What is the El Nino phenomenon? What caused it? When did it mainly appear? What's the impact on the climate in China?
It is only in the past 30 years that the sea-air anomaly accompanying El Nino gradually became clear. The earliest El Nino was only related to the disappearance of the cold water area in the eastern Pacific. Generally speaking, there is a large area of cold water in the waters south of the equator in the eastern Pacific Ocean. These cold waters flow from the depths of the ocean. Why cold water can be turned on here is discussed as follows. These upturned cold water contains a lot of nutrients, which attracts a large number of fish and shrimp to feed and lay eggs here. There is no doubt that this is a bumper harvest year for local fishermen. Once the cold water area disappears and the fish and shrimp don't come, even if the water temperature is high, a large number of fish and shrimp will die, which is undoubtedly a disaster year for local fishermen. The disappearance of the cold water area began around Christmas. The local people think that this is the misfortune that God asked his son to create for the world, so they call this phenomenon "the son of God" or "the infant" for short. Now no one thinks that El Nino has anything to do with God, but it just reflects some phenomena in meteorology, or a new term with specific content in meteorology. What people are concerned about now is far from some phenomena caused by El Niñ o, but its possible impact on climate and ecology. More and more people are studying the causes of El Nino.
I. Causes and formation process of El Niñ o
Let's talk about the main reasons for the formation of El Nino, and then explain them one by one: (1) global temperature rise, (2) the westerly belt is strengthened in spring, (3) the return point of Walker circulation moves eastward, and (4) the obstacles of the Andes to the returning Walker circulation. Of the above four reasons, the first two are global and the last two are regional. The key to El Nino is the change of Walker circulation, so before explaining these four reasons, we need to know something about atmospheric circulation.
The word atmospheric circulation often appears in weather forecast, because atmospheric circulation is one of the main driving forces leading atmospheric activities, and the change of atmospheric circulation is also one of the main reasons for climate change. The appearance and disappearance of El Nino is the result of a change called Walker Circulation. The atmospheric circulation mainly moves in the troposphere below 10 km, and there are many atmospheric circulation in different directions. It can be said that there is no place in the world where the climate is not affected by the change of a particular atmospheric circulation. So what can cause the change of atmospheric circulation? Fundamentally speaking, this is determined by the change of global atmospheric energy balance. Atmospheric energy includes the sum of atmospheric thermal energy and atmospheric kinetic energy. More than 99% of atmospheric energy comes from solar radiation. The measurement results of solar constant in the past hundred years show that the average temperature of the atmosphere is less than 0.0l℃ due to the change of solar radiation, but in fact, the annual temperature of the atmosphere can reach about 0.2℃, which shows that the main reason for the change of atmospheric energy comes from the inside of the atmosphere. The atmosphere converts about 50% of the absorbed solar radiation energy into kinetic energy and heat energy, which is the income part of atmospheric energy. The other 50% is reflected into space, which is the expenditure part of atmospheric energy? The income and expenditure of atmospheric energy is not fixed, and the annual variation range is within +0.05. The factors that cause this change are very complicated, including physical factors, chemical factors and dynamic factors. At present, the understanding of this aspect is not very clear, but one thing is clear, that is, the pollution of the atmosphere, ocean and land is one of the main factors leading to the change of atmospheric energy budget. Because of the instability of atmospheric energy budget, it is also the fundamental reason of atmospheric circulation change and climate change.
We only discuss the atmosphere that affects El Nino.
How does the cycle form and change? This kind of circulation is called Walker circulation, which was first discovered by Englishman Walker in 1969. Walker circulation belongs to the air-sea energy exchange circulation, which originated in the equatorial region of the western Pacific, and the land part mainly belongs to Indonesia and Malaysia. This is a rising hot air stream, which rises to a height of 6-7 kilometers from here and then moves to the southeast, and then drops near the tropic of Capricorn in the eastern Pacific Ocean. The reason why it descends here is that (1) is blocked by the Andes. The Andes here is 6000 meters high, which is undoubtedly a great resistance to Walker's circulation eastward. (2) Blocked by the updraft of South America, it belongs to tropical and subtropical areas, with strong updraft. During the decline of Walker circulation, Coriolis force makes the airflow shift westward, and the center of the airflow lands near Easter Island in the eastern Pacific Ocean. Because the airflow has a great influence on the falling process, it blows a large area of surface seawater south of the equator of the eastern Pacific Ocean to the west. At the same time, the deep cold water here was turned up, so a cold water area appeared here. After the Walker circulation drops, it will return to its birthplace and form a southerly wind in the equatorial region of the Pacific Ocean, which is called the "southeast trade wind". This southeast trade wind blows the surface water on the equator of the Pacific Ocean to the west. (3) The equatorial region of the western Pacific is an arc-shaped structure consisting of thousands of islands and peninsulas. The west is basically closed, and ocean water blown from the east accumulates here. Generally speaking, the sea level here is about 60 cm higher than that in the cold water area of the East Pacific. The accumulated water can reach 1 trillion cubic meters. Because the water here can't flow and has strong heat storage function, it has become the hottest water area in the Pacific Ocean. Generally, it is about 2℃ higher than that in the central Pacific Ocean, and about 6℃ higher than that in the east Pacific cold water area. This is why there are three phenomena in the Pacific Ocean during the non-El Nino period. (1) the cold water area near the equator of the eastern Pacific, (2) the southeast trade wind on the equator of the Pacific, and (3) the accumulation of hot water on the equator of the western Pacific.
The disappearance of the above three phenomena is the appearance of El Nino. Let's see how El Nino is formed. It is generally believed that the El Nino year must be a year with high temperature. Why? With the increase of temperature, the atmosphere will inevitably expand outward, which is the basic law of thermodynamics. When the atmosphere expands outward, the height of all atmospheric circulation will also rise. Every time the average temperature of the atmosphere increases by 0.65438 0℃, the atmosphere expands outward by an average of 20-30m. The atmosphere near the equator expands outward several times higher than the average level. When the Walker circulation rises, it will surpass the Andes and cross the Andes to continue eastward, but it is blocked by the updraft in South America. It is difficult to move eastward. Every winter and spring, the global atmosphere is thick. Driven by the strong westerlies, the Walker circulation that has crossed the Andes can move eastward. It was just a spent force and soon fell over the South American continent. When it fell back to its birthplace, it was immediately blocked by the Andes. At this time, all Walker circulations landed on the South American continent. Because there is a lot of water vapor in Walker circulation, there are often abnormal weather such as heavy rain and strong wind here. This is the first disaster that the "son of God" brought to the world after his descent. At the same time, the cold water area in the eastern Pacific on the west side of the Andes disappeared, and the southeast trade winds in the equatorial region of the Pacific also disappeared, and the hot water accumulated in the equatorial region of the western Pacific returned to the east. This is the appearance of El Nino. As mentioned above, this phenomenon must have started in spring. After about four months, this hot water flowed to the eastern Pacific Ocean, so the whole equatorial region of the Pacific Ocean became hot and El Niñ o reached its peak. This time, the season must be summer. At this time, the sea levels in the East and West Pacific also tend to be consistent.
When El Nino reached its peak, there was almost no excess hot water accumulation in the equatorial region of the western Pacific, and the source power of Walker circulation was greatly weakened. Walker circulation entering the South American continent began to retreat westward, and El Nino began to weaken. If the backward distance of Walker circulation is equal to the original eastward distance, then the original cold water area in the equatorial waters of the eastern Pacific will be restored. However, because the heat at the source of Walker circulation is much less than before, the distance of Walker circulation retreat is often farther than the distance of eastward advancement. In this way, the cold water area will expand westward, which is La Nina phenomenon. La Nina is inevitable after El Nino disappears. It can be said that La Nina is a "by-product" of El Nino. Figure 1d illustrates the reason why La Nina appears. La Nina usually occurs at the turn of summer and autumn, because at this time, the easterly wind in the global atmosphere is strengthened and the westerlies are weakened. This also provides some impetus for the westward retreat of Walker circulation.
Among the four conditions for the formation of El Niñ o, the Andes play a unique role. In the years when the temperature is not high, it blocks the eastward advance of Walker circulation. In the year of high temperature, the backflow of Walker circulation is blocked, which is one of the typical examples of geographical factors affecting climate. The Andes range from north latitude 10 to south latitude 50. The total length is about 9000 kilometers, the general height is about 3000 meters, and the highest point is about 6000 meters. Mainly located near the tropic of Capricorn, where Walker Circulation passes. It can be said that the formation of El Nino phenomenon is also a coincidence of many factors in nature.