What Is El Niño and La Niña? Causes, Effects & Everything You Need to Know

Various climate-related disasters such as floods, droughts, heavy rainfall, and cyclones are quite natural. However, lately all these disasters seem to be getting more and more intense. For example, when it rains, there is so much rainfall that it is turning into sudden floods. Again, during summer, the heat is so extreme that it is breaking records from any point in the past — what we call a heat wave or heat dome. Besides this, the earth is also facing various natural disasters including frequent wildfires and cyclones, which affect food production, the food supply chain, water supply, public health, and the global economy.

Now the question is: why are natural disasters getting more and more intense? Why is the same place sometimes experiencing heavy rainfall and sometimes drought? Why are events like heat waves and cold waves happening? To know the answers to all these questions, you need to understand El Niño and La Niña. Along with that, you also need to understand the jet stream. In today's video, we will talk about El Niño, La Niña, and the jet stream. I am Chayon Kumar, with you. You are watching News Monster.

The Discovery of El Niño

In the 17th century, fishermen in the coastal regions of Peru noticed that periodically, every few years around Christmas time, the water of the Pacific Ocean remained comparatively somewhat warm or heated. They named this phenomenon El Niño, which is a Spanish word meaning "boy" — through this they were essentially referring to the boy Jesus, since the warm water phenomenon arrived around Christmas time. After the warm water, however, they also noticed the presence of comparatively cooler water in the Pacific Ocean. The reason for this periodic difference in ocean water temperature was not known to people at that time. In fact, the reason for the temperature difference remained unknown for a long time afterward.

Finally, around 1960, people came to understand why the difference in water temperature occurs in the Pacific Ocean. Along with that, they also came to understand that the fluctuation of water temperature in the Pacific Ocean affects the weather of the entire earth — what is now called El Niño Southern Oscillation, abbreviated as ENSO. Since this phenomenon occurs periodically, it essentially has three phases or stages: first, the La Niña phase; second, the neutral phase; and third, the El Niño phase.

Understanding High Pressure and Low Pressure

However, to understand these things, you need to understand one very basic concept — and that is high pressure and low pressure. If you can understand just these two things, you will get a clear understanding of El Niño, La Niña, and various other weather-related matters.

Imagine two water-filled containers placed at a moderate distance from each other. Now if one of them is continuously heated and the other is continuously cooled, what will happen? The water in the container being heated will begin to evaporate and rise upward — meaning the air there will become lighter and rise up. Now, as this air rises upward, the pressure at this location will decrease — meaning a low pressure zone will form here. On the other hand, in the container where water is being cooled, since no steam will form, the air there will cool down and become denser, causing pressure to increase — meaning a high pressure zone will form in this area.

Now we know that any substance always wants to move from a high pressure region to a low pressure region — meaning here we will see airflow from the high pressure area toward the low pressure area. Now you might think this is a very natural thing, but this matter does not end here. The water vapor rising from the low pressure zone will cool down when it goes up, causing pressure to increase — meaning even though there is low pressure below, high pressure will form up above. But the air of this high pressure cannot descend directly down to the low pressure zone because air is already rising from here. So the high pressure air formed above will move sideways and come down. Now if we consider it moving in this direction, notice that a cycle of airflow is being created here — and this cycle is the driving force of Earth's airflow, rainfall, and overall weather diversity.

From Earth's surface, we mainly feel the airflow in the lower part. Since water vapor rises from this section, clouds will form in this area, causing rainfall near the hot container. On the other hand, there will be no rainfall in the cold water area, so drought or desert-like conditions will appear there.

The Role of the Sun

Now we understood that due to high pressure and low pressure, diversity in airflow including rainfall is seen — but initially, hot and cold water, or a hot and cold environment, is needed to create high pressure and low pressure. So how does the diversity of hot and cold form on Earth? Then comes the sun. Sunlight does not fall equally on all parts of the Earth. In the equatorial and surrounding regions, sunlight falls vertically, while in the polar regions it falls obliquely. Now, because sunlight does not fall with equal intensity on all parts of the Earth, the middle part of the Earth remains more heated and the polar regions remain less heated. Due to this difference in temperature, low pressure forms in the middle of the Earth and high pressure forms in the polar regions. As a result, wind flows from the polar regions toward the equatorial region.

Of course, wind does not directly come from the poles to the equatorial region — rather, certain cells form, and the airflow you see along these cells is, broadly speaking, the path of Earth's airflow. Now the question is: does this create the El Niño and La Niña phenomenon? The answer is no.

The Walker Circulation and Normal Pacific Conditions

Along with the airflow from Earth's poles to the equatorial region, there is another type of airflow — from east to west or from west to east — which is created due to Earth's rotation on its own axis. Due to Earth's rotation on its own axis, the wind coming from the poles to the equatorial region bends westward under the influence of the Coriolis force. As a result, in the equatorial and surrounding regions, there is airflow from east to west — and it is primarily due to this airflow that El Niño and La Niña occur.

El Niño and La Niña originate from the Pacific Ocean. In the equatorial region, the upper layer of water in the Pacific Ocean becomes heated due to sunlight. But the east-to-west wind created in the equatorial region due to Earth's rotation carries the upper layer of ocean water toward the west. As a result, the ocean water near Australia and the region above it remains comparatively warmer than the water near Peru and Chile. The result: the air in the Australian part becomes lighter and rises upward — meaning low pressure forms. On the other hand, high pressure prevails in this part. As a result, just like the hot and cold water mentioned earlier, a cycle of airflow forms here — which is called the Walker Circulation.

Now, under normal conditions, since there is more water vapor in the sky over this area, clouds form, causing rainfall. On the other hand, since high pressure prevails in this area, there is no water vapor in the sky, so there is no rainfall in the coastal regions of Peru and Chile. As a result, desert conditions can be seen in this part of Chile. Now, when this normal condition of the Pacific Ocean is disrupted, El Niño and La Niña come to the fore.

La Niña: When the Trade Winds Strengthen

Let's first talk about La Niña. If for some reason the eastward-to-westward airflow in the Pacific Ocean increases — meaning more warm water than normal goes from east to west — that is called La Niña. In other words, if what normally happens occurs with greater intensity, it will be called a La Niña event. In such a case, more warm water than normal accumulates in the Australian region, causing more water vapor to form, which causes much more rainfall than normal in this region — resulting in sudden floods as well. Along with that, more cyclones than normal are also formed. On the other hand, in the coastal regions of Peru and Chile, cold water from the bottom of the ocean rises to the surface — meaning more high pressure than normal forms there, making that area drier than normal. The result: drought-like conditions appear.

However, in this event, a particular benefit can be seen in the coastal regions of Peru — and that is, as the cold water from the ocean rises to the surface, food necessary for fish also comes up. As a result, fishermen can catch more fish than normal.

El Niño: When the Trade Winds Weaken

The opposite of La Niña is called El Niño — meaning if the eastward-to-westward airflow decreases below normal, that will be called El Niño. In such a case, since the westward airflow is less, the warm surface water of the ocean, instead of going westward, begins to come in the opposite direction — eastward — meaning toward Peru and Chile. As a result, a large amount of water vapor forms roughly in the middle of the Pacific Ocean, causing heavy rainfall in Peru, Chile, and the Pacific Ocean. This leads to sudden terrible flooding in Peru or Chile. On the other hand, dry conditions are seen in Australia or its surrounding regions — resulting in drought or wildfires.

Now it is not the case that only the weather in these two regions changes due to El Niño or La Niña. Rather, due to powerful El Niño and La Niña, changes are seen in the weather of the entire Earth — and you can understand how, if we mention the case of Bangladesh and South Asia.

Impact on South Asia and Bangladesh

According to the Walker Cell, high pressure will prevail in the Indian Ocean, so wind will rush from the Indian Ocean toward Asia — meaning at that time high pressure will prevail in the Indian Ocean and low pressure will prevail in Asia or on land. As a result, wind from the Indian Ocean will rush toward Asia, which will then evaporate due to the heat of the Earth's surface and rise upward, causing rainfall. Now if for some reason the Walker Cells change their position, the weather will change accordingly. For example, if due to the change in the Walker Cell, instead of high pressure, low pressure prevails in the Indian Ocean, then wind from South Asia will come toward the Indian Ocean — resulting in drought-like disasters in South Asia.

Now, as a result of El Niño or La Niña: first, the Walker Cell alone changes; and second, what is supposed to happen under normal conditions either happens with extreme intensity or the opposite happens. As a result of El Niño or La Niña, noticeable changes are seen in the weather of the entire Earth.

How Often Do El Niño and La Niña Occur?

El Niño and La Niña are seen occurring alternately roughly every few years, although their intensity is not always the same — sometimes less, sometimes more. For example, in the last 30 years, two extreme phases of El Niño have been seen: the first from 1997 to 1998, and the second from 2015 to 2016. Now the question is: why does the airflow in the Pacific Ocean increase or decrease, causing El Niño or La Niña to occur?

Generally speaking, ENSO is a very natural event — that can be seen by going through history. However, the intensity of ENSO seems to be constantly increasing, for which many people blame excessive greenhouse gas emissions. Actually, scientists are still not clear about the true cause of this.

Heat Waves, Cold Waves, and the Jet Stream

Lately, during summer, a phenomenon like a heat wave or heat dome is seen occurring — a time when an unbearably hot environment prevails. Again, during winter, more cold or cold waves than normal are seen. The influence of ENSO is also behind this happening — which influences the jet stream to create heat waves or cold waves.

Now the question is: how does the jet stream create heat waves and cold waves? At the meeting point of the winds coming from Earth's poles to the equatorial region, a type of intense airflow is created — which is called the jet stream. This jet stream is of two types: one is the subtropical jet stream and the other is the polar jet stream. What is important for us is the subtropical jet stream. The path of this jet stream is not straight — rather it is like a sine wave.

How Heat Domes Form

Now, in the part of the wave that goes upward, a large amount of air enters from below. On the other hand, from the part of the wave that comes down, a large amount of air exits. Now, in the place where air enters the jet stream, low pressure forms, and in the place where air exits and comes down, high pressure forms.

Now imagine that the warm air of some location wants to rise upward, but if the lower part of the jet stream is above that location, then the air wanting to rise upward will not be able to rise due to the air exiting from the jet stream — rather, the air exiting from the jet stream will continue to compress the air of that location, meaning it will keep increasing the pressure. Now we know that when gas or air is compressed, its temperature keeps rising. Now in this location, on one hand the hot air cannot go upward, and on top of that, due to the air coming from the jet stream, the air here is being compressed — and due to this compression, the temperature is increasing further. Now these two things together create a high pressure zone where hot air gets trapped — and this is called a heat wave or heat dome. The temperature of this heat dome rises several degrees above normal and creates an unbearably hot environment.

The opposite event — where a cold wave or cold front is seen — is called a cold wave.