The key to predicting short-term climate change by "Ground-synoptic" map is to determine the location and movement of geothermal vortex. Therefore, some characteristics of geothermal vortex are mainly analyzed in this section. Based on the 3.2 m monthly anomaly data from January 1980 to August 2017, 452 monthly anomaly maps were drawn. Contours are drawn on each 3.2 m monthly anomaly map of soil temperature, and the high temperature center with soil temperature anomaly T′ 3.2 m ≥ 0.5 °C could be analyzed. At least part of the heat generated by this high temperature zone comes from deep underground. When the disturbance of monthly soil temperature anomaly reaches 3.2 m, the amplitude of soil temperature will decrease to 0.05-0.15°C. The high temperature area often accompanied by earthquakes is defined as "Geothermal Area". We stipulate that on the T-map (Fig. 6), there are patches of T′ 3.2 m > 0°C areas, in which at least one station T′ 3.2 m ≥ 0.5 °C is surrounded by the low temperature zone T′3.2 m < 0°C, and the area T′3.2 m > 0°C is called "Geothermal Vortex". For example, the northwest region in Fig. 6.
As for the horizontal scale L of " Geothermal Vortex ", it is stipulated in this paper that since the shape of each "geothermal vortex" is approximately elliptic, as shown in Fig. 6 in Northwest China, the lengths of long axis (H) and short axis (S) can be calculated according to the longitude and latitude coordinates of the edge, then L= (H + S)/2 is the horizontal scale of " Geothermal Vortex ". After these specific definitions of "Geothermal Vortex" are given, its spatial characteristics can be statistically analyzed.
Table 1
Maintenance months of "Geothermal Vortex" (Unit: month).
Duration | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
Number | 19 | 20 | 18 | 27 | 23 | 26 | 24 | 16 | 10 | 8 | 6 | 5 | 3 | 1 | 2 | 4 | 1 | 2 |
Duration | 19 | 20 | 21 | 22 | 23 | 24 | 27 | 28 | 30 | 34 | 38 | 39 | 54 | 65 | 73 | 74 | 150 | 161 |
Number | 2 | 1 | 2 | 2 | 1 | 1 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
Table 1 shows the statistics of the maintenance time of "Geothermal Vortex". It can be seen that the most cases of "Geothermal Vortex" maintenance are 4–7 months, followed by 1–3 months. The number of cases of "Geothermal Vortex" maintenance for more than 1.5 years is very small, and the longest is more than 13.4 years. The average duration of each geothermal vortex is 9.7 months. The average number of "Geothermal Vortex" on each map is 5.2. There were obvious seasonal variations in the beginning and disappearing of 238 geothermal vortex process. Among them, 91 geothermal vortexes began to appear in winter, accounting for 38% of the total number of geothermal vortexes. 70 geothermal vortexes appeared in summer (29%), 41 in spring (17%), and 36 in autumn (15%). In addition, 79 geothermal vortexes disappeared in spring (33.2%), 59 in summer (24.8%), and 47 in autumn (19.7%).
Table 2
Horizontal scales of "Geothermal Vortex" (Unit: 100km).
Horizontal scale | < 2 | 2∼3 | 3∼4 | 4∼5 | 5∼6 | 6∼7 | 7∼8 | 8∼9 | 9∼10 | 10∼18.9 | Sum |
Number | 6 | 19 | 28 | 61 | 33 | 25 | 22 | 11 | 7 | 26 | 238 |
The horizontal scale of a "Geothermal Vortex" varies at different stages of its development. The "Geothermal Vortex" accompanying the earthquake is small in horizontal scale when it is formed at the beginning, and the largest when the earthquake occurs, and then decreases (Tang and Gao 1995). In this paper, a graph with the largest range of each "Geothermal Vortex" was taken to make statistics on the horizontal scale. Table 2 shows that the largest number of horizontal scale is 400-500km, followed by 500-600km. The number of "Geothermal Vortex" with the horizontal scale of 300-400km, 600-800km and 1000-1888km was the same, and the smallest number was less than 200km.The largest horizontal scale was 1888km, which appeared in Xinjiang, Qinghai and Tibet in August 2000.
From Table 2, the average horizontal scale of 238 geothermal vortexes is 604 km, which is close to the depth of the upper mantle (670 km). If the thickness of the lithosphere (70–80 km) is subtracted, it can be concluded that the thickness of the upper mantle is basically equal to the average horizontal scale of the geothermal vortex. The characteristic velocity of the "Geothermal Vortex" (V = L/t ≈ 746 km/a ≈ 2.4 cm/s) can be obtained from its average duration (t = 0.81a) and average horizontal scale (L = 604 km).
Table 3
Average horizontal scale of "Geothermal Vortex" in different regions (Unit: km).
Region | Qinghai-Tibet Plateau and Xinjiang Province | Southwest China | South China | East China | Northeast China | North China | Junction of Gansu and Shaanxi Province |
Horizontal scale | 1087.3 | 599.4 | 529.4 | 522.8 | 466.8 | 459.8 | 351.3 |
Number | 44 | 33 | 25 | 10 | 25 | 97 | 4 |
Table 3 shows the statistical characteristics of geothermal vortexes in different regions. It can be seen that the characteristics of geothermal vortexes in different regions are different during the development of geothermal vortexes. The scale of geothermal vortexes in Northwest China is the largest, followed by Southern China, with a horizontal scale of about 560 km, Eastern China of about 523 km and Northern China of about 460 km. Since there is a basically fixed "Geothermal Vortex" center at the junction of southeastern Gansu and Shaanxi province, this region is considered as a separate region. In addition, the largest number of cases occurred in North China, accounting for 41% of the total geothermal vortexes, followed by the Qinghai-Tibet Plateau and Xinjiang, accounting for 18.5%, and the remaining region accounted for 40.5%.
Figure 7 records the coordinates of the central points of 2353 geothermal vortexes on 452 maps. Different colors indicate the number of times that the central point of the "Geothermal Vortex" appears at this position. It can be seen that the distribution of the "Geothermal Vortex" is not uniform. It is concentrated in some areas, namely, northern Xinjiang, central Tibet, Eastern Gansu and Qinghai, northeastern China, central North China, southeastern Gansu, the middle reaches of the Yangtze River, Yunnan-Guizhou Plateau, and coastal areas of Fujian-Guangdong. Most of these areas are areas with strong crustal activity or high values of soil heat flow (Tang and Gao 1995). At the same time, it can be clearly seen that the center of the "Geothermal Vortex" moves horizontally from east to west or from north to south. However, most of the centers move eastward and tend to move toward higher latitudes, which is similar to the movement of atmospheric cyclones. As time goes by, one "GV" may split into two, or two "GV" may merge into one. The evolution of "GV" is complicated, and all "GV" moving on the same track can be called a "GV" family, with a life history of roughly 10 years. The vortex moves about 200-500km a year, almost as fast as a single one. Meanwhile, the path of geothermal vortex movement is similar to that of continental cyclone eastward moving out of the sea, mostly from the Yangtze Estuary to the East China Sea, from North China to the Yellow Sea or northeast to the east. Low-value systems in the atmosphere tend to form over geothermal vortexes (Tang and Gao 1995).