What Is El Niño and What Causes It To Occur?

I have been wanting to write an article for some time now explaining exactly what El Niño is, and what causes it to occur. For many decades, we have known that this phenomenon occurs every few years, and in today’s modern world, you hear a lot about El Niño on the news and on the internet when one is currently developing. But what is it exactly? Most everyone has heard of El Niño, but I bet if I randomly asked 100 people to tell me what El Niño is, only one or two people would actually be able to tell me. If you had asked me a few years ago what El Niño was, I wouldn’t have been able to explain it either. By the end of this article, you’ll have a basic understanding of what El Niño is, and what causes it to develop.

Let me start out by telling you what ENSO (El Niño Southern Oscillation) is. ENSO is the periodic changes in sea surface temperatures and sea surface pressure across the equatorial Pacific (the band of water along the equator in the Pacific Ocean). In other words, over a certain period of time (years), the surface waters along the Pacific Ocean at the equator fluctuate from warmer than average to cooler than average back to warmer than average, and this fluctuation continues on and on. In between those fluctuations, the ENSO is considered to be in a neutral state if sea surface temperatures over the equatorial Pacific are around average and are expected to stay that way for an extended period of time. Let me also explain what an anomaly is. An anomaly is basically a departure from the average. So if I show you a sea surface temperature anomaly map, that will show you how above or below average the sea surface temperatures are over a particular region. The reason that the water surface temperatures along the equatorial Pacific have to be monitored closely is because it can have major impacts on the weather around the globe.

This image shows the fluctuation between El Nino and La Nina since 1965.
This image shows the fluctuation between El Nino and La Nina since 1965.

Typically, trade winds blow from east to west across the Pacific Ocean at the Equator. These strong winds transport the water from east to west across the Pacific, and as this water moves west, solar radiation from the sun warms this water. These warmer waters begin to pile up over the western Pacific, and cooler waters deep in the eastern Pacific begin to surface and replace the water that has been transported west. This is why waters are typically cooler in the eastern Pacific than in the western Pacific. All of this drives the Humboldt Current, which is an ocean current that brings cooler waters northward to the coasts of Peru and Chile from the Antarctic regions. The reason that the Peruvian coast is one of the richest fisheries in the world is because of the upwelling that typically occurs off the coast, which brings to surface the nutrient-rich, cooler waters.

Conditions over the Pacific during a typical year.
Conditions over the Pacific during a typical year.

As an El Niño begins to develop, the conditions described above begin to change dramatically. The trade winds weaken, the cooler waters in the eastern Pacific begin to warm, and the waters in the western Pacific begin to cool. When the water begins cooling in the western Pacific, the pressure increases over that region, which decreases storminess, and when the waters begin warming in the eastern Pacific, the pressure decreases over that region, which increases storminess. This is a big indicator that El Niño is in the process of developing when this begins to happen.

Conditions over the Pacific during an El Nino year.
Conditions over the Pacific during an El Nino year.

So how do the warmer waters from the western Pacific move across the ocean to the central and eastern Pacific? The body of really warm water in the western Pacific builds up after the previous El Niño event. The really strong trades winds (remember, the trade winds normally blow from east to west across the equatorial Pacific), have been keeping that water confined to that region. Now that those winds have weakened, these waters are now free to begin pushing east. Earlier this year, low pressure systems on both sides of the equator allowed for the development of what is known as a Kelvin wave. These two low pressure systems lined up along the equator in the western Pacific to reverse the wind flow towards the east (remember, low pressure systems rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). This allowed these warmer sub-surface waters to freely push eastward from the western Pacific, and now that they are across the central and eastern Pacific, they are beginning to surface.

This image shows the above-average temperatures across the central and eastern Pacific during the strong 1997-98 El Nino.
This image shows the above-average temperatures across the central and eastern Pacific during the strong 1997-98 El Nino.

When watching to see if El Niño conditions are developing or have developed, there are four regions across the equatorial Pacific Ocean that are monitored. These regions are called Niño 1+2, Niño 3, Niño 3.4, and Niño 4. Portions of Niño 3 and 4 make up the Niño 3.4 region, and if temperatures stay above average in the Niño 3.4 region over a certain period of time, we are considered to be in El Niño conditions. There are different types of El Niño’s that can occur, which can have different effects on the weather around the globe, but I’ll go into all of that in a future article.

These are the 4 Nino regions across the Pacific.
These are the 4 Nino regions across the Pacific.

When the sea surface temperatures are 0.5 C (0.9 F) above-average across the Niño 3.4 region and those temperatures are expected to stay above that threshold, then we consider that to be a developing El Niño. The sea surface temperature anomalies over this region are averaged over a 3-month period, and if temperatures are at or above 0.5 C (0.9 C) above-average consecutively for five of these 3-month periods, then we are then officially in an El Niño. For example, March-April-May was the last 3-month average, and the next one will be April-May-June after we finish this month. This index is referred to as the Oceanic Niño Index (ONI). Sometimes NOAA is somewhat loose with their criteria if all the evidence shows that we’re going into an El Niño or La Niña.

This image shows that the sea surface temperatures are currently above average across all 4 Nino regions.
This image shows that the sea surface temperatures are currently above average across all 4 Nino regions.

You should now have a basic understanding of what El Niño is. There is still a lot that we don’t know about El Niño, but it is certain that it can have major impacts on the weather around the globe. In future articles, I’m going to explain what type of El Niño is likely developing this year, and how it will affect the upcoming hurricane season and upcoming fall and winter. The strength of this developing El Niño is key to what the weather will be like later this year, and I’ll explain some of the variables that we must not overlook this year. Please follow Firsthand Weather on Facebook, where I will be putting out plenty of useful information.