The test statistic Z c was calculated as follows: (3) Z c = { s - 1 v a r ( s ) , s > 0 0 , s = 0 s + 1 v a r ( s ) , s < 0 In this equation, | Z c | ≥ 1.96 and | Z c | ≥ 2.58 indicated that the sample sequence had a significant trend change at p < 0.05 and 0.01, respectively, Z c > 0 indicated a rising trend and Z c < 0 indicated a declining trend ( Ling et al., 2014 ).
Geopotential height at 500 hPa in the study area had an extremely significant increasing trend, positive anomalies of potential height corresponded to higher temperature and the regional climate clearly showed a trend of increasing extreme warm indices and reducing cold indices ( Figure 8A ).
Judging from the abrupt-changes test results, except for WSDI and FD which had extremely significant abrupt-changes in 2000, the other temperature indices had extremely significant abrupt-changes in the late 1980s and mid-to-late 1990s ( Table 1 ).
Tree-Ring Chronology in Two Regions The tree-ring chronology showed an increasing trend in the non-degraded but a decreasing trend in the degraded regions ( Figure 5 ).