So What is an El Ni~no, Anyway? A non-technical description An El Ni~no is a temporary change in the climate of the Pacific ocean, in the region around the equator. You can see its effects in both the ocean and atmosphere, generally in Northern Hemisphere winter. Typically, the ocean surface warms up by a few degrees celsius. At the same time, the place where hefty thunderstorms occur on the equator moves eastward.
Although those might seem like small differences, it nevertheless can have big effects on the world's climate. o What causes it? o What makes it stop growing? o What effects does it have? o How long does it last? o How often do we get them? o How well can we predict El Ni~no? o A more technical explanation What causes it? Usually, the wind blows strongly from east to west along the equator in the Pacific. This actually piles up water (about half a meter's worth) in the western part of the Pacific. In the eastern part, deeper water (which is colder than the sun-warmed surface water) gets pulled up from below to replace the water pushed west.
So, the normal situation is warm water (about 30 C) in the west, cold (about 22 C) in the east. In an El Ni~no, the winds pushing that water around get weaker. As a result, some of the warm water piled up in the west slumps back down to the east, and not as much cold water gets pulled up from below. Both these tend to make the water in the eastern Pacific warmer, which is one of the hallmarks of an El Ni~no. But it doesn't stop there.
The warmer ocean then affects the winds -- it makes the winds weaker! So if the winds get weaker, then the ocean gets warmer, which makes the winds get weaker, which makes the ocean get warmer... this is called a positive feedback, and is what makes an El Ni~no grow. Back to top So what makes it stop growing? The ocean is full of waves, but you might not know how many kinds of waves there are. There's one called a Rossby wave that is quite unlike the waves you see when you visit the beach. It's more like a distant cousin to a tidal wave.
The difference is that a tidal wave goes very quickly, with all the water moving pretty much in the same direction. In a Rossby wave, the upper part of the ocean, say the top 100 meters or so, will be sliding one way, while the lower part, starting at 100 meters and going on down, will be slowly moving the other way. After a while they switch directions. Everything happens very slowly and inside the ocean, and you can't even see them on the surface. These things are so slow, they can take months or years to cross the oceans. If you had the patience to sit there while one was going by, you'd hardly notice it; the water would be moving 100 times slower than walking speed.
But they are large, hundreds or thousands of kilometers in length (not height! Remember, you can hardly see them on the surface), so they can have an effect on things. Another wave you rarely hear about is called a Kelvin wave, and it has some characteristics in common with Rossby waves, but is somewhat faster and can only exist close to the equator (say, within about 5 degrees of latitude around the equator). El Ninos often start with a Kelvin wave propagating from the western Pacific over towards South America. Perhaps you saw, on the TV news, the movie (produced by JPL) for the El Nino of 1997/98? It showed a whitish blob (indicating a sea level some centimeters higher than usual) moving along the equator from Australia to South America.
That's one of the hallmarks of a Kelvin wave, the early part of the El Nino process. When an El Ni~no gets going in the middle or eastern part of the Pacific, it creates Rossby waves that drift slowly towards southeast Asia. After several months of travelling, they finally get near the coast and reflect back. The changes in interior ocean temperature that these waves carry with it 'cancel out' the original temperature changes that made the El Ni~no in the first place. I'm being deliberately vague here becase it's complicated; look at the 'For Further Reading' link or the 'More Technical Explanation' link for more information. The main point is that it shuts off when the these funny interior-ocean waves travel all the way over to the coast of Asia, get reflected, and travel back, a process that can take many months.
Back to top What effects does it have? A strong El Ni~no is often associated with wet winters over the southeastern US, as well as drought in Indonesia and Australia. Keep in mind that you aren't guaranteed these effects even though there is an El Ni~no going on; but the El Ni~no does make these effects more likely to happen. Back to top How long does it last?
A strong El Ni~no can last a year or more before conditions return to normal. If you read the bit above about Rossby and Kelvin waves (you did, didn't you?) then you know that it lasts more or less as long as it takes the interior-ocean waves to travel all the way over to the coast of Asia, get reflected, and travel back. You can also look at the Historical El Ni~no section, which has a plot showing the last 30 years of El Ni~nos, and judge for yourself. Back to top How often do we get them?
El Ni~nos happen irregularly, but if you want to impress people at cocktail parties, you might mention that we usually get one every three to seven years. Note the word 'usually': sometimes they turn up more frequently, sometimes less. You can also look at the Historical El Ni~no section, which has a plot showing the last 30 years of El Ni~nos, and judge for yourself (de ja vu). Back to top How well can we predict El Ni~no? On average, complex computer models designed to predict El Ni~no can successfully do so 12 to 18 months in advance.
However, it seems to vary by episode; sometimes El Ni~nos are predicted quite well, with plenty of advance notice from the models, while other times they are predicted poorly, with the models not picking them up until the El Ni~no has already started. Trying to fix up the models is one of our research topics here at Scripps. Back to top This isn't much of an explanation. A more technical explanation, complete with nifty graphics, has been created by the people at NOAA's Pacific Marine Environmental Laboratory. Check that out, or look at the 'For Further Reading' link for written material. web modified: 25 June 1997 Copyright (c) 2000 David W. Pierce. All rights reserved.
El Ni~no a warm current of water El Ni~no (Spanish name for the male child), initially referred to a weak, warm current appearing annually around Christmas time along the coast of Ecuador and Peru and lasting only a few weeks to a month or more. Every three to seven years, an El Ni~no event may last for many months, having significant economic and atmospheric consequences worldwide. During the past forty years, ten of these major El Ni~no events have been recorded, the worst of which occurred in 1997-1998. Previous to this, the El Ni~no event in 1982-1983 was the strongest. Some of the El Ni~no events have persisted more than one year. The surface water becomes progressively warmer going westward because of its longer exposure to solar heating.
El Ni~no is observed when the easterly trade winds weaken, allowing warmer waters of the western Pacific to migrate eastward and eventually reach the South American Coast (shown in orange). The cool nutrient-rich sea water normally found along the coast of Peru is replaced by warmer water depleted of nutrients, resulting in a dramatic reduction in marine fish and plant life. Animation by: Shao In contrast to El Ni~no, La Ni~na (female child) refers to an anomaly of unusually cold sea surface temperatures found in the eastern tropical Pacific. La Ni~na occurs roughly half as often as El Ni~no.
Instrumentation placed on Buoys in the Pacific Ocean after the 1982-1983 El Ni~no began recording abnormally high temperatures off the coast of Peru. Over the next couple of months, these strength of these anomalies grew. The anomalies grew so large by October 1997 that this El Ni~no had already become the strongest in the 50+ years of accurate data gathering. The image below displays the Sea Surface Temperature (SST) Anomalies in degrees Celsius for the middle of September, 1997. By this time, the classic El Ni~no pattern has almost fully ripened, with maxim a above +4 degrees Celsius. Image by: CPC ENSO Main Page Droughts in the Western Pacific Islands and Indonesia as well as in Mexico and Central America were the early (and sometimes constant) victims of this El Ni~no.
These locations were consistent with early season El Ni~nos in the past. A global view of the normal climatic effects of El Ni~no can be seen below. Image by: CPC ENSO Main Page The effects El Ni~no have on United States' weather is less obvious. Back in 1982-1983, the U.S. Gulf States and California received excessive rainfall. As the winter approached, forecasters expected excessive rainfall to occur again. Indeed, portions of central and southern California suffered record-breaking rainfall amounts.
Damage consisted not only of flooding, but mudslides Some mudslides destroyed communities in a flash -- causing many casualties. Other problems could be found in the Gulf states, as severe weather was above average. Even though no one particular storm can be blamed on El Ni~no, many forecasters do believe the event did increase the chances for such severe weather to occur. Upwelling the transport of deeper water to shallow levels One oceanic process altered during an El Ni~no year is upwelling, which is the rising of deeper colder water to shallower depths. The diagram below shows how upwelling occurs along the coast of Peru. Because of the frictional stresses that exist between ocean layers, surface water is transported at a 90 degree angle to the left of the winds in the southern hemisphere, 90 degrees to the right of the winds in the northern hemisphere.
This is why winds blowing northward parallel to the coastline of Peru 'drag's surface water westward away from shore. Nutrient-rich water rises from deeper levels to replace the surface water that has drifted away and these nutrients are responsible for supporting the large fish population commonly found in these areas. The effectiveness of upwelling and its ability to support abundant sea life is greatly dependent upon the depth of the thermocline. The thermocline is the transition layer between the mixed layer at the surface and the deep water layer. The definitions of these layers are based on temperature. The mixed layer is near the surface where the temperature is roughly that of surface water.
In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature. The mixed layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two. A deeper thermocline (often observed during El Ni~no years) limits the amount of nutrients brought to shallower depths by upwelling processes, greatly impacting the year's fish crop. Non El Ni~no Years colder water in the eastern tropical Pacific The easterly trade winds of the tropics drag the surface waters of the eastern Pacific away from the coastlines of the Americas. As it moves away, the water is deflected northward (in the northern hemisphere) by the Coriolis force and southward (in the southern hemisphere), causing water to move away from the equator in both directions. Upwelling in the eastern Pacific brings colder water up from deeper levels to replace the surface water that has been dragged away.
Sea surface temperature (SST) data reveals the presence of colder water in the eastern tropical Pacific. The following plot of average sea surface temperatures from 1949-1993 shows that the average December Sst were much cooler in the eastern Pacific (less than 22 degrees Celsius) than in the western Pacific (greater than 25 degrees Celsius), gradually decreasing from west to east. The trade winds accumulate warm surface water around Indonesia, raising the sea level roughly half a meter higher in the western Pacific. As upwelling persists, the level of the thermocline rises to shallower depths off the South American coast and is depressed in the western Pacific.
The up welled water is rich in nutrients and supports an abundance of fish and marine life. As surface water propagates westward, it is heated by the atmosphere and the sun, allowing warmer waters to accumulate in the western Pacific. The cooler water in the eastern Pacific cools the air above it, and consequently the air becomes too dense to rise and produce clouds and rain. In the western Pacific however, the overlying air is heated by the warmer waters below, destabilizing the lower atmosphere and increasing the likelihood of precipitation. This is why during most non El Ni~no Years, heavy rainfall is found over the warmer waters of the western Pacific (near Indonesia) while the eastern Pacific is relatively dry. El Ni~no Events results from weakening easterly trade winds The easterly trade winds are driven by a surface pressure pattern of higher pressure in the eastern Pacific and lower pressure in the west.
When this pressure gradient weakens, so do the trade winds. The weakened trade winds allow warmer water from the western Pacific to surge eastward, so the sea level flattens out. This leads to a build up of warm surface water and a sinking of the thermocline in the eastern Pacific. The deeper thermocline limits the amount of nutrient-rich deep water tapped by upwelling processes. These nutrients are vital for sustaining the large fish populations normally found in the region and any reduction in the supply of nutrients means a reduction in the fish population.
Convective clouds and heavy rains are fueled by increased buoyancy of the lower atmosphere resulting from heating by the warmer waters below. As the warmer water shifts eastward, so do the clouds and thunderstorms associated with it, resulting in dry conditions in Indonesia and Australia while more flood-like conditions exist in Peru and Ecuador. El Ni~no causes all sorts of unusual weather, sometimes bringing rain to coastal deserts of South America which never see rain during non-El Ni~no years. The flooding results in swarming mosquitoes and the spread of disease. The air-sea interaction that occur during an El Ni~no event feed off of each other.
As the pressure falls in the east and rises in the west, the surface pressure gradient is reduced and the trade winds weaken. This allows more warm surface water to flow eastward, which brings with it more rain, which leads to a further decrease of pressure in the east because the latent heat of condensation warms the air... and the cycle continues. Atmospheric Consequences of El Ni~no influencing weather patterns worldwide During an El Ni~no year, tropical rains usually centered over Indonesia shift eastward, influencing atmospheric wind patterns world wide. Possible impacts include: a shifting of the jet stream, storm tracks and monsoons, producing unseasonable weather over many regions of the globe. During the El Ni~no event of 1982-1983, some of the abnormal weather patterns observed included: . Drought in Southern Africa, Southern India, Sri Lanka, Philippines, Indonesia, Australia, Southern Peru, Western Bolivia, Mexico, Central America Heavy rain and flooding in Bolivia, Ecuador, Northern Peru, Cuba, U.S. Gulf States Hurricanes in Tahiti, Hawaii The 1982-83 El Ni~no strengthened the upper-level ridge that was present off the West coast of the United States.
(This intensification is represented by the increased amplitude of the wave in the right panel below). Normal Winter El Ni~no Winter Images by: DAS, University of Washington The amplification led to a warming in the near-Pacific regions of North America, extending from Alaska to the northern Plains of the United States (orange shading). Simultaneously, the deepening of the winter upper-level trough (typically found over the eastern US) produced heavier than normal rains in the southern states (blue shading). As a result of the 1982-83 El Ni~no event, wide spread flooding occurred across the southern United States. Economic Consequences of El Ni~no and the influence on prices worldwide The coast of Peru is one of five major fishing grounds in the world (along with the coastal waters of California, Namibia, Mauritania, and Somalia). The abundance of fish is supported by the upwelling of nutrient rich waters from deeper levels (below the thermocline).
During non-El Ni~no years, the southeast trade winds, drag surface water westward away from shore. As surface water moves away, upwelling brings up colder waters from depths of 40-80 meters or more. This deep sea water is rich in nutrients which can sustain large fish populations. During an El Ni~no event, the southeast trade winds weaken and so does the amount upwelling in the eastern Pacific. The deeper thermocline means that any upwelling that does occur is unable to tap into the rich nutrients found in deeper waters. Consequently, warm nutrient-poor water predominates the region and a decrease in the fish population is observed.
A reduction of the fish population reduces the amount of fishmeal produced and exported (by local industry) to other countries for feeding poultry and livestock. If the world's fishmeal supply decreases, more expensive alternative feed sources must be used, resulting in an increase in poultry prices worldwide.