El Niño is building in the Pacific Ocean. What does that mean for Arizona?

Scientists are predicting that El Niño conditions are likely to develop this year — forecasters put the chances at 55% for a strong cycle — bringing the potential for severe weather, higher temperatures and the chance of flooding in Arizona and across the globe.

El Niño is the name climate and weather researchers have given the warm phase of the El Niño-La Niña Southern Oscillation pattern. This phenomenon happens in a naturally occurring cycle when sea surface temperatures across the east-central equatorial Pacific Ocean reach above-average temperatures. They are typically associated with extreme weather and above-average precipitation in the western U.S.

ENSO is the most dramatic year-to-year variation of the Earth’s climate system, according to National Oceanic Atmospheric Administration. The El Niño part of the cycle has the potential to rain down effects on a range of sectors including agriculture, public health, freshwater availability, power generation and economic activity in the United States and around the globe.

Though a weak El Niño is possible, there is currently an 80% chance it will be moderately strong and a 55% chance it will be strong, NOAA said. There is also a 90% chance that the system will linger into 2024.

Scientists will declare an El Niño if sea surface temperatures are one-half a degree Celsius above average for three months straight. Scientists say conditions are on track for that threshold.

Earlier this month NOAA's Climate Prediction Center found that global ocean temperatures reached record highs in April 2023, at 1.55 degrees Fahrenheit above the average from 1982-2011. This marked the second-highest monthly ocean temperature for any month on record, just 0.02 of a degree shy of the record-warm ocean temperatures set in January 2016, during the last El Niño.

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How does an El Niño develop?

Typically, during El Niño winters, the powerful jet stream that develops over the equator will push weather events into California, Arizona and the Southwest and dump large amounts of rain or snow on the area.

Sea surface temperatures across the Pacific can warm by 1–3°F or more, and these warmer conditions can last anywhere from a few months to two years. The unusual warmth is coupled with a slowdown of the easterly trade winds, as well as increased rainfall and a drop in surface air pressure in the central tropical Pacific.

An El Niño pattern is typically associated with extreme weather, though how much rain it will bring and its effects on temperatures are unclear and dependent on when and where the system develops. The position of the jet stream and other related atmospheric and oceanic oscillations also will affect precipitation totals. Usually, the stronger the El Niño, the greater the chance for extreme weather.

"No two El Niños or La Niñas are perfectly alike," said Antonietta Capotondi, a CIRES scientist working at NOAA's Physical Sciences Laboratory, in a news release. "We've seen how diverse ENSO events can be. This diversity adds another degree of complexity for understanding how climate change will influence future ENSO events.”

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What does this mean for Arizona?

Precipitation records over the last few decades in Arizona show dramatic swings from very wet one year to very dry the next. Such climate variability can often be tied back to ENSO.

For Arizona, an El Niño event means increased probability of receiving above-average winter precipitation, generally between October and March.

And while rain is always welcomed in drought-stricken Arizona to recharge aquifers and top off reservoirs, too much in a short period of time can lead to flooding and landslides that can turn deadly and cause millions of dollars in damage.

The 1993 El Niño left an estimated $50 million in damage, according to the Arizona Geological Survey. U.S. Highway 95, just east of Yuma, was closed for nearly six months as the swollen Gila River drained of flood waters brought on by excess precipitation.

Arizona and other parts of the West are already seeing greater mountain runoff from a wetter-than-average winter. The potential for more precipitation would also increase the chances of landslides and erosion.

El Niño would likely increase snow above the Mogollon Rim and could bring more rain to southern Arizona due to the change in the jet stream, according to researchers at the University of Arizona. Experts say El Niño's effects on the monsoon in Arizona are more difficult to predict.

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How has climate change affected ENSO patterns?

While these weather events have been occurring for centuries or longer, climate change will likely influence the way the storms develop and the impacts they bring.

A recent report from the World Meteorological Organization found that worldwide, temperatures will likely surpass record levels within the next five years due to heat-trapping greenhouse gasses.

The WMO predicts that there is a 98% likelihood that at least one of the next five years, as well as the five-year period, will be the warmest on record.

“Extreme El Niño and La Niña events may increase in frequency from about one every 20 years to one every 10 years by the end of the 21st century under aggressive greenhouse gas emission scenarios,” said Michael McPhaden, senior scientist with NOAA’s Pacific Marine Environmental Laboratory in Seattle. “The strongest events may also become even stronger than they are today.”

With warmer atmospheric and ocean temperatures, rainfall extremes are projected to shift eastward along the equator in the Pacific Ocean during El Niño events. Arizona and California lie east of the wet band where El Niño typically forms.

The 2015-2016 El Niño brought an increase in Pacific tropical storm activity likely due a warmer climate, but that did not correlate to more precipitation over land, experts said.

How does NOAA monitor and track El Niño?

Scientists, governments and nongovernmental organizations all collect data about El Niño using a number of technologies, including a network of scientific buoys monitored by NOAA. These buoys measure ocean and air temperatures, currents, winds, and humidity. The buoys are in about 70 locations in the southern Pacific Ocean, from the Galapagos Islands to Australia.

These buoys transmit data daily to researchers and forecasters around the world. Using data from the buoys, along with visual imagery they receive from satellite imagery, scientists are able to more accurately predict El Niño and visualize its development and impact around the globe. They hope this data will help more accurate predictions these oceanic patterns in the future and for better preparation for those of us on land.

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Jake Frederico covers environment issues for The Arizona Republic and azcentral. Send tips or questions to jake.frederico@arizonarepublic.com.

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This article originally appeared on Arizona Republic: As El Niño builds in the Pacific Ocean, Arizona braces for effects