The El Niño Southern Oscillation (ENSO) is a natural climate cycle of fluctuating ocean temperatures and changing storm patterns over the tropical Pacific and is the dominant interannual signal of climate variability, affecting weather patterns worldwide. The two extreme phases of ENSO are El Niño and La Niña, which influence severe weather events like hurricanes, flooding, and drought.

For California, an El Niño year often yields rain, but how does it affect the rest of the world?

A new study examines the effects of El Niño and La Niña events on flooding across the globe. The paper, titled “Strong influence of El Niño Southern Oscillation on flood risk around the world,” was recently published in the journal Proceedings of the National Academy of Sciences. Climate researcher Michael Dettinger of Scripps Institution of Oceanography, UC San Diego, collaborated with colleagues from the Netherlands and Finland to develop models of rivers, floods, and flood effects to answer questions about ENSO’s influence on flood risk. The study found that during El Niño and La Niña years, flooding and drought become more extreme across approximately a third of the earth’s surface.

“The models used are remarkably detailed. This is something that couldn’t have been done a few years ago. For example, the flood flows and inundations are modeled and then imposed on socioeconomic maps with resolutions down to about a half kilometer at the latitude of California,” said Dettinger. “This extreme model detail allows us to make the step from river flows to economic impacts in ways that we haven’t been able to in the past.”

The topic of the study differs from previous research on ENSO in its focus on real-world socioeconomic impacts. The cascade of models was used to assess the impacts of floods in terms of exposed population, exposed domestic product, and urban and agricultural damage. It found that summarizing these kinds of impacts at the scale of whole countries masks important regional variations in ENSO impacts, especially in large countries such as the United States. Different areas within a large country can experience opposing effects from ENSO that end up canceling out when aggregated with each other. Thus, this new, detailed study was able to more accurately describe effects of flood volume across the globe during opposing ENSO signatures.

“During historical El Niño years, the study reports particularly high levels of flood damage to cities and agriculture across the central latitudes of South America, parts of the Horn of Africa, northeastern-most India, central Eurasia, and elsewhere. On the other hand, a large number of regions have suffered lower than average flood damages, prime examples being the Sahel in northern Africa, southern Africa, broad areas of Canada, and eastern Australia,” said Dettinger.

Previous work has only focused on the negative impacts of ENSO events. This study also evaluated the positive impacts, which were widespread. Understanding both the positive and negative impacts of ENSO on climate and socio-economics could aid policy makers worldwide.

“Combining forecasts of ENSO conditions in the tropical Pacific with this new socioeconomic knowledge may help global and regional humanitarian, insurance, and flood management institutions to better plan and mobilize for the impacts and hazards to which they are more likely to be responding in upcoming months,” said study lead author Philip Ward of the VU University Amsterdam.

Dettinger said this work adds to a long tradition of research at Scripps by scientists such as Tim Barnett, Dan Cayan, and Dean Roemmich, among others, who have contributed greatly to science’s understanding of the physical processes and impacts of ENSO, which scientists regard as a key climate phenomenon to study due to its global reach, its reflection of dynamic ocean-atmosphere coupling, and the potential for it to change in a warming world. Another important characteristic of ENSO is its cyclical nature, which, though irregular, enables prediction of El Niños and La Niñas with increasing accuracy with a few months’ lead time.

Information about climate variability’s impact on society and the ability to predict these natural disasters can aid successful mitigation and prevention. The applied nature of this recent study could pave the way to directly link climate prediction with appropriate disaster risk management.

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