Like water sloshing in a giant bathtub, El Niño begins an inevitable retreat

A few weeks ago, the Australian Bureau of Meteorology declared that the Pacific Ocean is no longer in an El Niño state and is back to “neutral”. American scientists at the National Oceanic and Atmospheric Administration are more hesitant, but estimate that there is an 85% chance that the Pacific Ocean will enter a neutral state in the next two months and a 60% chance that a La Niña event will begin by . August.

After an El Niño that was one of the three strongest in the last 40 years and brought a wet winter to the US — and California, in particular — this transition could mean a big change in weather as we approach with the summer.

The progression from El Niño to La Niña, which is part of a broad system known as the “El Niño Southern Oscillation,” or ENSO, is a result of conditions in the tropical Pacific Ocean. During the neutral phase, which is or will soon be in effect, the so-called trade winds move from east to west along the equator. These winds push warm surface water with them, bathing Indonesia and New Guinea in the balmy waters of the “Pacific Warm Pool” and forcing cold water to rise from the deep ocean along the coast of South America.

As the El Niño phase begins, these winds weaken, so that warm sea surface temperatures move eastward toward South America. This can cause climate changes around the globe: landslides in Peru, drought in Australia, fish die-offs in the eastern Pacific and more frequent atmospheric rivers in Southern California. These changing weather patterns further reduce the trade winds, which leads to more warm water off the coast of South America, which weakens the winds, and so on.

So what prevents El Niño events from continuing to strengthen forever?

Well, you can think of the Pacific Ocean sort of like one giant bathtub, and El Niño like a wave of warm water sloshing from one end of the bathtub to another. When that wave reaches the coast of Ecuador, it bounces back, carrying the warm water back towards Asia and the Ocean, which strengthens the trade winds, pushing the warm water faster, until it reaches the wave on the other side of the “bathtub” – this is the La Niña phase, when the western Pacific Ocean is particularly warm and the eastern Pacific Ocean particularly cold – at which point the process repeats itself. This “oscillation” gives ENSO its name, and is why a strong La Niña event is often followed by a strong El Niño.

Read more: Why California’s average snowpack this year isn’t cause for celebration

This winter’s El Niño event had sea surface temperature anomalies of 3.6 degrees (2 degrees Celsius), qualifying it for the unofficial status of “very strong El Niño.” As usual, El Niño’s warm waters produced high global temperatures, but due to the unprecedented effects of climate change, these temperatures were only typical. In December, when El Niño was at its peak, global surface temperatures were 0.45 degrees (0.25 degrees Celsius) above the next warmest December on record.

This increase may not seem so unusual given the current era of ever-climbing temperatures, but when you consider that the difference between the coldest Christmas on record (back in 1916) and the second warmest (in 2016), it’s much more alarming — so surprising that leading climate scientists are publicly wondering if there are elements missing from our understanding of climate change.

Fortunately, the onset of neutral ENSO conditions, followed by a likely La Niña, should begin to reduce global temperatures, at least temporarily. This will be little consolation for the United States, as the National Weather Service predicts above-average summer temperatures for most of the country. In addition, La Niña events are associated with drier conditions across the southwestern US that may continue into next winter. Although this year’s generous Sierra snowpack should be able to protect California from the effects of a scorching summer, the state is more than one winter below average from drought.

Read more: The warmest April on record extends the planet’s warmest stretch to 11 months

There could also be implications for the rest of the country — La Niña has been linked to increased hail and tornado activity in the Southeast and an increase in hurricanes in the Atlantic and Gulf of Mexico. In fact, many experts are predicting a “hyperactive” hurricane season in the tropical Atlantic, with one forecast going as high as an unprecedented 33 named storms. On the flip side, though, it’s likely to be a slow hurricane season in the Eastern Pacific, with the possibility of Hurricane Hilary revisiting Southern California last August.

Of course, there are all these forecasts – that La Niña tends to create dry conditions in Southern California, that this place will get more hurricanes and that region will get more hail, and even how strong an El Niño or La Niña event can be – established. regarding correlations and theories that researchers have painstakingly developed using data from the last half century.

But given the rapidity of recent climate change, there are no guarantees that past trends will continue into the future. In situations like this, climate scientists generally look to computer models to understand how phenomena like ENSO might change over time.

Unfortunately, many climate models have not yet developed the ability to accurately predict ENSO — representing its complexity and the fact that the ocean and atmosphere are moved out of it is a huge challenge. together. This means that the future of ENSO remains uncertain as we move into a new era of accelerating climate change.

Ned Kleiner is a scientist and disaster modeler at Verisk. He has a doctorate in atmospheric science from Harvard.

This story originally appeared in the Los Angeles Times.

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