The El Niño Southern Oscillation (ENSO)

The El Niño Southern Oscillation (ENSO), which originates in the tropical Pacific Ocean, is Earth’s most dominant source of year-to-year climate variability and exerts a profound impact that extends well beyond the Pacific basin.  Knowledge about ENSO and its impacts help to strengthen prediction tools used in support of early warning systems. Therefore, improving our understanding of ENSO is fundamental in reducing societal vulnerabilities. Despite considerable progress in our understanding of the impact of climate change on many of the processes that contribute to ENSO variability, it is not yet possible to say whether ENSO activity will be enhanced or damped, or if the frequency or character of events will change in the coming decades.

As changes in ENSO have the potential to be one of the largest manifestations of anthropogenic climate change, such changes will have profound impacts on the reliability of regional attribution of climate variability and change. Two main reasons can be invoked for shortcomings in predicting such changes. First there is a lack of long and comprehensive enough observations of the various ENSO processes to be able to reliably detect past changes. It may be that we need to observe ENSO for another several decades to detect and attribute significant ENSO changes. Second, as ENSO involves a complex interplay of numerous ocean and atmospheric processes, accurately modeling this climate phenomenon with Climate Global Circulation Models (CGCMs), and understanding, anticipating, and predicting its behavior on seasonal to decadal and longer time scales remains a great challenge. Even though the ability of CGCMs to simulate El Niño has largely improved over the last few years, the diversity of model simulations of present-day El Niño characteristics indicate current limitations in our ability to model this climate phenomenon and anticipate changes in its properties on a range of time scales. In this context, there is a pressing need to assess how we can use models to anticipate potential changes to ENSO in a warming climate.

During the boreal spring of 2014, a remarkable increase in warm water volume with a series of westerly wind bursts alerted ENSO experts to the possibility of a strong event, one which some thought could rival the intensity of the 1997-98 event, generating news headlines worldwide.  However, while the equatorial Pacific remained anomalously warm, the expected Mega El Niño did not form.  That failed expectation may in part be a reflection of our incomplete knowledge of extreme El Niño and its predictability, or perhaps the very nature of the ENSO system itself.  Finally a very strong ENSO event did develop during 2015-16 (considered the third largest on record after the events of 1982-83 and 1997-98). However the expected impacts in countries like Ecuador and Peru never materialized.

Against this backdrop of progress, uncertainties, and ensuing greenhouse warming, it is timely to ask - what is the current state of understanding of ENSO in terms of its diverse behavior, extremity, impacts and teleconnections?

Some of the remaining uncertainties are related with processes not well understood yet, which could provide potential contribution to improve model prediction skills. Some of these processes have been well explained by Kessler et al, 2014 in the White Paper: “ENSO Research: The overarching science drivers and requirements for observations” as part of the Tropical Observing System 2020 project:

  • Equatorial upwelling and rapid atmospheric feedback – scales and fronts
  • Mechanisms by which subsurface ocean dynamics drive SST
  • Atmospheric processes relevant for Tropical Observing System.
  • Large-scale feedbacks driving ENSO variability
  • Diurnal cycle and penetration of surface fluxes into ocean
  • Recharge and discharge to subtropics Low latitude western boundary currents in the Pacific
  • Tropical instability waves

This background highlights the need to address ENSO scientific challenges in the context of international climate research, assessments, and operational prediction efforts.  As a catalyzing activity in this process, CLIVAR and CIIFEN propose the IV International Conference on El Niño Southern Oscillation: ENSO in a Warmer Climate. This conference is a follow-on to the III International Conference on ENSO: “Bridging the gaps between Global ENSO Science and regional processes, extremes and impacts” held in November 2014; the Second International “Workshop on ENSO, Decadal Variability and Climate Change in South America: Trends, teleconnections and potential impacts” held in October 2010; and the First International ENSO Conference: “The El Niño phenomenon and its global impact” in May 2005, All held in Guayaquil, Ecuador.