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Understanding ENSO by City Tech Blogger Monil Pandya

Editor’s Note: This is a very concise explanation of the El Niño/La Niña cycles. According to The Guardian  these cycles have been changing because the planet is getting warmer; and  a study just published in Geophysical Research Letters addresses how that can impact weather in the future.

ENSO is an abbreviation where “EN” stands for the El Nino effect and “SO” stands for the Southern Oscillation. The ENSO phenomena occurs at the Southern Hemisphere, right below the equator and the Southern Oscillation is the oscillation of the ocean temperature from warm to cold temperatures and vice-versa. The Pacific Ocean is a large body of water that stretches from the western part of the American continents to the eastern part of the Asian and the Australian continent. As we know, the equator receives the greatest amount of sunlight hence why this large pool of water gets intensely warm. So far we know what ENSO is and where ENSO takes place, now it’s time to understand how it all happens. To understand how ENSO occurs we will need to break this phenomenon into three phases; The Neutral phase, El Nino phase and La Nina phase. The first phase is a neutral phase. There is no such thing as a neutral phase when it comes to any of nature’s phenomena.

To look at the bigger picture, nature has a constant ongoing cycle that has never stopped but for our own understanding, we created a neutral state where we can see how this thing starts and goes on. In the Neutral phase, the Central Pacific Ocean is warm. Well during this phase, trade winds blow all over the tropical Pacific from the east to west. These winds accumulate warm water in the western part of the Pacific. On the other hand, water temperatures in the east are lower because the trade winds trigger the cool water to rise up from the deep. The temperature alteration across the tropical Pacific Ocean causes air to rise from northern Australia, and descend near the western part of the South American continent. This creation of the cycle is called the Walker Circulation. While a neutral phase brings normal weather to Australia, droughts, and floods are certainly still possible. When we move into a La Niña, it’s a bit like the neutral phase has gone into overdrive. The hard blowing trade winds intensifying the warm pool of water on the Australian side of the tropical Pacific, and cooling the oceans towards South America.

 

 

This escalates the east to west temperature difference and makes the Walker circulation even stronger causing the trade winds to blow even harder again. This is called a feedback loop, and once it starts La Niña won’t leave until at least the following autumn. Because of the higher ocean temperatures, greater evaporation occurs, more cloud and rain in the western Pacific. In Australia, this increases the risks of lower daytime temperatures, widespread flooding, and more tropical cyclones. On the other end of the scale, we have El Niño, which is almost the opposite of La Niña.

 

During El Niño the trade winds weaken, or start to reverse, allowing  the warmer water to flow back towards the east. The change in the ocean temperature patterns causes the Walker circulation to  interrupt, resulting in even weaker trade winds, causing the temperature to warm up even more in the east. This feedback point towards the setting in of the El Niño. With the warm water moving east, the evaporation, cloud, and rain follow behind slowly shifting away from Australia. That means a greater risk of drought for northern and eastern Australia, higher temperatures, heatwaves, and fewer tropical cyclones. El Niño can hold a ton of energy. The 1997 El Niño moved 35 million billion Joules of energy into the eastern Pacific, as much as, 100 times the energy used by everyone on Earth in a year, 1/14th as much as the meteor impact that killed the dinosaurs! That’s a lot, and when that energy gets transferred to the atmosphere, where weather happens, it can have effects that reach around the world. The El Niño during the Northern Hemisphere winter of 1997 and 1998 was the strongest ever on record. Rain and mudslides in California and Peru killed dozens and left thousands homeless, Kenya’s annual rainfall was 40 inches above normal. And on the other side of the Pacific, Indonesia had massive droughts, temperatures in Mongolia hit 108˚ F. And while El Niño usually means a quieter Atlantic hurricane season, it can mean more and stronger storms in the Pacific. In 1997, Hurricane Pauline dumped 27 inches of rain on western Mexico in a single day. While there are scientific definitions for El Niño and La Niña no two events, and no two sets of impacts, are the same. We also know some impacts will emerge as an ENSO event is developing, and some will persist even if an El Niño or La Niña never fully forms.

 

 

 

 

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