The El Niño Southern Oscillation(ENSO), which originates in the tropical Pacific Ocean, is Earth’s mostdominant source of year-to-year climate variability and exerts a profound impactthat extends well beyond the Pacific basin.  Knowledge about ENSO and itsimpacts help to strengthen prediction tools used in support of early warningsystems. Therefore, improving our understanding of ENSO is fundamental inreducing societal vulnerabilities. Despite considerable progress in ourunderstanding of the impact of climate change on many of the processes thatcontribute to ENSO variability, it is not yet possible to say whether ENSOactivity will be enhanced or damped, or if the frequency or character of eventswill change in the coming decades.  

As changesin ENSO have the potential to be one of the largest manifestations ofanthropogenic climate change, such changes will have profound impacts on thereliability of regional attribution of climate variability and change. Two mainreasons can be invoked for shortcomings in predicting such changes. First thereis a lack of long and comprehensive enough observations of the various ENSOprocesses to be able to reliably detect past changes. It may be that we need toobserve ENSO for another several decades to detect and attribute significantENSO changes. Second, as ENSO involves a complex interplay of numerous oceanand atmospheric processes, accurately modeling this climate phenomenon withCGCMs, and understanding, anticipating, and predicting its behavior on seasonalto decadal and longer time scales remains a great challenge. Even though theability of CGCMs to simulate El Niño has largely improved over the last fewyears, the diversity of model simulations of present-day El Niñocharacteristics indicate current limitations in our ability to model thisclimate phenomenon and anticipate changes in its properties on a range of timescales. In this context, there is a pressing need to assess how we can usemodels to anticipate potential changes to ENSO in a warming climate.

During the borealspring of 2014, a remarkable increase in warm water volume with a series ofwesterly 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 equatorialPacific remained anomalously warm, the expected Mega El Niño did not form. That failed expectation may in part be a reflection of our incompleteknowledge of extreme El Niño and its predictability, or perhaps the very natureof the ENSO system itself.  Finally a very strong ENSO event did developduring 2015-16 (considered the third largest on record after the events of1982-83 and 1997-98). However the expected impacts in countries like Ecuadorand Peru never materialized.

Against thisbackdrop of progress, uncertainties, and ensuing greenhouse warming, it istimely to ask - what is the current stateof understanding of ENSO in terms of its diverse behavior, extremity, impactsand teleconnections?

Some of theremaining 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 theWhite Paper: “ENSO Research: The overarching science drivers and requirementsfor 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

Thisbackground highlights the need to address ENSO scientific challenges in thecontext of international climate research, assessments, and operationalprediction efforts.  As a catalyzing activity in this process, CLIVAR and CIIFENpropose the IV International Conference on El Niño SouthernOscillation: ENSO in a Warmer Climate. This conference is a follow-on to theIII International Conference on ENSO: “Bridging the gaps between Global ENSO Scienceand regional processes, extremes and impacts” held in November 2014;the Second International “Workshop on ENSO, Decadal Variability and ClimateChange in South America: Trends, teleconnections and potential impacts” held inOctober 2010; and the First International ENSO Conference: “The El Niñophenomenon and its global impact” in May 2005, All held in Guayaquil, Ecuador.