OC103 Lesson #14: El Niño and La Niña An unusual oceanic phenomenon, called El Niño, occurs every 2–10 years in the tropical Pacific Ocean, and has widespread affects on global weather patterns. El Niño effects in the western U.S. include unusually wet, stormy winters in southern Oregon and northern California (to the point of sometimes causing heavy flooding and coastal erosion), and unusually dry winters in northern Oregon and Washington. This lesson covers what El Niño is, how it happens, and what some of its effects are. As scientists study El Niño more and collect more data, it is becoming clear that El Niño is an important climate feature. El Niño-related weather sometimes leads to damage that costs hundreds of millions of dollars and affects several important sectors of the economy. It is an example of a “coupled ocean-atmosphere phenomenon”, and demonstrates how the ocean, atmosphere, and climate are interconnected. El Niño • Unusual oceanic phenomenon: abnormally warm surface water in the equatorial Pacific Ocean. • First described by a Peruvian scientist in 1891 based upon accounts by local fisherman along the coasts of Ecuador and Peru; they named it “The (Christ) Child” because they often first noticed it there around Christmas time. • Characteristics: o Normally starts in western tropical Pacific and develops progressively eastward. o Often results in natural disasters (flood, droughts, fires); but is also thought to suppress hurricane development in the tropical Atlantic Ocean. o Occurs every 2–10 years: the most recent and severe events were in 1953, 1957–58, 1965, 1972–73, 1976–77, 1982–83, 1991–92, 1997–98, 2002–03, 2009–10, and 2015–16. o The U.S. National Weather Service's official definition of an El Niño requires tropical Pacific surface ocean water temperatures at least 0.5°C above normal for 5 consecutive months. o Moderate El Niños in 2006 and 2009 may have been part of the reason why the 2006 and 2009 Atlantic hurricane seasons were so mild (and the Pacific hurricane/typhoon seasons were so severe). • We don’t know everything about El Niños yet. • We can’t really predict one yet, although we now know some early warning signs to watch for.Abnormal Wind and Ocean Conditions of El Niño In a normal year in the Pacific, strong east-to-west tradewinds near the equator push warm surface water to the west (see figure below). This allows cold deeper water to “upwell” (rise to the surface) along the South American coast. This upwelling brings nutrients from below up into the photic zone, where they can be utilized by microscopic algae. The rich fishing industry off the west coast of South America is ultimately supported by this upwelling of nutrients. Normal year In an El Niño year, the tradewinds weaken or fail, and tropical winds along the equator reverse: blowing toward the east instead of to the west (see figure below). This wind slowly switches direction like a giant seesaw, taking 3–15 months to “oscillate” from normal to abnormal conditions. Meteorologists called these abnormal atmospheric conditions the Southern Oscillation, and they noticed that it changed weather patterns so that normally dry areas became flooded and normally wet areas experienced drought. El Niño yearHow the Ocean Responds to El Niño Wind Conditions • As the “normal” tradewinds weaken, the warm equatorial surface water that is normally pushed to the west stays where it is (in the eastern equatorial Pacific), so sea surface temperatures there are unusually warm. To someone in a boat off the west coast of South America, where it normally upwells cold water, it seems like warm water has “moved in” from the western tropical Pacific, although what really happened was that the warm surface water was not "moved out" by tradewinds. Peruvian fisherman first noticed the unusually warm surface waters around Christmas time, so they named this ocean phenomenon El Niño. • Because the tradewinds no longer push surface water to the west, the upwelling of nutrient-rich deep waters along the South American coast shuts down. The figure below shows normal (top) and El Niño (bottom) temperature profiles across the equatorial Pacific during August. These show the temperature structure of the top 500 m of the Pacific in a slice along the equator (looking north, so South America is on the right). Notice how during normal conditions the warm surface water (darkest red color; the temperature bar on the right is labeled in °C) is concentrated on the west (left) side of the Pacific, whereas during an El Niño the warmest surface waters are in the central part of the profile. Also, the normal profile shows cold deep water (blue) upwelling almost to the surface on the eastern end, while the El Niño profile shows those cold waters do not get nearly as close to the surface (notice that there is no "orange" water at the surface on the east end of the normal profile, and the "blue" water gets up as shallow as 50m, whereas on the east end of the El Niño profile there is "orange" water at the surface and the "blue” water only gets up as shallow as about 100m). • When upwelling shuts down or is reduced, the supply of nutrients for marine life near the surface is diminished. This can be very severe. Off the coast of Peru the shutdown of upwelling results in fish and sea birds either starving to death or moving elsewhere. • Scientists now understand that the Southern Oscillation phenomenon in the atmosphere and the El Niño phenomenon in the ocean are closely linked and that they interact with one another. This has been described as a big “climate engine”, and the term for this joint phenomena is the ENSO (for El Niño-Southern Oscillation). Normal El NiñoEffects of ENSO Globally: The map at right shows how an El Niño affects global winter weather, variously causing droughts and floods in different parts of Africa and South America, and droughts in Australia and Indonesia. As you can see, the affects are most severe for coastal regions near the ocean, but they can also affect people living far inland. Here is a link to a YouTube video about the effects of El Niño https://www.youtube.com/watch?v=7FVZrw7bk1w In North America: the effects of ENSO include unusually wet, stormy winters in southern Oregon and northern California (to the point of causing heavy