Atmospheric motion

First, the effect of atmospheric circulation on climate.

Between high latitude and low latitude, between ocean and land, due to the uneven cold and hot air pressure, the atmospheric circulation on the earth is formed under the action of pressure gradient force and geostrophic deflection force. Atmospheric circulation guides the generation and movement of air masses, fronts, cyclones and anticyclones with different properties, which is of great significance to the formation of climate. The equatorial belt is controlled by low pressure all the year round, mainly by updraft, with abundant precipitation and dense forests; On the contrary, under the control of high air pressure, the subtropical zone, which is dominated by subsiding airflow, has little precipitation and forms a desert. Air masses from high latitudes or inland areas are cold and dry, while air masses from low latitudes or oceans are mild and humid. When an area is controlled by two different air masses in a year, the climate has obvious seasonal changes. For example, the climate in China is cold and dry in winter and hot and rainy in summer, which is the result of alternating control of polar continental air mass and tropical ocean air mass in winter and summer. In short, on a global scale, the atmospheric circulation is carrying out a lot of heat and water transport between high and low latitudes and between land and sea. In the meridional heat transfer, the heat transferred by atmospheric circulation accounts for about 80%.

Under the combined action of atmospheric circulation and ocean currents, the tropical temperature drops by 7 ~ 13℃, and the mid-latitude temperature rises, reaching as high as 20℃ in high latitudes above 60 n. 

The transport of atmospheric circulating water also plays an important role. The quantity, direction and speed of moisture transport in the atmosphere are closely related to the circulation situation. In the northern hemisphere, the water vapor is transported from the westerly airflow to the middle and high latitudes around 30 N. It was transported southward by the trade winds to low latitudes. There are two main types of water vapor transport in China: one comes from the Bay of Bengal, the Indian Ocean and the South China Sea and enters China with the southwest airflow; The other comes from the Atlantic Ocean and the Arctic Ocean and enters China with the northwest airflow. The transportation capacity of the south branch is large, while the transportation capacity of the north branch is small. The dividing line between Huanghuai and Qinling is basically equivalent to the dividing line between humid and semi-humid climate.

The formation of precipitation is inseparable from the weather system, the input of clouds and water vapor, and the vertical upward movement of air. All this is closely related to the circulation situation. For example, the amount of precipitation is related to the amount of water vapor entering various weather systems. The inflow of warm and humid equatorial airflow can produce 1 00 mm precipitation in a few hours or1hour; The precipitation of thunderstorms can be proportional to the amount of water vapor flowing into cumulonimbus clouds.

The distribution of global precipitation has two peaks and two low peaks, namely, two rainy areas and two rainless areas. The positions of the two rainy areas are basically consistent with those of the equatorial convergence area and the polar front convergence area. The two rainless zones are in the same position as the subtropical high and the polar high.

Atmospheric circulation plays an extremely important role in the formation of climate. Under the control of different circulation, there will be different climates. Even if the intensity of the same circulation system changes, the climate in the area it controls will change. If the circulation is abnormal, the climate will be abnormal.

The change of atmospheric circulation can be expressed by the intensity and transformation of meridional circulation and zonal circulation. There is an average condition for the change of atmospheric circulation in a certain area over a long period of time. When the duration and switching frequency of meridional circulation and zonal circulation in a certain period of a year greatly exceed the average situation in that period, the atmospheric circulation in a certain period of a year is called circulation anomaly. For example, the main circulation characteristics of 1972 indicate that there are two stable and powerful long-wave troughs in the northern hemisphere. From February to March, there are blocking high pressure in Europe and the North Pacific, and low trough in the West Atlantic and Asia. From May to September, there are blocking high pressure in Europe and western North America, and there are large troughs in eastern North America and East Asia. Throughout the year, the North Atlantic, the North Pacific, eastern and northeastern Europe and most parts of western Asia are under the control of strong blocking high, so 1972 is an abnormal circulation year for the northern hemisphere.

Due to the abnormal circulation, the numerical values of meteorological elements such as pressure field, temperature field and humidity field will inevitably be obviously deviated, which will lead to abnormal precipitation, cold and warm, drought and flood, continuous cold and other climate anomalies.

The World Meteorological Organization pointed out in its annual report 1972 that "1972 is one of the most abnormal years in history". In this year, in June of 65438+ 10, the rainfall and snow amount in Marley, Gong Sheng, Michigan, USA reached 135 1.3mm, which was more than ten times that in normal years. In February, a strong snowstorm hit the south of Yilan. In Ardakan area, many villages are buried under 8 meters of snow. From March to May, the north-central United States and the Mediterranean countries in Europe were successively hit by strong winds, rain and snow, while several snowstorms accompanied by strong low temperature and freezing injury occurred almost simultaneously in the Middle East and the Near East. From May to June, India was extremely hot, with the highest temperature exceeding 50℃, and a once-in-a-century torrential rain occurred in Hong Kong. From July to August, there are endless icebergs floating in the Arctic Ocean, four times more than normal. In the past two months, high temperature and rainy weather occurred in the Soviet Union and Europe, which led to peat self-immolation and forest fire. In Western Europe, the continuous low temperature caused the special phenomenon that the highest temperature in London during the summer solstice 1972 was lower than that during the winter solstice 197 1. In autumn, the low temperature in eastern Asia and Europe generally advances the first frost; In winter, Sweden in northwest Europe experienced a rare warm winter in 200 years, and the Soviet Union also experienced an unusually warm winter. Mushrooms in the suburbs of Moscow actually broke ground in winter, and a once-in-a-century "winter thunderstorm" appeared in Leningrad. In West Africa, India and the European region of the Soviet Union, there has been almost a serious drought that lasts all the year round. In West Africa, drinking water for people and livestock has become a problem.

In China, due to the continuous and stable blocking situation in Europe and West Asia, there are few opportunities for cold and warm air to cross in China, resulting in less rain and severe drought in parts of northern and southern China during flood season.

Therefore, in the case of abnormal circulation, drought may occur in one area, flood may occur in another area, or extreme heat may occur in one area and extreme cold may occur in another area.

Atmospheric circulation plays an important role in climate formation. It not only affects the zonality of climate through the zonal distribution of circulation, but also expands the influence range of land, ocean and terrain through the transport of heat and moisture, and destroys the zonality of climate. When the circulation situation tends to the long-term average situation, the climate is normal; When the circulation situation is abnormal in individual years or seasons, it will directly affect the weather and climate of that period and make it abnormal.

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