It is very likely that El Nino Southern Oscillation rainfall variability, used for defining extreme El Ninos and La Ninas, will increase significantly by the second half of the 21st century
On July 4, 2023 the World Meteorological Organization declared that El Nino conditions had developed in the tropical Pacific for the first time in seven years, setting the stage for a likely surge in global temperatures and disruptive weather and climate patterns.
This can have widespread impacts on climate and weather patterns, with changes in temperature and rainfall in various parts of the world.
El Nino is likely to fuel further global temperature increase. It is one facet of the El Nino Southern Oscillation (ENSO), a naturally occurring large-scale climatic phenomenon involving fluctuating ocean temperatures in the central and eastern equatorial Pacific, coupled with changes in the overlying atmosphere.
ENSO occurs in irregular cycles of two to seven years, and presents three phases: El Nino, La Nina, and a neutral phase.
El Nino episodes usually last nine to 12 months, and up to 18 months, and are associated with warming of the ocean surface temperatures in the central and eastern tropical Pacific Ocean, and changes in atmospheric.
Sea surface temperature (SST) variability of El Nino underpins its planetary scale impact, and its future change is a complex and long-standing science issue.
First things to note, it is virtually certain that ENSO will not only exist in a warming world, but that it will continue to play a huge role in affecting Earth’s climate patterns.
The Intergovernmental Panel on Climate Change’s Sixth Assessment Report (IPCC AR6) goes a step further and indicates that “it is very likely that ENSO rainfall variability, used for defining extreme El Ninos and La Ninas, will increase significantly, regardless of amplitude changes in ENSO SST variability, by the second half of the 21st century” in most emission scenarios. This is consistent with findings by previous studies of a stronger rainfall response to ENSO SST variability, even if ENSO SST variability itself does not change.
Furthermore, IPCC AR6 highlights that there is no model consensus for a systematic change in intensity of ENSO SST variability over the 21st century in any of the emission scenarios assessed (medium confidence).
But regardless of changes in ENSO SST, in emission scenarios, it is very likely that rainfall variability over the east-central tropical Pacific will increase signif-icantly. Therefore, we might expect El Nino in the future to be wetter in this region and La Nina to be drier.
There are number of issues with the interplay between climate change and ENSO which the climate research community actively investigates and seeks clarifications on.
For instance, the IPCC AR6 assessment on ENSO change relies on the evolution of ENSO SST variability in the central-to-eastern equatorial Pacific (Nino3.4 area: 5°N-5°S, 170°W-120°W) over 30-year periods, normalised by averaged variability over the 20-year current climate of 1995-2014 simulated by climate models under various emission scenarios.
However, sampling ENSO SST variability over relatively short 30-year running periods would be subject to influence from decadal climate fluctuations that can mask the influence of greenhouse emissions.
A study published by a group of scientists in January 2022, in the journal Nature Climate Change, indicates that contrary to the IPCC AR6 assessment, ENSO SST variability is stronger in the 21st century than in the 20th century. Under greenhouse warming, an increased air-sea coupling arising from enhanced upper equatorial ocean stratification underpins the increase in ENSO variability.
Surely, we have a good understanding of recent past El Nino teleconnection and precipitation impacts from the scientific literature. However, the complexity of interactions between ENSO and climate change would require the climate research community to pursue investigations on this topic.
Wilfran Moufouma Okia is head of Regional Climate prediction Services Division, World Meteorological Organization
This was first published in the 1-15 August, 2023 print edition of Down To Earth
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