Science For Everyone: Welcome back to "Science For Everyone", ABP Live's weekly science column. Last week, we discussed how India contributes to climate change, and what may happen by 2030 if appropriate steps are not taken to control it. This week, we discuss what El Niño and La Niña are, and how these events impact weather patterns across the globe.
El Niño conditions developed this year by early June. The atmospheric response to warmer-than-average Pacific sea surface temperatures emerged in May, the National Oceanic and Atmospheric Administration (NOAA) said in a statement dated June 8, 2023. The NOAA expects El Niño to continue into winter, and there is a 56 per cent probability of El Niño becoming a strong event at its peak. There is an 84 per cent likelihood of at least a moderate event occurring due to El Niño.
According to the NOAA's Climate Prediction Center, weak El Niño conditions emerged in May, evident from the above-average sea surface temperatures becoming strengthened across the equatorial Pacific Ocean.
When the conditions in the Pacific Ocean are normal, trade winds blow east to west along the equator, and take warm water from South America towards Asia. Cold water rises from the depths to replace this warm water. This process is called upwelling. However, El Niño and La Niña are two opposing climate patterns that break these normal conditions, and have global impacts on weather, ecosystems, wildfires and economies.
El Niño and La Niña episodes occur every two to seven years, on average, and usually last nine to 12 months, but can sometimes last for years.
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What is El Niño?
El Niño is the warm phase of the El Niño-La Niña climate pattern, also known as the El Niño-Southern Oscillation (ENSO), which is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation. This, in turn, influences temperature and precipitation across the globe.
The ENSO has three phases: El Niño, "neutral", and La Niña. El Niño and La Niña are two opposite phases, and require certain changes in both the ocean and the atmosphere because ENSO is a coupled phenomena. Meanwhile, "neutral" is the middle phase of ENSO.
The ENSO encompasses the central Pacific region.
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El Niño refers to the warming of the ocean surface, or above-average sea surface temperatures in the central and eastern tropical Pacific Ocean, and is a phenomenon which changes global atmospheric circulation in known ways, because of which climate scientists can predict potential upcoming weather and climate patterns. When there is a stronger El Niño event, global temperature, rain and other patterns are more likely to reflect the expected El Niño impacts.
The expected impacts of El Niño include rainfall over Indonesia becoming reduced, and that over the tropical Pacific Ocean increasing.
Trade winds weaken during El Niño, as a result of which warm water is pushed back east, toward the west coast of the Americas.
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According to the Naval Postgraduate School, normal conditions are those in which a warm pool of ocean water builds up in the western equatorial Pacific Ocean, because trade winds blow east to west. There is a pressure gradient between the western Pacific and eastern Pacific, the former having lower pressure, and the latter higher pressure. Due to this pressure gradient, the trade winds move from east to west. They are called the easterly trade winds.
When El Niño conditions develop, the surface pressure weakens, and the trade winds reverse. As a result, the warmer water propagates eastward via Kelvin Waves, a special type of gravity wave affected by Earth's rotation and trapped at the equator and along lateral vertical boundaries such as coastlines or mountain ranges.
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Since warm water is pushed eastward toward the west coast of the Americas, the thermocline sinks. The transition layer between the warmer mixed water at the surface and the cooler deep water below is called thermocline.
The sinking of the thermocline causes upwelling, or the rise of cold water from the depths, to occur at a deeper level in the ocean.
According to the Naval Postgraduate School, this is critical at the western coasts of the Americas because without upwelling of nutrient rich bottom water, the fish population is greatly reduced.
El Niño has been observed and recorded at least since the 1600s, off the coast of Peru. Since the abnormally warm waters occasionally appeared around Christmas, Peruvian fishermen called the phenomenon El Niño De Navidad, which means "Christ child" in Spanish.
La Niña
La Niña refers to the cooling of the ocean surface, or below-average sea surface temperatures in the central and eastern tropical Pacific Ocean.
La Niña tends to increase rainfall over Indonesia, and decrease it over the central tropical Pacific Ocean.
La Niña is the opposite of El Niño. A La Niña event is one in which trade winds are even stronger than usual, as a result of which more warm water is pushed westwards, toward Asia. Since the easterly winds along the equator become even stronger, and warm water is pushed westwards toward Asia, upwelling increases off the west coast of the Americas, and cold, nutrient-rich water is brought to the surface. A strong pressure gradient drives the movement of trade winds.
Cold water moves toward the west coast of the Americas via Rossby Waves, which naturally occur in rotating fluids. Also known as planetary waves, Rossby Waves form within the Earth's atmosphere and ocean as a result of the planet's rotation.
Since the trade winds are stronger than usual, the warm pool of water formed in the western Pacific is shifted even more to the west.
La Niña means Little Girl in Spanish, and is sometimes called El Viejo, which means "a cold event".
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How El Niño and La Niña affect global climate
El Niño affects the weather significantly because the warmer waters pushed back east cause the Pacific jet stream to move south of its neutral position, and spread further east. As a result of this shift, areas in the Northern United States and Canada are drier and warmer than usual. However, during these periods, the US Gulf Coast and Southeast are wetter than usual, and have increased flooding.
Due to the warming of the surface air above the warm water, convection occurs. Since the warm water propagates eastward, the surface air above these regions becomes warm. As a result, heavy rains are caused by convection in the US Gulf Coast and Southeast.
Australia and Southeast Asia usually experience drier conditions due to El Niño.
Due to this shift in the convection, there is a major change in the global weather pattern. Droughts, floods, storms and other weather anomalies occur in many regions of the world.
The marine life off the Pacific coast is strongly affected due to El Niño. This is because during normal conditions, upwelling brings cold and nutrient-rich water from the depths of the ocean to the surface, but during El Niño, upwelling weakens or stops altogether, and without the nutrients from the deep, there are fewer phytoplankton off the coast.
Since there are fewer phytoplankton off the coast, the fish that eat phytoplankton are affected, and in turn, everything that eats fish is affected.
Tropical species like yellowtail and albacore tuna may venture into areas that are normally too cold due to the movement of warmer waters toward the east.
The normal pressure gradients and trade winds set up again within one to two years after the initiation of El Niño. As a result, cold, nutrient-rich water again upwells off the coast of Peru. This reversal of air pressure, known as the Southern Oscillation, occurs along with El Niño, and the two phenomena are a part of the ENSO.
Since more warm water is pushed toward Asia during La Niña, upwelling increases off the west of the Americas, as a result of which cold, nutrient-rich water comes to the surface.
According to the NOAA, these cold waters in the Pacific push the jet stream northward, and this leads to drought in the southern US, and heavy rains and flooding in the Pacific Northwest and Canada.
Winter temperatures are warmer than normal in the South during a La Niña year, and cooler than normal in the North. There could be a stronger and more severe hurricane season in the Atlantic basin due to a La Niña event.
Since the warm pool of water that occurs in the western Pacific is shifted to the west, the convection effect is also shifted westward. This happens because the warming of the surface air above the water drives the convection effect. As a result, heavy rains caused by the convection also shift westward. This alters the global weather pattern, and results in droughts, floods, storms and other weather anomalies in different regions of the world.
Waters off the Pacific Coast are colder than normal during La Niña. As a result, the waters, which contain more nutrients than usual, support marine life, and attract more cold-water species such as squid and salmon to places like the California coast.