Spatial and Temporal Variations of Climate on Different Flanks and Elevations in the Qinling-Daba Mountains of China from 1969 to 2018

Climate change presents great variations on different anks and elevations in the same mountain range. Analysis of the spatial and temporal variations of climate on different anks and elevations in the north-south transition zone--the Qinling-Daba Mountains (QDM) are of great signicance for exploring the complexity of climate geography pattern and phenology in China. This paper collected 118 weather stations data on daily temperature and precipitation during 1969-2018 from the National Meteorological Information Center, and then explored the temporal and spatial distribution characteristics of climate on different anks and elevations. The results showed that: 1) On the whole, there was a signicant warming trend in the four seasons in the past 50 years. Precipitation did not show signicant variation at most stations with an exception to 8 weather stations having signicant changes in trend in different seasons. 2) The temperature showed an increasing trend in different seasons, but its increasing rate varied greatly on different anks. Generally speaking, climate change was greater on the northern anks than that on the southern anks in the four seasons in the QDM. Temperature tendency is greater in spring and winter than in summer and autumn on different anks in the QDM. 3) The increase rate of temperature varied greatly at different elevations in different seasons. The warming rate was greater in the higher altitude regions than that of the lower altitude regions in summer, autumn and winter, while, in spring, however, it is higher in the lower altitudes than in the higher altitudes.


Introduction
The Qinling-Daba Mountains (QDM), located at the transition zone between the north and the south, and between the Tibetan Plateau and the plain in China, is considered as the natural boundary between the warm temperate zone and north subtropical zone, and an ecological corridor connecting the Tibetan Plateau and the eastern plains in China 1, 2 .
The climate of the QDM is accordingly characterized by complication and diversity on different anks and terrains [3][4][5] . The southern ank of the Qinling Mountains, serving as the windward slope of moisture-carrying air from the southeast, sunny slope, and relatively at slope affected by fold belts, is wet and warm, while the northern ank, as leeward, shady, and steep slope formed by fault belts, is relatively dry and cold 1,6 . Accordingly, climate change and its in uence varied greatly from different anks and altitudes. It has been recognized that the humidi cation rate, climate warming is greater on the northern ank than on the southern ank in the Qinling Mountains 7, 8 , but vegetation NPP improvement is higher on the southern than on the northern ank in the Qinling Mountains below 2000m 4,7 . However, most of the previous studies mainly focus on climate change of partial regions, such as the Qinling Mountains 3, 7, 9 , Huaihe basin 10 , the Daba Mountains 11 in the QDM. Although the QDM is an integrated geographic region connecting the warm temperate zone to the north subtropical zone, little attention is paid to climate change in the entire region 1 . This paper aims at revealing climate change and its complexity of different anks and altitudes in the QDM, which contribute to the exploration of the cause of north-south regional differentiation in China. In this paper, we rst collected and compiled temperature data and precipitation data at 1969-2018 from weather stations, and then carried out the Manne-Kendall method to analyze the heterogeneity and similarity with climate change on different anks and altitudes in the QDM from 1969 to 2018.

Study area
The QDM are located in central China, range from 30° N to 36° N and from 101° E to 115° E, and span six provinces, namely, Gansu, Sichuan, Shaanxi, Chongqing, Henan, and Hubei (Fig. 1). The Qinling-Daba Mountains consisted primarily of the Qinling Mountains and the Daba Mountains separated by the Han River Valley and form a natural boundary of between the warm temperate zone and the subtropical zone. Generally speaking, the Qinling Mountains is roughly coincident with the 0°C isotherms in January, the 800mm isohyet and the 2000-hour sunshine duration contour in China 12 . Variation on these climate factors causes vegetation to change gradually from a deciduous broad-leaved forest zone to an evergreen broad-leaved forest zone across the QDM from north to south 13,14 . West Qinling, however, is dominated by cold temperate grassland because of its higher altitude and drier climate in some valleys 15 .
A dataset of 118 weather stations are distributed over different anks in the QDM. Among them, 31 and 20 stations are located on the southern and northern anks of the Qinling Mountains. In addition, 13 and 20 stations are found on the southern and northern anks of the Daba Mountains, respectively. Thirty-four stations are present on western ank of the Qinling-Daba Mountains (Fig. 1).

Weather stations
A dataset of 118 weather stations for daily temperature and precipitation data from 1969 to 2018 in the QDM was collected from the National Meteorological Information Center (https://data.cma.cn/), and then the temperature and precipitation data were averaged by year, season and month. As shown in Fig 1 weather stations are mainly distributed over 6 provinces including Gansu, Shaanxi, Henan, Sichuan, Hubei and Chongqing.

DEM data
In this paper, the digital elevation model (DEM) data was rst downloaded from the USGS website (https://earthexplorer.usgs.gov/) by the location of different blocks of the QDM, and then spliced into a complete regional elevation data covering the whole QDM with a spatial resolution of 1000m.In order to analyze the association between climate change and altitude, DEM data were classi ed into four levels: 0 1000m, 1000 2000m, 2000 3000m and 3000 4000m. The variation characteristics of weather stations (1969 2018) were determined in different classi cation segments and the relationship between climate change and elevation was explored in the QDM.

Exposure data
To decipher the association between climate change and different anks in the QDM, we divided QDM into ve parts. The southern and northern anks of the Qinling Mountains, and the southern and northern anks of the Daba Mountains, and the west Qinling-Daba Mountains. We classi ed and divided the QDM parts on the basis of positions of the main ridge lines and major rivers (Jialing River and Han River) in (Fig.1). Temperature and precipitation change rates were calculated by statistic analysis in the ve regions to explore the similarity and dissimilarity of climate change on different anks in the north-south transitional zone of China.

Method
The widely-used Manne-Kendall method is an effective test to detect the long-term change in time series 16 . The detailed treated method is described in Xu et al. 17 . In this study, the Manne-Kendall method was applied to detect the long-term trend of temperature and precipitation changes on different anks and elevations based on these 118 meteorological data in the QDM.

Results
Temperature changes on different anks in the QDM from 1969 to 2018 The Mann-Kendall test revealed 91.53% (108 stations) of stations presenting a signi cant increasing trend in spring temperature (Fig. 2), while 51.69% (61 stations), 77.12% (91 stations) and 86.44% (102 stations) of weather stations showed substantial trend in summer, autumn and winter temperatures, respectively. The greater proportion of stations showed signi cant temperature trends on the west Qinling-Daba Mountains than on the other anks in different seasons. In this case, 97.06%, 88.24%, 94.12% and 97.06% of stations were found to have a notable warming trend on the West Qinling Mountains in spring, summer, autumn, and winter, respectively, while stations are less than 89.29%, 35.71%, 69.05%, 80.95% on the other anks. The northern anks have had more proportion of weather stations showing signi cant warming trend in different seasons as compared to the southern ank in the Qinling Mountains. For instance, 95%, 40%, 75% and 95% of stations signi cant warming trend in the former in spring, summer, autumn, and winter, respectively, while 93.55%, 35.48%, 64.52% and 87.10% of stations showing signi cant warming trend in the latter in the four seasons, respectively. The southern ank in Daba Mountains, with 7.69% stations depicted a signi cant negative trend in summer, autumn and winter.
The trend analysis for the mean temperature of different seasons revealed that there was a signi cant warming trend on different anks in the QDM from 1969 to 2018 (Fig.3, Table1). However, the increasing rate varied greatly from different seasons and anks. The temperature increasing rate was greater on the northern ank of the Qinling Mountains and the Daba Mountains as compared to the southern anks in the four seasons (Fig. 3). Among them, the temperature tendency is greater in spring and winter than in summer and autumn on different anks in the QDM. Climate warming was least in summer with temperature increasing rate as low as 0.123°C/10a, 0.074°C/10a,  The signi cance statistics of trends in mean temperature showed that stations detected signi cant increasing trend varies greatly varied on different seasons and altitudes in the QDM (Fig.4) (1000 2000m, 2000 3000m). In different seasons, it was discovered that high altitude areas have a higher proportion of warming area than low altitude areas.
The temperature trend analysis for different seasons revealed that there was a signi cant warming trend at different elevations in the QDM from 1969 to 2018. (Fig. 5, Table 2). However, the rate of increase in mean temperature varied greatly depending on the season and elevation. In higher elevation, we detected an increasing rate in the mean temperature of summer, autumn, and winter (Fig. 5, Table 2). At elevations of 3000 4000m, the increasing rate could reach 0.440°C/10a, 0.39°C/10a, and 0.456°C/10a, respectively, while at 0 1000m, it was 0.205°C/10a, 0.218°C/10a, and 0.303°C/10a, respectively. The increasing rate in spring is higher in the lower altitude areas (at 0 1000m and 1000 2000m) than in the higher altitude areas (at 2000 3000m and 3000 4000m).  As shown in Fig.7 and Table 3, there were two weather stations, Zigui and Shiquan, that showed signi cant increasing trends in precipitation in summer, and 1 weather station (Diebu) that showed signi cant decrease trends, with change rates as high as 47.36mm/10a, 22.91mm/10a, and 19.43mm/10a, respectively, which were higher than other stations in other seasons. In contrast, four weather stations (Zigui, Beichuan, Taibai, and Hezuo) indicated increasing and decreasing changes in precipitation in winter. It is important to note that Mount Hua weather station recorded a signi cant decrease in precipitation in spring and autumn, with rates of 14.60mm/10a and 11.50mm/10a, respectively. Only in spring, the Songpan weather station depicted an increasing trend, i.e., 7.63mm/10a in precipitation.

Discussion
Our analysis con rmed that the temperature had an increasing trend in the QDM in the last 50 years, but the climate increasing rate presented great variations on different anks and elevations. On the whole, climate changing rate was greater on the northern anks than the southern anks in the four seasons in the QDM. Current ndings highlighted the variation in climate warming in different seasons. Increasing temperature rate was recorded in higher altitudes in summer, autumn, and winter. On the contrary, it was higher in the lower altitudes than higher altitudes. In addition, the temperature tendency is greater in spring and winter than in summer and autumn on different anks in the QDM. Present ndings indicated the climate warming was more pronounced in spring and winter.
These results coincide with previous results by Gao Xiang

Conclusion
We used climate statistical analysis methods to analyze the spatiotemporal trends of temperature and precipitation during 1969-2018on different anks and elevations in the QDM. The following conclusions were obtained: (1) On the whole, there was an obvious warming trend in different seasons in the past 50 years in the QDM, but seasonal temperature increasing rate varied greatly on different anks and elevations. The precipitation did not change signi cantly in most areas from 1969-2018, except for 8 weather stations showing signi cant precipitation changes to a certain degree in different seasons in the QDM.
(2) Seasonal climate warming presented great variations on different anks. Generally speaking, climate change was greater on the northern anks than that on the southern anks in the four seasons in the QDM. Among them, the temperature tendency is greater in spring and winter than in summer and autumn on different anks in the QDM.
(3) There was an increasing trend of seasonal temperatures at different elevations in different seasons in the QDM. As the elevation increases, the increasing rate of mean temperature showed a tendency of rising in summer, autumn and winter, while, in spring, however, it is higher in the lower altitudes than in the higher altitudes.
Declarations Figure 1 The study area and the spatial distribution of weather station in the QDM. Pink dots represent weather stations on the southern ank of the Qinling Mountains (31)  Signi cance analysis of mean temperature based on Mann-Kendall test at the 5% signi cance level at different weather stations in the QDM during 1969 2018