4.1. Connection between Tree-Ring δ18O and Precipitation δ18O
In China, ASM brings high amounts of water vapor from marine sources. As the water vapor moves inland, δ
18O decreases via Rayleigh fractionation with continuous rain [
24]. The water in trees comes from the soil, and the isotopic signals in trees come from the isotopic signals of rainfall [
14]. The tree-ring δ
18O inherits the δ
18O of atmospheric precipitation and can reflect large-scale water vapor cycle processes [
14]. In our study, we compared tree-ring δ
18O on Mt. Hasi with data from two nearby stations (Xi’an and Shijiazhuang, data from GNIP). Because the records were limited (about 10 years) and there were some defects, we chose a reliable period for comparison, and the results were shown in
Figure 9. Because of the amount effect (the negative relationship between the amount of precipitation and δ
18O
p) [
41], although there were some differences in the δ
18O
p values of both stations, the variation patterns of δ
18O and δ
18O
p during the observation period were similar, which indicated that the tree-ring δ
18O on Mt. Hasi integrated the δ
18O signal of precipitation on a large scale [
25].
4.2. Variation Features of RH Reconstruction in the Mt. Hasi Region and Its Relationship with ASM
Our reconstruction is shown in
Figure 6. The mean RH
JA was 73.10% and the standard deviation (σ) was 4.41%. Anextremely dry year was defined as one with a humidity value lower than the mean − 1σ (68.69%), and an extremely wet year as having a value higher than the mean + 1σ (77.51%). In the past 328 years, there were 48 extremely dry years and 55 extremely wet years. From 1930 to 2012, there were only 5 extremely wet years, but 20 extremely dry years, accounting for 24.10% of the period, indicating the trend of aridification. This trend was also evident in Mt. Xinglong precipitation reconstruction [
18], Mt. Yaoshan and Mt. Shimen RH reconstruction [
24,
25]. All these results suggest that the aridification tendency is widespread in NC.
Although Mt. Hasi is located in northwest China, it is still within the monsoon area. The ASM system contains the East Asian summer monsoon (EASM) and the South Asian summer monsoon (SASM, Indian summer monsoon), which are independent but interplay with each other [
7]. The ASM usually affects the coast of southern China in early March, and then moves northward in two ways: gradual and rapid. It reaches the north of the Yellow River in July, which is the peak period of the ASM. In early September, it begins to retreat from north to south and wholly withdrawn in mid-October. The precipitation from July to August in NC is mostly affected by the strength of the ASM. Considering that the mean precipitation controls RH
JA in this region, RH
JA should have a connection with the ASMs. We calculated the correlations between RH
JA and several representative EASM indices [
42,
43] and SASM indices [
44,
45]. The results showed that the reconstructed sequence was significantly positively correlated with the ASMs (
Table 5).
Next, we focused on SASM as an example for further analysis. Although the seasonal precipitation over South Asia shows complicated patterns, the Indian summer monsoon rainfall time series can represent the index of Indian Summer Monsoon (ISM) intensity on interannual timescales [
46]. Therefore, we selected the summer core-monsoon Indian rainfall (1871–2008 CE) data to represent the ISM intensity variations and compare with the reconstructed RH
JA sequence (
Figure 10).
The two series were significantly positively correlated with
r = 0.21 (
n = 138,
p < 0.01), and the decadal correlation coefficient was 0.33. In particular, for the 1930–2008 CE period (the red border area in
Figure 10), the decadal correlation coefficient was 0.72. A possible reason for this is that the relation between water transport from the ISM and that over East Asia in NC is an antiphase relationship [
47]. Since the 1930s, ASM seems to have been weakening, ISM water vapor transmission is weak and more ISM moisture reaches East Asia, meaning that the correlation between NC monsoon precipitation and ISM will increase. There is a close relationship between the ISM water vapor transport and the intensity of the western Pacific subtropical high in its southwestern part; the weaker (stronger) the ISM water vapor transport, the stronger (weaker) the western Pacific subtropical high in its southwestern part, which leads to more (less) water vapor being transported to East Asia [
47]. We demonstrated that the reconstructed RH
JA series had a significant positive correlation with ASM from multiple angles (
Table 5 and
Figure 10), and it represented the intensity variations of ASM on a large time scale. This relationship could be explained by the strong monsoon bringing more precipitation and thus high humidity, and vice versa [
7].
The extremely dry period in the last three centuries was 1710–1720 CE, and 1713 (reconstructed RH
JA was 62.03%) was the driest year. According to the historical records, Jiuquan, Ganzhou, and Dongle experienced a drought in summer and autumn in 1713. Pingliang, Jingzhou, and Huating were extremely dry, crops were not harvested, and the people starved to death. Jingyuan suffered drought, the government exempted people from taxation and distributed food [
48]. The wettest period was from 1865 to 1870. Two flood events were recorded in1867 (reconstructed RH
JA was 77.35%) and 1868 (reconstructed RH
JA was 83.03%). According to the literature, “In the summer of 1868, heavy rain from July to mid-August, and the Yellow River flooded many houses and farmland” [
48].
Since the mean precipitation from July to August controls RH
JA in the study area, we calculated the spatial correlation between RH
JA and global grid precipitation CRUTS 4.03 in 1958–2012 CE (
Figure 11). The significant spatial correlation covered central Gansu Province, northern Ningxia, northern Shaanxi, and central Inner Mongolia. This result indicated that our reconstruction was able to represent the summer hydrological climate variations over a vast region in NC. Additionally, several paleoclimate reconstructions have been published near our study area, which provided an excellent opportunity to verify our reconstruction sequence and obtain more information about climate changes. The reconstructed RH
JA was in reasonable agreement with the tree-ring-based reconstruction of precipitation from previous-year July to current-year June on the western Loess Plateau (
r = 0.26,
n = 328,
p < 0.001) [
49], and the decadal correlation coefficient was 0.42 (
Figure 12b). We also compared our reconstruction with the nearest dryness/wetness index (DWI, 38°45′ N, 106°15′ E) [
50] from 1685 to 2000 (
Figure 12c), and the two series were significantly negatively correlated with
r = −0.22 (
n = 316,
p < 0.001). Moreover, our reconstruction was significantly positively correlated with the tree-ring δ
18O-based RH
JJA reconstruction in Ordos [
7] with
r = 0.37 (
n = 205,
p < 0.001). After a 10-year low-pass filtering process, these correlations all increased. We noted the drying trends in all sequences since 1930, which also existed in other hydroclimate records in NC [
51] and South China [
52]. All these results indicate that the weakening of the ASM since the 1930s is a widespread climate phenomenon, which can be attributed to anthropogenic aerosol [
53]. The aerosol emissions from Asia alone can affect the intensity of the global summer monsoon, mainly limited to the East Asia because of its proximity to the emission source. Liu [
49] reconstructed the regional precipitation on the Loess Plateau and based on modeling results, put forward the suggestion that the weakening trend of ASM from 1934–2013 was mainly caused by increasing anthropogenic aerosol emissions.
4.3. Exploration of the Juvenile Effect
Studies on the juvenile effect of tree-ring stable isotope series have been mostly based on empirical analyses, and the mechanism is still being explored [
54]. It is known that tree-ring width series can be influenced by the juvenile effect, because the growth of a tree is not only affected by external climate changes, but also controlled by its genetic characteristics. Therefore, the ring-width sequence also contains physiological information about the tree. The ring width is relatively narrow when the tree is young, and then increases rapidly with tree age, until it declines again after a maximum. As for whether the juvenile effect affects δ
18O series, this remains unclear. There was no juvenile effect found in the oak cellulose δ
18O in western France, but there was a pronounced juvenile effect in its δ
13C and width series [
55,
56]. Leavitt [
57] found that the tree-ring δ
13C exhibited a downward trend in the early period of tree growth, but cellulose δ
18O did not. The tree-ring δ
18O values of young juniper trees exhibited a juvenile effect in northern Pakistan [
20]. Labuhn [
58] reported that the δ
18O of young oak trees in southwestern France showed a 2‰ upward trend during the first 30 years. In contrast, the δ
18O values of pines in Spain changed by −0.089‰ every 10 years in the first 100 years [
21]. In NC, most results to date show that cellulose δ
18O values have no noticeable juvenile effect [
7,
51,
59]. We found a downward trend (−0.07‰/year) in the first 20 years (1685–1705 CE); whether this was the effect of climate signals or the existence of the juvenile effect, it was important to differentiate this for accurate climate research.
We hypothesized that the downward trend of cellulose δ
18O was caused by actual climate changes, and that from 1685 to 1705, there must have been a process of RH increase in the study area, because the tree-ring δ
18O was significantly negatively correlated with RH (
Figure 4). This wetting process was captured by the surrounding paleoclimate reconstructions (area I in
Figure 12). For example, the ring width-based reconstruction on the western Loess Plateau exhibited a 2.75 mm/year increase from 1685 to 1705 (
Figure 12b). The nearest grid point of DWI (38°45′ N, 106°15′ E) showed a downward trend of 0.005/year (
Figure 12c). We also prepared a short sample (32B) to verify whether there was a juvenile effect in the tree-ring cellulose δ
18O in this study (
Figure 3). As shown in
Table 1, 32B and the other three cores showed similar variations at the common interval (1914–2012 CE), the correlation coefficients were between 0.65 and 0.69. This was a further evidence indicating that our δ
18O chronology was not affected by the juvenile effect. Additionally, during the young period (1914–1934 CE) of the short core (32B), the RH of the study area followed a pattern of slowly becoming wet first, then rapidly drying, and then quickly becoming wet again. All surrounding reconstructions captured this process (area II in
Figure 12). The same dry and wet changes also appeared in the precipitation reconstructionseries of Mt. Helan [
26] and Ningwu areas [
60]. The above comparisons indicated that the tree-ring δ
18O series in this region was not affected by the juvenile effect.
4.4. Possible Factors Affecting RH Changes in the Mt. Hasi Region
We analyzed the periodicities of the reconstructed sequence by MTM method (
Figure 7), the 6.3, 5.1–5.2, 4.2–4.3, 3.4–3.5, 3, and 2.5–2.6 quasi-cycles belonged to the ENSO cycle (2–7 years) [
61,
62,
63]. Previous studies have described the evolution of precipitation anomalies influenced by the two action centers of ENSO during the outbreak and recession processes. One of the significant negative correlations affects summer and autumn precipitation in NC. In the developing phase of the El Niño event, convection around the Philippines weakened, and the subtropical high was more southwest, leading to more monsoonal precipitation over the Yangtze–Huaihe River Basin in China, Japan, and South Korea, while the precipitation in NC decreased and the climate was dry. During the attenuation phase of the El Niño, the opposite occurred [
64].
ASM and ENSO are inextricably linked [
65], and the SST is an indicator of ENSO. We considered that the observed SST data after 1950 was more reliable [
66], so we calculated the correlation coefficients between the RH
JA sequence and SST, MEI, and SOI (
Table 6). Our reconstruction was significantly negatively correlated with NIÑO1+2 and NIÑO3 SST, while the correlations with the NIÑO3.4 and NIÑO4 regions were not as good as East Pacific (EP) regions obviously. Additionally, the 2.8–6.1 cycle in IMF1 and IMF2 (
Figure 8, the variance contribution to the RH
JA series accounted for 63.29%) also corresponded to the ENSO cycle. All of these results indicate that the ENSO strongly affects the RH
JA variations in the Mt. Hasi area. Previous studies used instrumental data to explain the significant negative correlation between NC precipitation and SST in the EP region [
1,
67], and the key to connect them was a barotropic cyclone over northeast Asia, which comprised two tropical heat sources: one was over South Asia, and another one was over the west of the North Pacific. Anomalous South Asia heating caused a zonal wave pattern over mid-latitude Asia [
1], and anomalous western North Pacific heating caused a meridional teleconnection pattern [
67]. During the developing phase of ENSO, the cyclone over northeast Asian displaced southwestward, while the anomalies on the west side of the cyclone reduced the water vapor supply to NC, triggering precipitation below the normal level [
68].
Although long-term Pacific SST records are not obtainable for the last three centuries, a number of El Niño and La Niña events have been reported [
62]. As shown in
Table 7, extremely dry years were quite matched with El Niño events, and most of the extremely dry years in NC followed El Niño events that occurred in the previous or current year. In particular, some extreme (E) and very strong (VS) events seemed to influence the climate of NC for the preceding 2 years [
68], such as El Niñoin 1718 (E) and 1719 (S) matched with the extremely dry year 1719, and El Niño in 1926 (E) led to the drought in 1926 and 1927. A possible reason for this is that the ENSO events generally last 18–24 months [
62]. The relationships between extremely wet years and La Niña events are shown in
Table 8. All the above results suggest that ENSO has a considerable influence on hydroclimate variations in NC.
Furthermore, the inter-decadal period of 20–30 years found by MTM and IMF4 (27.3 year, the variance contribution to the reconstructed series is 10.92%) was very close to the PDO cycle. There is a clear relationship between PDO and ENSO in both time and space scales, and the PDO can be regarded as the inter-decadal climate variability of ENSO [
69]. PDO is a robust recurring pattern of ocean-atmosphere climate variability with multi-decadal cycles centered over the mid-latitude North Pacific basin [
70]. Many studies have reported that the observed PDO has great impacts on climate changes in NC [
71,
72,
73,
74]. A comparison between our reconstruction and reconstructed PDO series by MacDonald and Case [
75] indicated that there was a close connection over the past three centuries (
Figure 13). The correlation was significantly high (
r = 0.61,
n = 288,
p < 0.001) at the inter-decadal scale by using a 25-year moving average because of the 20–30 cycle of PDO [
76]. Although the comparison was not very good in some years such as 1920 and 1970, this may be caused by regime shifts of PDO. Overall, PDO has a significant impact on our study area. During the warm phase, the middle and northwest Pacific become cool and EP warms, leading to NC precipitation decrease. During the cool phase, the opposite situation occurs [
77,
78]. However, the mechanism of this connection is still unclear, which requires more comprehensive observed data for climate simulation and diagnostic analysis.