Every 2-7 years hits the most famous swing in Earth's climate, El Niño, which in many countries often bring heavy rains and even floods. Maybe the next one would not be as strong as in 1997 or in 1983, but either way one of the realities about this climate change is that it is still very little understood.
In 1895, during the Sixth International Geographical Congress in London, Federico Alfonso Pezet made notice of the following discovery:
"In the year 1891, Señor Dr. Luis Carranza, President of the Lima Geographical Society, contributed a small article to the 'Bulletin' of that Society, calling attention to the fact that a counter-current flowing from north to south had been observed between the ports of Paita and Pacasmayo."
"As this counter-current has been noticed on different occasions, and its appearance along the Peruvian coast has been concurrent with heavy rain in latitudes where it seldom, if ever, rains to any great extent, I wish, on the present occasion, to call the attention of the distinguished geographers here assembled to this phenomenon, which exercises, undoubtedly, a very great influence over the climatic conditions of that part of the world"
"Captain [Camilo] Carrillo says that the Paita sailors, who frequently navigate along the coast in small craft, either to the north or to the south of that port, name this counter-current the current of "El Niño" (the Child Jesus), because it has been observed to appear immediately after Christmas."
(Federico Alfonso Pezet, "The counter-current 'El Niño', on the coast of northern Perú", in "Report of the Sixth International Geographical Congress. Held in London, 1895. With maps and illustrations.", edited by Hugh Robert Mill & John Scott Keltie, John Murray Publishing House, Albermarle Street, London, 1896, pages 603-606.)
"El Niño" remained a scientific curiosity for nearly a century, repeated at intervals of between 2 and 7 years, until 1983, when it presented itself with unusual force and attracted public attention. In the last 20 years remarkable progress has been made in the mental image we have of this phenomenon, especially thanks to new scientific instrumentation and sophisticated meteorological buoys and satellites. Now we known that El Niño affects not only the Peruvian coast, but has consequences on the entire planet.
IN THE PACIFIC OCEAN
The Pacific Ocean is the largest ocean on Earth; it is almost twice as wide as the Atlantic Ocean. In 1923, physicist and statistician Gilbert Walker, Director of Observations in India, by analyzing enormous scientific database noted that when atmospheric pressure was low in Darwin, Australia, on the border between the Indian and Pacific oceans, it was high in Tahiti, French Polynesia, halfway towards South America. This was the first recognition that changes in the tropical Pacific and beyond are not isolated phenomena but are connected as part of an even larger swing.
This pressure difference causes air to move from South America to Australia and Asia, generating almost constant winds, formerly known as "trade winds" by the Spaniards who sailed towards the Philippines.
In 1969 Professor Jacob Bjerknes, from the University of California, Los Angeles, showed that these surface winds go upwards when reaching Australia and Asia, therefore becoming cooled, which makes water molecules in the air bind together and fall as rain. Then the air returns by above, coming back through the upper altitude levels to South America, where it descends to the surface and the cycle restarts again going to Australia and Asia.
Most notable of these winds is that, apparently, over the months and years it is also able to move the surface of the sea. It pushes surface water from South America to Australia and Asia, to the point that this water will accumulate on the other side of the Pacific Ocean: the sea on that side is about two meters higher than at the South American coast.
These winds might carry more than just water: they also carry heat. Sea water heated by the tropical sun is wiped off the South American coast, exposing colder water from the depths. The average temperature of the sea at the South American coast is about 23 degrees Celsius, while that of the opposite end, near Australia and Asia, rises up to 29 degrees Celsius.
All these mean less evaporation and therefore less rainfall in Perú and Chile, so these countries have desert landscapes. At the same time, it means more evaporation and rainfall in tropical Asia and Australia, with a corresponding difference in scenery.
But what has been noticed is that every 2-7 years, these "trade winds" blowing from South America to Asia and Australia cease, and the hot water stored at one end of the ocean begins to recede towards the center of the Pacific. The heat comes back.
The interaction between the atmosphere and the oceans is very complicated. On one hand, winds carry heated water from one side to another, but hot water also causes air to rise and generate winds. What moves what? It's like the (old) chicken-or-egg riddle. Only in recent decades the study of the Earth's climate had become multidisciplinary, with meteorologists and oceanographers working together.
IN THE REST OF THE WORLD
In the case of El Niño, it is clear that when the winds stop and the water recedes rainfall in South America increases and drought in Australia and Asia makes its appearance. But the effects go far beyond these regions.
The Pacific Ocean is so big, the water masses that are repositioned so large, and the amount of heat that is redistributed so much that their effects can be felt thousands of kilometers away. Especially, because they affect the high-altitude wind belts which exist in areas where tropical air masses encounter polar air masses, the so-called jet streams. They are "rivers of air" that travel at about 11-kilometers altitude and at over 100 km/h, around the globe. With the distribution of Pacific storms greatly affected, these "rivers of air" are also altered, and this has its effect on the whole world.
Caribbean hurricanes tend to decrease when El Niño appears, there is more rain in the southern United States and winters are less cold than usual in Canada. Even Africa is affected, with more rain in the Nile River basin, and drought to the south of the continent. It is suspected that El Niño has to do with droughts in the northeast tip of Brazil, and the non-arrival of the monsoons in India.
WHY DOES THIS OSCILLATION HAPPEN?
There is much confusion among the public about the relationship between global warming, the thinning of the ozone layer and El Niño. Firstly, we should make it clear that El Niño is not caused by humans; El Niño is a completely natural event. In fact, from studies of sediment in the bottom of lakes of the Andes, from growth rings in trees and the chemical composition of ancient Pacific coral skeletons (very sensitive to temperature) it is known that El Niño has been happening since at least the past 125 000 years ago, so it is part of the natural functioning of the Earth. Sure, if we ever alter the balance of our planet we might very well end up altering El Niño too.
One of the things that draws attention from researchers is that this is not an event that is exactly periodic, but rather highly variable, with a period of between 2 and 7 years. The intensity is also highly variable: those of 1982-1983 and 1997-1998 were the strongest ever recorded, but the one of 2002-2003 was much more moderate. It was also striking that between 1990 and 1995 the phenomenon was almost continuous, though weak. There is also a debate whether after an El Niño an opposite-conditions period exists, with exaggerated rain to Australia and Oceania, and very dry to South America.
The truth is that we have a very small database in relation to El Niño events. The first global observations come from 1949, with complete records only since 1970; in total, we have studied a dozen cases. That is too few to make reliable statistics, and even more so predictions. It is not like predicting storms, where there are a lot of them each year, facilitating the study.
There are basically two types of theories on El Niño: one is that the oscillation itself is periodic, but becomes irregular due to interference with hurricanes, storms and other phenomena. The other theory is that El Niño is the product of a lot of circumstances isolated from each other, but when mixed in the right proportion, trigger the phenomenon. If this is the case it would be literally unpredictable.
But what we do know is that once it starts, usually 6 months to a year goes by before everything gets back to normal. So we must be prepared for the effects it will have during that time. Economically, it is known that the fish production extracted from the cold Humboldt Current (the one of Perú), which accounts for about 20 % of the world production, falls considerably. Meanwhile, there are farmers in Australia who already know how to do business by paying attention to this oscillation.
For many countries, we can expect summers more rainy than normal. How much rainier? And for how much longer? We are not yet able to answer. But that is precisely one of the objectives of Science: to understand how Nature works, in order to use it to our advantage.
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Originally published in ABC Color, on 10 December 2006. Photograph: Technicians of the National Oceanic and Atmospheric Administration of the United States of America preparing a buoy equipped with sensors to study changes in the Pacific Ocean. It is expected that data collected by equipment like these will some day help us understand why El Niño happens. The oscillation has been observed for more than a century, but only since 1949 we have global data. The two highest peaks correspond to the two largest events recorded in historical times: that of 1982-1983 and that of 1997-1998. For the months of El Niño, drought is expected in southeast Africa, Southeast Asia, northern Australia, and Northeastern Brazil; concurrently, a lot of rain for the Nile River basin, Polynesia, Perú, southern United States, and the Río de la Plata basin, while Japan, Alaska and Canada tend to have a warmer winter than normal. Photo credit: National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, Tropical Atmosphere / Ocean (TAO) Project Office, Dr. Michael J. McPhaden, Director. With permission from Michael McPhaden.