursts on ENSO dynamics and forecasts. Yet there remains surprisingly little convergence among the available observational and reanalysis estimates of the tropical Pacific wind stress. Improved collaboration among satellite and in situ observational communities is urgently needed to resolve these discrepancies.

  • Work is also needed to explore what observations are required to improve the physics of the models as well as initialize ENSO forecast models. This will require close collaboration between the ‘Model and Data Assimilation’ task team of TPOS and the CLIVAR Research Focus group.

  • There is a need for continued monitoring of the air-sea fluxes of heat, sunlight, momentum, and fresh water, and for dedicated field studies to understand the atmospheric and oceanic building blocks of tropical climate and variability (atmospheric convection and clouds, oceanic
    upwelling and mixing, diurnal cycle, tropical instability waves, and the
    space-time structure and nonlinearity of basin-scale feedbacks).

  • Although the community would welcome enhanced tropical Pacific moorings with new instrumentation for monitoring ocean-atmosphere interactions, biogeochemical and carbon studies, and other aspects, TPOS should be cautious about making fundamental changes to the moored buoy array configuration, which has proved highly effective in providing a well-calibrated, reliable climate record and in supporting ENSO research and forecasting for the past 30 years.

Report



Please find herunder the report of the event.