Hot Topic: Effects of Climate Variability on Fisheries
Part A: Short Term Variability: El Niño
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Satellite image showing global sea surface temperatures during the El Niño of 2016. Warmer areas are red. The red band moving east across the equator shows El Niño conditions. Image courtesy ofNOAA/NESDIS.
El Niño is an abnormal warming of surface ocean waters in the eastern tropical Pacific that occurs approximately every 3 to 7 years. The phenomenon was named El Niño (meaning "The Boy Child" or "The Christ Child") by South American fishermen because it typically arrives around Christmastime.
Under normal conditions, east-to-west winds drag warm waters westward. This results in a pile-up of warm water in the western Pacific, just east of Indonesia, and northeast of Australia. At the same time, cold water from deep in the ocean rises to the surface along the South American Coast.
Every few years, the trade winds change direction. This allows the pool of warm water to move eastward, where it blocks the rising cold water. These changes help trigger the global weather changes associated with El Niño.
1. Watch this brief video to learn more about El Niño.
The oscillating warming and cooling pattern described in the video is referred to as the El Niño-Southern Oscillation or ENSO cycle. This cycle directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world. El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called non-El Niño or ENSO-neutral.
The images below show the direction of trade winds (yellow arrows) and the location of warm ocean waters (red) under normal ocean conditions (left) and during an El Niño event (right).
Image Source: NOAA (and USA Today(www.usatoday.com). Graphic by John Herne.
Image Source: NOAA (viaUSA Today (www.usatoday.com).Graphic by John Herne.
Upwelling is when deeper colder water from the bottom of the ocean moves up toward the ocean's surface away from the shore. This cooler water is nutrient-rich and is responsible for supporting high levels of primary productivity (such as phytoplankton), diverse marine ecosystems, and major fisheries found in this area.
Why are the waters warmer and deeper in the western Pacific than they are in the eastern Pacific during a normal/non-El Niño year?
Trade winds push surface warm water westward toward Indonesia, causing the sea level to be roughly half a meter higher in the western Pacific than in the eastern Pacific. Thus you have warmer, deeper waters near Indonesia and cooler, shallower waters near the coast of South America.
In normal years, clouds and rainfall are fueled by the rising air over the warmest areas in the ocean near Asia. As the Pacific's warmest water spreads eastward, the rainfall moves with it. Rains that normally would fall over the tropical rain forests of Indonesia fall over the deserts of Peru. While the western Pacific has drought and the possibility of forest fires, South America has flooding. The eastward movement of atmospheric heat also alters the global atmospheric circulation. An El Niño can impact weather far away from the equatorial Pacific.
3. Examine the graph below of California squid landings from 1940-2018. The blue bars show the years in which particularly strong El Niño events occurred.
Reduced Catches of Squid During El Niño Warming Events
Squid has become an increasingly important product in California. Landings have been increasing since the 1960s; however, we don't know if the population has actually been increasing in the warm period since 1976, or if the increased landings reflect increased fishing effort and markets for squid. This increase has been interrupted several times by major El Niño events that raise ocean surface temperatures along the California coast. Landings dropped markedly during the 1958, 1983-84, 1992, and 1998 El Niños. Squid are short lived, reproducing and then dying within 1 year. During El Niño events they are not found on their normal spawning grounds or farther north. They may be spawning in deeper colder water during these years. So far, the population has always recovered after the El Niño has passed.
Stop and Think
1: Squid has only recently become a commercially important seafood. Explain how other species with long histories of commercial fishing and economic importance might be affected by an El Niño event.
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Satellite image showing global sea surface temperatures during a La Niña event. Cooler areas in blue cover the same areas that are warm during El Niño conditions. Image courtesy ofNOAA/NESDIS.
Scientists are trying to determine if squid populations have a positive reaction during La Niña, which is considered the opposite cycle to El Niño. Strong La Niña conditions occurred in 1999-2000 and 2010-2011 and squid landings during those times reached new highs. Read the following to learn more about What is La Niña.
Checking In
Why might market squid landings increase during La Niña conditions?
During a La Niña the upwelling along the South American coast intensifies, which means that there is more nutrient-rich cold water available in the eastern Pacific. If squid are sensitive to ocean temperatures during El Niño events and warm normal years, La Niña years, which bring cooler ocean waters, could encourage their growth, aid ability to find food, or simply cause them to move more widely and be more easily found by fishermen.
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