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17.7: El Niño and La Niña

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    Weather versus Climate

    People often confuse weather and climate; they are not identical. According to the American Meteorological Society (AMS), the weather is defined as the state of the atmosphere at some place and time, usually expressed in terms of temperature, air pressure, humidity, wind speed and direction, precipitation, and cloudiness. Meteorologists study the atmosphere, processes that cause weather, and the life cycle of weather systems.

    Climate is defined in terms of the average of weather elements (such as temperature and precipitation) over a specified period of time. (The World Meteorological Organization defines the typical time period of time as 30 years). The planet’s climate has changed many times over Earth’s long geologic history. For example, over the past million years, Earth has experienced several glacial periods interspersed with interglacial (warmer) periods. The relatively constant and favorable interglacial period of climate experienced over the past 8000 years has made human civilization’s advancement possible.

    Climate also encompasses weather extremes for a particular place. Climate patterns are long-term atmospheric conditions that influence life on Earth. One important example of a climate pattern is the El Niño-Southern Oscillation. This cyclic climate pattern greatly affects the weather in Southern California as well as the rest of North America.

    El Niño-Southern Oscillation

    Upwelling of the coastal water off of Peru makes that region one of the world’s most productive fishing grounds. But every so often, the conditions in the region are very different. Every few years, the cold, nutrient-rich water is replaced by unusually warm water that is low in nutrients, leading to a decline in fish populations. In addition, the normally dry areas receive lots of rain. Because this phenomenon occurs in the northern winter close to Christmas, it is called El Niño (the child). More formally, the event is referred to as El Niño-Southern Oscillation (ENSO). The Southern Oscillation portion refers to the fluctuating atmospheric conditions that lead to the localized ocean warming of El Niño. While the exact reasons for the oscillation events are unclear, it is easier to understand how they lead to an El Niño.

    Under normal conditions in the equatorial Pacific, the trade winds blow towards the west, moving large amounts of warm surface water towards the western Pacific around Southeast Asia. As the surface water moves west, it is replaced by cold, nutrient-rich deep water through upwelling. The coastal upwelling leads to a shallow thermocline in the eastern Pacific. In terms of atmospheric conditions, the trade winds are part of a convection cell called the Walker Cell. There is low pressure over the western Pacific, leading to rising moist air and significant precipitation in the region. In the eastern Pacific near South America, there is high pressure, leading to drier conditions.

    Cross section of the Pacific Ocean from 120°E to 80°W in normal conditions. The thermocline rises in a broad J shape from left to right. Arrows on the ocean surface indicate movement from right to left. The surface water is color coded with warmer (red) about 75% of the way to the left and cooler (blue) on the right. Above the red surface water there is low pressure and clouds with rain. The air rises through the clouds, travels right horizontally, descends on the right in high pressure, the travels horizontally left over the surface of the ocean.
    Figure \(\PageIndex{1}\): Normal conditions in the equatorial Pacific. Low pressure in the western Pacific and high pressure in the eastern Pacific cause the trade winds to move surface water to the west, leading to upwelling and a shallow thermocline near South America (Modified by PW from Fred the Oyster (Own work) [Public domain], via Wikimedia Commons).

    El Niño

    During an El Niño-Southern Oscillation, the high pressure system over the eastern Pacific diminishes, so the trade winds are weakened, or in extreme cases will even reverse. When this happens, warm surface water begins to flow east across the Pacific towards South America, warming the coastal South American water by up to 8°C in strong ENSO years. This influx of low-density warm water deepens the thermocline and prevents upwelling, which dramatically reduces productivity and can devastate populations of fish and other marine life.

    Cross section of the Pacific Ocean from 120°E to 80°W in El Niño conditions. The thermocline rises gently from left to right. Arrows on the ocean surface indicate movement from left to right. The surface water is color coded with warmer (red) in the middle and cooler (blue) on the outside. Above the red surface water there are clouds with rain. The air rises through the clouds, diverges at elevation, travels both left and right horizontally, descends on both sides, then travels horizontally over the surface of the ocean to the clouds.
    Figure \(\PageIndex{2}\): El Niño conditions in the equatorial Pacific. Weakening or reversal of the trade winds transport warm surface water eastward towards South America, disrupting coastal upwelling (Public domain, via Wikimedia Commons).

    In the atmosphere, the low pressure system in the western Pacific is replaced by high pressure, bringing dry or even drought conditions to Southeast Asia and Australia. The low pressure system moves east across the Pacific, potentially reaching as far as South America in strong El Niño years. The low pressure over the eastern Pacific brings lots of rain and flooding to South America. But the effects of El Niño are not just limited to the Pacific; it can influence weather patterns throughout the globe.

    La Niña

    Because the Southern Oscillation is a cyclic pattern, the eastern Pacific is not subject just to unusually warm conditions. There are also periods of abnormally cold water in the region known as La Niña events. During La Niña, the trade winds are unusually strong, leading to increased upwelling and transport of deep, cold water to the surface. The effects of La Niña are essentially the opposite of El Niño, bringing cooler and wetter conditions to the northwestern United States and Canada, while the southeastern U.S. receives below-average precipitation. Monsoon seasons in Asia are drier during El Niños but wetter during La Niña events.

    Cross section of the Pacific Ocean from 120°E to 80°W in La Niña conditions. The thermocline rises steeply from left to right. Arrows on the ocean surface indicate movement from right to left. The surface water is color coded with warmer (red) in the left and cooler (blue) on the right. Above the red surface water there are clouds with rain. The air rises through the clouds, travels right horizontally, descends on the right, then travels horizontally over the surface of the ocean to the clouds.
    Figure \(\PageIndex{3}\) La Niña conditions. Stronger trade winds promote more intense upwelling in the eastern Pacific, leading to cooler than usual water temperatures (Public domain, via Wikimedia Commons).

    El Niño and La Niña events alternate, although the presence of one does not always mean the other will automatically follow. El Niño occurs roughly every 2-7 years, and each event may last from a few months to a year or more. Although we do not understand exactly why or when the ENSO events will occur, we can anticipate their arrival by monitoring a number of ocean and atmospheric phenomena that make up the Multivariate ENSO Index (MEI). Examination of the MEI over time demonstrates the cyclic nature of ENSO events.

    figure9.6.5.png
    Figure \(\PageIndex{4}\): Multivariate ENSO Index over time. Positive (red) values indicate warmer than normal conditions (typical of El Niño), while negative (blue) values represent conditions that are cooler than average (typical of La Niña). The greater the deviation from zero, the stronger the event. Note the intense El Niño events in 1983, 1997-1998, and 2015 (NOAA, https://www.esrl.noaa.gov/psd/enso/mei/).

    Figure \(\PageIndex{5}\) shows a comparison of sea surface temperatures in the equatorial Pacific during El Niño and La Niña periods.

    Two globes with color coded oceans where blue indicates cooler than average ocean temperatures, white indicates average temperatures, and red indicates warmer than average temperatures. Top globe shows a streak of blue near the equator in the Pacific Ocean for a La Niña event in December 1988. Bottom globe shows a large wedge of red near the equator from South America to the middle of the Pacific Ocean for an El Niño event in 1997.
    Figure \(\PageIndex{5}\): Maps of sea surface temperature anomaly in the Pacific Ocean during a strong La Niña (top, December 1988) and El Niño (bottom, December 1997). Maps by NOAA Climate.gov, based on data provided by NOAA View.

    Impacts of El Niño

    ENSO events affect global atmospheric circulation such as the Polar and Pacific jet streams. This affects temperatures and precipitation worldwide. During El Niño in North America, the Pacific jet stream moves south from its normal position. This results in warmer, drier conditions than usual in the north, and wetter and colder conditions than usual in the south. During La Niña in North America, the Polar jet stream moves north from its normal position. The opposite conditions occur with colder and wetter conditions in the north, and drier and warmer conditions in the south.

    Top: Map of North America during El Niño. There is low pressure in the Pacific Ocean. Canada and east Alaska are colored orange and labeled 'warmer.' The Pacific jet stream makes a broad red wave across the south. The south is colored green and labeled 'wetter.' The central and eastern south region is colored blue and labeled 'colder.' The central eastern region is colored purple and labeled 'drier.' Bottom: Map of North America during La Niña. There is blocking high pressure in the Pacific Ocean. Canada and east Alaska are colored blue and labeled 'colder.' The Polar jet stream dips from Canada to the east coast in a blue wave. The south is colored purple and labeled 'drier.' The central and eastern south region is colored orange and labeled 'warmer.' The central eastern region is colored green and labeled 'wetter.'
    Figure \(\PageIndex{6}\): The effects of El Niño and La Niña on the jet stream and regional weather patterns during winter in North America. Images by NOAA Climate.gov.

    The 2014-2016 El Niño was one of the strongest ENSO events on record (Figure \(\PageIndex{4}\)). Some of the recorded global impacts of this El Niño included:

    • Widespread droughts in the Philippines and many South Pacific island nations.
    • Severe coral bleaching on the Great Barrier Reef in Australia.
    • One of the most destructive bushfire seasons in Australia, in part due to low rainfall.
    • High rainfall in the southeastern United States and parts of California, leading to flooding.
    • Mild, low-precipitation winter in the New England region of the United States.
    • Severe flooding in Peru and Argentina.
    • Droughts in many portions of southern Africa.
    • Nearly 100 million people worldwide suffered a lack of food or water from flooding and droughts.
    • Peru suspended its second anchovy fishing season due to low biomass, and an anticipated 20% reduction in the yearly catch.

    Although many of the impacts are negative, there are some positive impacts such as lower heating bills for those areas that experience warmer than usual conditions in the winter. Also, a change in ocean temperatures near the coast can attract fish species that are not normally found in the area. Warmer waters can bring tropical fish such as yellowtail and albacore tuna, and colder waters can bring cold-water fish such as squid and salmon.

    Additional links for more information

    This page titled 17.7: El Niño and La Niña is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Paul Webb via source content that was edited to the style and standards of the LibreTexts platform.