Climate variability in the northern and southern Altai Mountains during the past 50 years

The Holocene drying trend in the northern Altai Mountains and the wetting trend in the southern Altai Mountains inferred from the paleoclimatic studies indicated it is needed to understand the modern climatic characters in this region. However, a detailed analysis of modern climate variations in the northern and southern Altai Mountains is lacking. Here, we investigate the monthly temperature and monthly precipitation data from seventeen meteorological stations during 1966–2015 in the northern and southern Altai. The result shows that temperature increases significantly in the northern (0.42 °C/10 yr) and in the southern (0.54 °C/10 yr). The precipitation decreases insignificantly (−1.41 mm/10 yr) in the northern, whereas it increases significantly (8.89 mm/10 yr) in the southern. The out-of-phase relationship of precipitation changes is also recorded at different time-scales (i.e., season, year, multi-decades, centennial and millennial scales), indicating the Altai Mountains are an important climatic boundary. Based on the analysis of modern atmosphere circulation, the decreased precipitation in the northern corresponds to the decreasing contribution of ‘Northern meridional and Stationary anticyclone’ and ‘Northern meridional and East zonal’ circulation and the increased precipitation in the southern are associated with the increasing contribution of ‘West zonal and Southern meridional’ circulation.

In terms of the seasonal consistencies or inconsistencies of the changes in temperature, increasing trends are statistically detectable for all seasons among nine stations of the northern Altai Mountains (Fig. 2, Table 2). Spring (Fig. 2b) is the season when the temperature increases most dramatically with a rate of 0.68 °C/10 yr (P < 0.01), and the second increased season is winter (0.42 °C/10 yr, P < 0.01). The fastest increasing temperature of spring is observed in Kyzyl-Ozek (0.72 °C/10 yr, P < 0.01) and the lowest is in Mugur-Aksy (0.43 °C/10 yr, P < 0.01). The increasing rate of winter temperature at Ust-Coksa is much lower (0.69 °C/10 yr, P < 0.01) and no significant increase of temperature is observed in winter in Kara-Tyurek (0.18 °C/10 yr). The temperature in summer and autumn is also increased significantly with average rates of 0.26 °C/10 yr and 0.31 °C/10 yr, respectively (Fig. 2c,d).
In addition, the obvious altitudinal differences of seasonal temperature are showed among nine stations in the northern Altai Mountains though no obvious differences in annual changes. Specifically, the increased rates of spring and winter in high-altitude stations (i.e., Mugur-Aksy, Kara-Tyurek and Kosh-Agach) are lower than that in low-altitude stations (i.e., Rubcovsk, Zmeinogorsk, Soloneshnoe, Kyzyl-Ozek, Yailu and Ust-Coksa), whereas the increased rates of summer and autumn of high-altitude stations are higher than that of low-altitude stations.
Temperature variations in the southern Altai Mountains. Consistent with the increasing temperature in the northern Altai Mountains, the mean annual temperature also increases significantly (P < 0.01) with a rate of 0.54 °C/10 yr in the southern Altai Mountains over the past 50 years (Fig. 3, Table 3). Fuyun among the stations has the highest temperature tendency (i.e., increasing rate, 0.81 °C/10 yr), and Aletai has the lowest climate tendency (i.e., increasing rate, 0.33 °C/10 yr). The seasonal pattern in the southern Altai Mountains increases most dramatically during spring with a rate of 0.79 °C/10 yr (P < 0.01) and then winter with a rate of 0.61 °C/10 yr (P < 0.01), being consistent with seasonal changes in the northern Altai Mountains but being larger than the latter. This feature suggests that temperature increases more pronouncedly in the cold season. In addition, the temperature in summer and autumn increases significantly with average rates of 0.34 °C/10 yr and 0.42 °C/10 yr, respectively (Fig. 3c,d). It should be noted that we don't consider the attitudinal gratitude of temperature in the southern Altai Mountains because of all stations located in the low-altitude region (below 1500 m a.s.l.).
In terms of the seasonal consistencies or inconsistencies of the changes in precipitation, various trends are also statistically detectable for all seasons during the past 50 years ( Fig. 4b-e). Specifically, precipitation in spring has no obvious changes and that in summer experiences a slightly and insignificantly increased rate of 2.45 mm/10 yr. Interestingly, precipitation in autumn among almost all stations shows a decreasing trend with a maximum decreased rate (−11.66 mm/10 yr, P < 0.01) in Kyzyl-Ozek although precipitation insignificantly decreases in other stations.

Precipitation variations in the southern Altai Mountains.
Compared with the varied trends of precipitation in the northern Altai Mountains, the precipitation significantly (P < 0.01) has kept a consistent rising trend with a rate 8.89 mm/10 yr in the southern Altai Mountains (Fig. 5a, Table 5), being similar with the average rate (about 8.40 mm/10 yr) in Central Asia for the same period 3 . The maximum increased rate of precipitation in Habahe is 14.71 mm/10 yr, the minimum rate in Tacheng is only 2.27 mm/10 yr.
For precipitation in all seasons, increasing trends are statistically detectable for all stations during the past ~50 years (Fig. 5b-e). However, the Manne-Kendall trend test showed that the increase precipitation in winter is significant, while the increase in spring, summer and autumn is insignificant. The Manne-Kendall trend test also showed that the precipitation in spring significantly increases in Habahe, while that in summer significantly increases in Fuyun and Qinghe. It is worth noting that winter is also the season when the precipitation increased most dramatically among four seasons. The fastest increasing winter precipitation is observed in Habahe (8.38 mm/10 yr) and the lowest in Hoboksar (1.91 mm/10 yr). In general, an increase of precipitation is the main character of precipitation in the southern Altai Mountains.

Comparisons and Discussions
Based on the above-mentioned analysis, we can find that the temperature experiences an obviously increased trend by a rate of 0.42 °C/10 yr in the northern Altai Mountains and by a rate of 0.54 °C/10 yr in the southern Altai Mountains during 1966-2015. The increased temperature rate in the southern is larger than that in the northern, which might be attributed to higher annual precipitation amounts in the northern. The markedly increased temperature rates in the northern and southern Altai are both larger than the average of northwest China (0.34 °C/10 yr) 4-6 , the whole China (0.25 °C/10 yr) 7 and the entire globe (0.175 °C/10 yr) 8 . In terms of the seasonal changes in temperature of the northern and southern Altai, the temperature increases most dramatically in the cold season (spring and winter). Under the warming condition, the mean annual precipitation decreases insignificantly by −1.41 mm/10 yr in the northern Altai during 1966-2015, consistent with the results in the eastern Altai Mountains 9 . Though no consistently changeable trends in spring, summer and winter, the changes of precipitation in autumn consistently reduce. Conversely, the mean annual precipitation increased significantly by a rate of 8.89 mm/10 yr in the southern Altai Mountains, being similar with the changeable trend of precipitation in northwest China (including north Xinjiang) 3-5,10-12 and Central Asia 13,14 . In terms of seasonal changes, winter is the season when the precipitation increased most dramatically. Overall, the decrease of precipitation in the northern Altai during 1966-2015 is attributed to the decrease of autumn precipitation, while the increase of precipitation in the southern Altai could result from the increase of winter precipitation.
According to the changeable trends of temperature and precipitation during the past 50 years (1966-2015), the climate experiences a drying trend in the northern Altai, whereas the climate exhibits a wetting trend in the southern Altai. We therefore propose that the climate changes are out-of-phase between the northern Altai and the southern Altai. This proposal is supported by tree-ring-and ice core-recorded (Fig. 1)  in the past two hundred years [21][22][23][24] . Specifically, Fig. 5a presents two tree-ring oxygen isotope-indicated summer precipitation amount curves of the past two hundred years in the northern 21 and southern Altai Mountains 22 . The result reveals that changes of summer precipitation amounts in the northern (red curve in Fig. 6a) and in the southern (dark curve in Fig. 6a) are totally opposite. The opposite trend is also recorded in annual precipitation variations in the northern (red curve in Fig. 6b) and in the southern (dark curve in Fig. 6b) during the past two hundred years 23,24 . It should be noted that summer and annual precipitation both have a decreasing trend in the northern Altai and experience an increasing trend in the southern Altai, providing a strong support to the aforementioned proposition that the climate change is out-of-phase between the northern Altai and the southern Altai.

precipitation variations
Furthermore, the out-of-phase relationship of precipitation in the northern and southern Altai Mountains also exists in the last millennium 25,26 and the Holocene epoch [15][16][17][18][19] . In detail, the climate is characterized by a    During the Holocene epoch, the climate is featured by a wet condition in the early Holocene warm period (between ~10,000 and ~5000 cal. yr BP) and that by a dry condition in the late Holocene cold period (between ~5000 and 0 cal. yr BP) in the northern Altai Mountains and the totally converse Holocene climatic condition has been showed in the southern [15][16][17][18][19] . Therefore, we can conclude that the out-of-phase relationship of precipitation change at different time-scales (i.e., season, year, multi-decades, centennial and millennial scales) indicates that the Altai Mountains are an important climatic boundary. The vegetation evolution 2 and oxygen isotopes of precipitation 27 strongly support this result. Specifically, the northern Altai Mountains are dominated by the densely covered forests and the relatively depleted oxygen isotope of precipitation (averaged −12‰), whereas the southern are dominated by the Asian cold steppe and the relatively enriched oxygen isotope of precipitation (averaged −7‰).
Under the large-scale wetting condition during 1966-2015 in Central Asia (including the Altai Mountains) 1-14,28-33 , the reason of diverging (i.e., opposite) trend of precipitation in the northern and southern Altai Mountains over the past 50 years should be taken into consideration. Our attention firstly turns to the seasonal characters of precipitation in the northern and southern Altai (Fig. 7). The unimodal distribution of precipitation in the northern Altai mostly concentrates in summer and autumn with 55-84%. The largest percentage in Kosh-Agach is about 83.89%. The bimodal distribution of precipitation in the southern Altai Mountains is featured by two maximum in April-September (50-68%) and in November-December (13-21%). The largest percentage of April-September precipitation is about 68.05% in Fuhai, and the largest percentage of November-December is about 21.22% in Tacheng 34 .
Our attention in turn changes to the resource of water vapor in the northern and southern Altai Mountains. Three supply channels of water vapor shape regional climate of the Altai Mountains and they are (1) the eastward water vapor flow transferred by western and northwestern intrusions from the Atlantic Ocean; (2) the

Conclusions
The temperature and precipitation variations are investigated from seventeen meteorological stations during 1966-2015 in the northern and southern Altai Mountains. The results show that the temperature experiences a consistently increasing rate in the northern and southern Altai Mountains. The precipitation in the northern Altai insignificantly decreases, whereas that in the southern Altai significantly increases. The out-of-phase relationship of precipitation variations is also recorded at different time-scales (i.e., season, year, multi-decades, centennial and millennial scales), indicating that the Altai Mountains are an important climatic boundary. Our works are not only conducive to understand the past climatic differences in the Altai Mountains and also imply that the protection of ecology and water resources should take the climatic differences in the whole Altai Mountains into account. Additionally, the influence mechanism of differences between north and south in-depth from the scale of atmospheric circulation are needed to further analyse in the different scales (i.e., season, year, multi-decades, centennial and millennial scales).

Data Source and Methods
In this study, the monthly temperature and precipitation from seventeen meteorological stations are utilized to investigate their variations during 1966-2015 in the northern and southern Altai Mountains. The selected locations of seventeen meteorological stations are shown in Fig. 1 and detailed information are shown in Table 1. The widely-used Manne-Kendall method is an effective test to detect the long-term change in time series 42 . The detailed treated method is seeing in Xu et al. 12 . In this study, it was applied to detect the long-term trend change of temperature and precipitation of these seventeen meteorological data.