Climate anomaly refers to a rare climate phenomenon that has not occurred in the recent complete 30 year meteorological data, or that has occurred in a region for 30-50 years, or that has only one or two opportunities in a hundred years.[2]Climatic conditions where the anomaly of climatic elements reaches a certain order of magnitude (such as more than 1-3 mean square deviations).
The abnormal climate is relative to the normal climate.The so-called normal climate means that the climate change is close to the average situation for many years, which is more conventional and more suitable for human activities and agricultural production.Exceptions: infrequent, such as seriousdrought, extra largerainstorm, serioushail, Super strongtyphoonEtc.Abnormal climate will have a great impact on human activities and social economy.
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The causes of climate anomalies can be summarized into three categories: (1) astronomical causes: including changes in solar radiation, Earth's orbital parameters (i.e. tilt of the Earth's axis, orbital eccentricity, changes in precession, etc.), and tidal changes;(2) Geographical reasons: including polar movement, sea and land changes, continental drift, polar ice growth and decline, and volcanic activity;(3) Human reason: burning fossil fuels makes CO in the atmospheretwoIncrease in content and the impact of artificially increased aerosols[1]。
(1) Solar activity: The fundamental cause of climate change is solar activity, which can cause changes in solar radiation. If the solar radiation activity increases, the direct radiation flux of the sun will increase, and the temperature of the earth's surface will rise, and the temperature of the equator will rise higher than that of the poles, so that the temperature gradient between the equator and the poles will increase, thus strengthening the atmospheric circulation.The increase of temperature and circulation will lead to the increase of evaporation, cloud cover and precipitation.The increase in cloud cover increases the solar radiation returning to the sky.In addition, the increase of cloud cover reduces the temperature difference between day and night, between winter and summer, and between sea and land. During the period of enhanced solar radiation, the climate is oceanic.During the weakening of solar radiation, the climate is continental.
（2）COtwoAnd aerosols: with the worldwide industrialization and other human activities, the content of micro dust in the atmosphere increases, reducing the solar radiation directly obtained from the earth atmosphere, thus reducing the surface temperature.
(3) Volcanic activity: After volcanic eruption, the volcanic dust curtain formed by volcanic dust entering the lower stratosphere can spread to the whole hemisphere.At low latitudes, the eruption can spread to the whole world, and maintain the maximum concentration at middle and high latitudes. At last, at the polar cap, the impact is greatest at the middle latitudes.The life span of aerosol particles in the lower stratosphere is generally 3 to 7 years, which can reach 15 years. Therefore, volcanic eruption will have a certain impact on global climate change.
（4）El Nino Phenomena andLa NinaPhenomenon: El Ni ñ o phenomenon refers to the continuous abnormally high sea water temperature in the sea off Peru and the equatorial Pacific region, which is a prominent manifestation of the interaction between the ocean and the atmosphere.In general, the surface water in the western tropical Pacific is warm, while the water temperature in the eastern part is very low.This temperature gradient between the eastern and western Pacific oceans, together with the easterly trade winds, constitutes a quasi equilibrium state of the ocean atmosphere system.Once this equilibrium state is broken, the warm hot air flow in the western Pacific moves eastward with thunderstorms, warming the water temperature of the entire Pacific waters and causing abnormal climate.
La Nina phenomenon is a phenomenon of abnormal drop of water temperature in the eastern Pacific near the equator, which is accompanied by global climate anomalies.Its characteristics are opposite to that of El Ni ñ o, and together with El Ni ñ o, it has become the strongest signal for forecasting the anomaly of the global climate system.The La Nina phenomenon has changed the atmospheric dynamic, thermal and moisture conditions, thus affecting the situation of atmospheric circulation, intensifying the tropical easterly jet, and causing climate anomalies.
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Usually, after El Nino, due to the weakening of Walker circulation and the rising temperature of the equatorial Pacific sea water, the normal cold high pressure cannot be formed, resulting in the weak summer monsoon in China, the monsoon belt is southward, and the Yangtze River basin and Jiangnan region are prone to floods.For example, 1931, 1954 and 1998 all occurred in the next year of El Nino.El Ni ñ o was one of the most important factors affecting the catastrophic flood in China in 1998.Drought and high temperature often occur in summer in northern China.After the strong El Nino in 1997, the drought and high temperature in northern China were very obvious.
After La Nina, the opposite is true.The strong summer monsoon and the rapid northward movement of the rain belt caused floods in the north and drought in the south.
In addition, El Nino and La Nina phenomenon will also causetyphoonException of.
