Influence of Qinghai-Tibet Plateau on Climate in China

1. Mechanical blocking effect

The Qinghai-Tibet Plateau has a high altitude and a large area, standing at 29? Between D40°N, north and south span about 10 latitude, and east and west span about 35 longitude. Its area is quite large, with an altitude of over 5,000 meters and a series of peaks over 7,000 meters? D8000m, occupying the middle and lower troposphere, is like a huge island in the atmosphere and ocean, which is an insurmountable obstacle to the cold air with stable stratification and small thickness in winter. The cold wave invading China from Western Siberia generally passes through Junggar Basin, Hexi Corridor and Loess Plateau, resulting in the winter temperature in the tropical and subtropical areas in eastern China being much lower than that in the northern Indian Peninsula, which is blocked by the Qinghai-Tibet Plateau. Table 6? In 10, stations A, C and E were located in the northern part of Indian Peninsula, and the average monthly temperature in winter was higher than that of stations B, D and F at the same latitude and height, especially stations C and D. This is because Yuanling Station D is located in the plain in the eastern part of the plateau, and the cold wave is unimpeded, while Delhi Station C is located in the middle of the southern part of the plateau, and the barrier effect is very obvious.

In winter, the westerly airflow was forced to branch and walk around the Qinghai-Tibet Plateau. It can be clearly seen from the monthly average temperature maps of 700hPa and 500hPa in winter in the northern hemisphere that the northwest is warmer than the northeast in winter, while the southeast of the southern half of the plateau is warmer than the southwest, which is obviously caused by the combined action of cold, warm and advection of the above branches. Because the west wind branches on the west side of the plateau, the northwest side of the plateau is warm advection and the southwest side is cold advection. After bypassing the plateau, the airflow converges, with cold advection in the northeast and warm advection in the southeast.

In summer, the Qinghai-Tibet Plateau can also block the warm and humid air flow from the south to the north, but the warm and humid air flow is generally unstable and it is easier to climb mountains than cold air. It can be seen from the monthly average temperature distribution map in summer that there are always two warm tongues extending from Assam in the north and northeast of Pakistan to Tibet, and some warm and humid air flows pass through mountain passes or valleys in the south of the plateau and enter the south of the plateau, which is an important reason for the formation of a warm belt extending from east to west in the Yarlung Zangbo River basin.

The blocking effect of Qinghai-Tibet Plateau on temperature not only appears in the lower troposphere, but also affects the middle troposphere. According to the comparison of the average temperature of Quxian and Delingha in the same latitude last month, it can be seen that the temperature of Quxian and below 500hPa is lower than that of Delingha, especially in winter.

2. Thermal action

Compared with the free atmosphere at the same altitude, the surface temperature of the Qinghai-Tibet Plateau is lower in winter and higher in summer. According to the analysis and calculation of observation data, it is shown that the heat transported to the surrounding atmosphere by the plateau geogas system month by month is shown in Table 6? 11.165438+10 to February of the following year, the surrounding atmosphere provides heat for the plateau geogas system. At this time, the Qinghai-Tibet Plateau is a cold source, and its intensity is the highest in 65438+February and 65438+1October, and it absorbs more than 600 J/cm2d from the surrounding free atmosphere. The Qinghai-Tibet Plateau is a powerful heat source in spring and summer, and its intensity is the highest in June and July, which provides more than 850J/cm2d heat for the surrounding atmosphere. As far as the annual average is concerned, the geogas system of the Qinghai-Tibet Plateau is a heat source. In winter, the cold area of Qinghai-Tibet Plateau is located in the west of the plateau. In summer, the warm zone is very vast. The temperature of the whole troposphere is higher than that of the surrounding area, and then it extends to the upper warm region. In 100hPa layer, the temperature distribution is warm at high latitudes and cold at low latitudes.

Judging from the surface temperature of the Qinghai-Tibet Plateau, it has the following characteristics:

(1) The roof of the world: The Qinghai-Tibet Plateau is at the latitude of subtropical zone and warm temperate zone due to its high altitude, but the average surface temperature of Qilian Mountain in the northern part of the plateau and the eastern part of Bayan Kara in June is 5438+0-1 6? The closed isotherm of D- 18℃ shows that the average temperature in summer and July is lower than 8℃, and the average temperature in winter and summer is lower than that in the eastern plain of the same latitude 18. D20 degrees Celsius

(2) The diurnal range is larger than that of annual range: the diurnal range of the Qinghai-Tibet Plateau is larger than that of the eastern plain and Sichuan basin at the same latitude, larger than that of the free atmosphere at the same altitude, and larger than that of annual range, but slightly smaller than that of the eastern plain at the same latitude due to its higher altitude.

(3) The temperature changes rapidly in season, and the temperature in spring is higher than that in autumn: the Qinghai-Tibet Plateau has a strong warming intensity in spring, especially after the snow melts and before the rainy season, the high temperature is heated rapidly by strong sunlight, and the temperature in autumn drops rapidly, and the temperature in spring is higher than that in autumn, such as Bango 4 on the plateau. The temperature difference in d 65438+ 10 is 2.8℃, while the temperature difference in Hankou at the same time is-1.4℃.

All these conditions indicate that the plateau temperature has the characteristics of continental climate.

Second, the plateau monsoon

Due to the thermal difference between the Qinghai-Tibet Plateau and the surrounding free atmosphere, the wind system prevailing in winter and summer is called plateau monsoon. Cold high pressure appears on the plateau in winter and hot low pressure appears in winter. Its horizontal range is large in the lower floor and small in the upper floor, and its thickness is larger in summer than in winter. Generally speaking, the seasonal variation of wind starts at the earliest in the northern plateau, and is the latest in the plateau, followed by the eastern plateau and the southern plateau.

Plateau monsoon has great influence on circulation and climate. First of all, it increases the thickness of the lower tropospheric monsoon in winter and summer in China. Southwest China is located in the southeast of Qinghai-Tibet cold high pressure circulation and hot low pressure circulation in winter and summer, so northeast monsoon and southwest monsoon should prevail respectively, which is completely consistent with the direction of low-level monsoon formed by the thermal difference between land and sea, and the superposition of the two makes the southwest monsoon particularly thick.

The greater influence of plateau monsoon is to destroy the planetary pressure belt and planetary circulation in the middle troposphere. Because the cold high pressure in winter and the hot low pressure in summer are quite strong, the thickness of the plateau can reach 5km and 5? D7km, so from sea level to 5? At the height of D7km, the air diverges from the plateau in winter and converges to the plateau in summer. The forced action of plateau topography makes the deep air layer in the plateau fluctuate, forming a strong monsoon meridional circulation. A circulation similar to Hadley's circulation appears in winter. In summer, there is a circulation opposite to Hadley's circulation. Air rises on the plateau, flows to low latitudes at high altitude, sinks, and turns to higher latitudes after reaching the ground, which also plays a great role in regulating air quality in the northern and southern hemispheres.

Third, the impact on precipitation.

I. Impact on the surrounding areas

The Qinghai-Tibet Plateau has a wide influence on the precipitation distribution in Asia. According to the latest climate model, if there is no Qinghai-Tibet Plateau, the southwest monsoon in summer can only reach the southern Indian Ocean, and most parts of China are westerly winds and northwest winds, which are controlled by downdraft. Therefore, the mainland will have a dry climate with little water vapor, and even India and Myanmar will not have such abundant rainfall. The influence of the existence of the Qinghai-Tibet Plateau on large-scale airflow first induces the tropical southwest monsoon to invade India and Myanmar, resulting in the plateau rainy season. At the same time, part of the southwest monsoon went deep and reached the eastern part of China, forming the Jiang Nanyu area. Without the Qinghai-Tibet Plateau, the drought in the west of China would be more serious, and there would be a dry climate in the east. Before the uplift of the Qinghai-Tibet Plateau, it was a vast arid climate zone from the north of China to the Yangtze River basin about tens of millions of years ago.

Second, the precipitation distribution of the plateau itself.

In summer, the southern slope of the Qinghai-Tibet Plateau is just in the windward slope of the southwest monsoon from the Indian Ocean, and the precipitation is extremely abundant. The most famous one, such as Kilapanchi, has an average annual precipitation of over 1 1000mm, with the highest annual precipitation as high as 2646 1.2mm, including 9300mm in July. When the southwest monsoon reaches the plateau, the water content has been greatly reduced, so the summer rainfall in the plateau is not large. For example, Dingri, located at the northern foot of the main peak of the Himalayas, is about 4300m above sea level, and the annual precipitation is only 318.5mm. [After crossing the plateau, the precipitation is even less 100mm.