Influence of Qinling Mountains on Climate

The remarkable climate characteristics of modern continental monsoon in Shaanxi and the drought trend at the northern foot of Qinling Mountains are mainly determined by tectonic movements of different scales. According to the theory of wave mosaic structure, the geological structure in China is mainly characterized by the interweaving of the NE-trending Pacific Rim tectonic active belt and the NW-trending Tethys tectonic active belt, resulting in an oblique grid-like China tectonic network. Any part of the tectonic network has the dual characteristics of Pacific Rim and Tethys structure. Tectonic belts interweave with each other to form a tectonic network, which is called a tectonic node and is a strong part of tectonic activity. Qinling Mountain is located in the center of China tectonic network. Block belts interweave with block belts to form a structural network, called blocks, which is the easing part of tectonic activity; The interweaving of tectonic belt and block belt shows that the tectonic belt has a single dominant tectonic direction, forming a tectonic network line, which is called tectonic segment, and its tectonic activity is between block and tectonic knot. Shaanxi is located in the middle of China tectonic network, and the Pacific Rim tectonic activity zone is divided into three zones, namely Bohai-Sichuan-Yunnan block zone, Liao Yan-Taihang-Longmenshan block zone and Songnen-Shaanxi-Gansu-Ningxia block zone from southeast to northwest. From southwest to northeast, the three belts of Tethys tectonic active belt are the interweaving parts of Qaidam-Sichuan block belt, Qilian-Qinling-Dabie block belt and Junggar-Hehuai block belt, forming the Qinling tectonic knot in central Shaanxi. ?

Qinling Mountain has had strong tectonic activity since Archean, because it is located in the fulcrum zone where the crust of South China and North China swings in balance. Since then, it has been repeatedly impacted by crustal fluctuations around the Pacific Ocean and Tethys, and many structures have been superimposed.

The imprint of the movement, since the Himalayan movement, is mainly a half-horst-half-graben block wave caused by alternating tension and compression. Since the Late Tertiary, the Qinling Mountains were once again controlled by the Tethys tectonic wave system, and the Himalayan orogeny or later tectonic movement further squeezed and lifted the Qinling Belt. In the early Quaternary, the Qinling Mountains did not reach its present height, nor was it the boundary of climate. Even in the warm and humid period when the temperature is slightly higher, the broad-leaved forest with subtropical characteristics may have developed in Guanzhong area. By the late Pleistocene, the Qinling Mountains basically rose to its present height, and its northern climate gradually became dry and cool, which became the dividing line between subtropical and temperate zones in central and eastern China. ?

1, the influence of Qinling tectonic belt on climate?

The influence of Qinling tectonic belt on climate is mainly the dynamic action and mechanical retardation of low-level airflow. The airflow crosses the mountains and climbs along the windward slope, and the water vapor cools and condenses, with more clouds and more precipitation; The leeward slope air current sinks and the precipitation is less. Qinling Mountain effectively blocks the cold air of polar continental air mass prevailing in northern China in winter, and its influence on climate is mainly manifested in the temperature difference. Under the control of the polar continental air mass, the temperature difference between north and south in winter in Qinling Mountains of Shaanxi Province is 3℃ ~ 4℃, especially when strong cold air passes through the border, this blocking effect is more obvious, and the temperature difference between north and south can exceed 65438 00℃. Tropical marine air masses prevail in China in summer. Because the mainland is basically low in heat and the temperature difference between north and south is not big, the barrier function of Qinling Mountain is mainly manifested in the difference of precipitation. ?

At the same time, the tectonic uplift of Qinling not only makes the horizontal distribution of various meteorological elements at the same altitude different from north to south, but also makes the vertical changes different. The extreme minimum temperature at the northern foot of Qinling Mountain is much lower than that at the southern foot of Qinling Mountain, which shows that the continental climate at the northern foot of Qinling Mountain is much stronger than that at the southern foot of Qinling Mountain, making the continental climate at the northern foot of Qinling Mountain stronger and the continental climate at the southern foot of Qinling Mountain weaker. The latitude difference between Xi 'an and Hanzhong is only 1 14', and Hanzhong is still west of Xi 'an, but the continental degree of Xi 'an (the degree to which the climate is influenced by the mainland) is larger than that of Hanzhong 10%[ 13]. On the other hand, from the foot of Qinling Mountain to the top of the mountain, the continental degree has decreased. In this sense, the protruding peaks have the characteristics of maritime climate with small annual temperature difference. This strengthens the northern subtropical zone in western China (concrete performance

Shaanxi, south of the southern slope of Qinling Mountain, is 800 m above sea level, which is basically the same as the hydrothermal index of eastern Chinese mainland near the ocean at the same latitude, and its typicality and stability are stronger than that of eastern Chinese mainland at the same latitude. The southernmost valleys and dams in Shaanxi even have the characteristics of hydrothermal assemblage in subtropical ecological environment. ? According to the principle of weather dynamics, the airflow will generate vibration waves after crossing the mountains, and when it has certain humidity conditions, it can produce precipitation, and the higher the mountains, the stronger the leeward fluctuation. The water vapor transport of precipitation in Shaanxi mainly depends on the southerly airflow at the lower level, and the Qinling Mountains above 3 000 m have a strong dynamic disturbance to this airflow. Above the average height of the mountain range, the peak of precipitation appears in the leeward of the mountain range to the south; Below the average height of the mountain range, the peak precipitation appears in the north of the leeward side of the mountain range. In summer, large-scale heavy rain and heavy rain on the north side of Qinling Mountains are the result of large-scale warm and humid inclined updraft from south to north, dry and cold airflow from north to south and minor leeward disturbance. Under the joint action of two different scale systems, a strong upward movement is generated, which increases the precipitation efficiency in Shaanxi. However, this fluctuation effect corresponds to the distribution of the southern slope of Qinling Mountain and the southern side of Yan 'an as upward movement areas, while the upward movement corresponds to the distribution of Guanzhong as downward movement areas. Guanzhong area is located on the leeward side of Qinling Mountains, with low terrain, which hardly produces airflow fluctuation effect. Therefore, the fluctuation effect produces the least precipitation in the rainy season in July, but the western part of Guanzhong can also produce a strong leeward fluctuation effect because it is surrounded by mountains on three sides. The terrain in the north of Guanzhong area is gradually rising, and the precipitation caused by leeward fluctuation is also gradually increasing. There is a sub-peak area of precipitation near the northern side of Qinling Mountains about one and a half latitudes (south of Yan 'an). ?

Qinling Mountain is a semi-horst-semi-low graben mountain formed by the intense uplift of neotectonic movement, and the warm and humid airflow in the southeast gradually rises, making this area a rainy area in Shaanxi. Due to the structural characteristics of Qinling Mountain, the air flow is in a strong subsidence area after crossing the main ridge of Qinling Mountain, and the precipitation decreases sharply. Therefore, although the total area of the Yangtze River basin south of the Qinling Mountains only accounts for 35.4% of the total area of the province, the effective rainfall area is large, and the annual runoff accounts for 73.4% of the province. The area of the Yellow River basin north of Qinling Mountains accounts for about 62.6% of the total area of the province, but the annual runoff accounts for only 26.6% of the province. The multiple impacts of Qinling Mountains on climate are the fundamental reasons for water shortage in most areas of northern Shaanxi and Guanzhong. At the same time, the Shaanxi-Gansu-Ningxia block is inclined to the southeast, and the Weihe fault basin is inclined to the south, which is divided into many smaller fault blocks, most of which are also inclined to the south (such as Lishan and Qinshui fault blocks). Therefore, the tributaries of the main rivers north of Qinling are generally long in the north and short in the south, and the water system north of Qinling meets the Guanzhong area, which makes up for the water resources in Guanzhong area to some extent. ?

2. Comprehensive climatic effect of X-shaped structural pattern of Qinling tectonic junction?

Geologists generally believe that Qinling-Huaihe River is an important dividing line between China's climate and the warm temperate zone and tropical climate in North Asia. At the same time, because China's precipitation is mainly affected by the warm and humid air flow in the southeast of the Pacific Ocean, experts generally believe that climate indicators such as precipitation and temperature mainly decrease step by step from southeast to northwest, but the local contradiction is obvious. It is found that the above phenomenon is caused by the X-shaped topographic features restricted by the X-shaped structural pattern formed by the interweaving and overlapping of two oblique structural uplift belts in the Qinling structural junction. ?

Dryness is the ratio of evaporation power to precipitation. Usually 1.0, 1.5, 2.0 and 4.0 are used as the boundaries of humid, semi-humid, semi-arid, arid and dry climate. The dryness of the Loess Plateau is greater than 10 except Wutai Mountain, which is a humid climate. 2.0 The isoline generally passes through the area along the Great Wall, and the south of this line has a semi-arid and semi-humid climate; Most areas in Shanxi Plateau, northern Shaanxi Plateau and west of Liupanshan Mountain in Gansu belong to semi-arid climate. Longdong, western Guanzhong, southern Shaanxi and southern Jinzhong are the areas with the best humid conditions on the Loess Plateau, with an annual dryness less than 1.5, which belongs to a semi-humid climate. The dryness of the northern foot of Qinling Mountain is basically greater than 10 except Huashan Mountain, and the dryness of the southern slope of Qinling Mountain is basically less than 10. ?

Previous studies believe that the Qinling Mountains near the east-west direction is the dividing line between humid climate and semi-humid climate, but it cannot explain the following distribution of dryness: Hanzhong, which is relatively far from the sea, has a humid climate, while Ankang, Baihe, Shangzhou and Shanshan, which are located in the east, have dryness less than 10. The dryness in Yang and other areas is greater than 10, which is a semi-humid climate. The dryness of Baoji, which is relatively far away from the sea, is less than 1.5, and that of Longdong and southern Shaanxi, which are far away in the west and north, is less than 1.5, which is a semi-humid climate, while that of Weinan, Tongguan, Sanmenxia and Mianchi is close to or greater than 1.5, which is a semi-arid climate. If we look at the X-type modern tectonic landform of Qinling tectonic junction, this problem will be solved easily. ?

The southeast warm and humid air flow that affects the precipitation in Shaanxi and its surrounding areas is first lifted by the East Qinling-Dabie structural section in the southeast wing of the Qinling structural junction, and forms a precipitation center in Lushan and Luanchuan of Henan Province on its windward slope, which is close to1000 mm; ; Shangluo and other areas in the transition from Shaanxi to Henan in the southeast of Shaanxi are relatively flat, and the western pass is low, which makes the north and south gas? Current is easy to communicate, but it is difficult to form a front. At the same time, as the summer in this area is the compensation area for the anticyclone descending flow in North China of China under the control of subtropical high, a relatively high temperature and little rainfall area is formed in the south of the main ridge of Qinling Mountains, and the annual precipitation of warm and humid airflow after crossing the high Qinling Mountains does not exceed 750 mm, forming a strong airflow settlement area, which leads to obvious foehn effect in Weinan and Tongguan, with high temperature and little rainfall. This airflow, together with the eastward airflow, was blocked by the mountains in the north of Guanzhong. At first, it moved to the west of Guanzhong where neotectonic uplift was weak. In Baoji area, influenced by the uplift of the northwest wing of Qinling tectonic knot, its precipitation is much higher than that in the eastern part of Guanzhong, with an annual precipitation of 7065438 0 mm. Foping, Ningshan, Liuba, Hanzhong and other areas in the central and western Qinling Mountains are backed by the strong uplift area of the high Qinling structural junction center, forming a strong precipitation center in Shaanxi, with an annual precipitation of about 900 mm. Two westerly airflow streams on the east side of the Qinghai-Tibet Plateau are attached to each other, forming a quasi-static front at low altitude. The rain belt enters the south of Shaanxi and Gansu along the foot of Qionglai Mountain-Longmen via Beichuan, Qingchuan and Guangyuan, and the warm and humid airflow climbs to the southwest wing of Qinling tectonic junction, resulting in increased rainfall. Therefore, there is more rainfall in the western Qinling Mountains, such as Lueyang and Liuba. Although it is in the west of southern Shaanxi, it is larger than the East Qinling Mountains. Fengxian County, Shaanxi Province and liangdang county City, Gansu Province are located in the leeward slope of the southwest wing of the Qinling tectonic junction, and at the edge of the rainless zone of the vertical circulation settlement compensation area around the Qinghai-Tibet Plateau, so the precipitation is less than that of the adjacent mountainous areas in the east and south. The Beishan area in the south of northern Shaanxi is another uplift area where the warm and humid airflow in the southeast resonates. However, due to the low mountain area, there is no obvious zonal obstacle, except for a semi-humid area in the south of the Loess Plateau, the dryness tends to increase from southeast to northwest. ?