Abstract:

Based on the daily mean air temperature at 9 meteorological stations in the Yarlung Zangbo River and its two tributaries (BRTT) of Tibet from 1980 to 2020, the relationships between the spatial-temporal distribution characteristics of the negative accumulated temperature (NAT) and its corresponding factors, including air temperature, snow cover and permafrost, were explored with the methods of linear regression, Mann-Kendall test and correlation analysis. The study results indicated that NAT decreased generally from west to east in BRTT, and it also decreased with altitude decreasing. The initial day of NAT was delayed at a rate of 3.11 d/10 a, while the final day was advanced at a rate of 2.53 d/10 a. The duration days decreased at a rate of 6.03 d/10 a, with the significant increase of NAT at a rate of 24.93(℃·d)/10a. The NAT exhibited an increasing trend at the decadal scale, with a low value in 1980s—1990s and a significant high value in 2000s—2010s. The trends of decadal variation features, such as the delayed initial day, the advanced final day, the shortened duration days and the elevated NAT, had become significant since the beginning of the 21st century. The abrupt changes detected by the M-K mutation test indicated that the abrupt changes of the final day of NAT was earlier, occurred in 1993. The abrupt change of both the initial day and the duration days of NAT occurred simultaneously in 1996, while the abrupt change of NAT was detected in 1995. After the mutation, the initial day of NAT was delayed by 9 days, while the final day was advanced by 6 days. The duration was shortened by 17 days, and the NAT was reduced by 68.6℃·d accordingly. The increase of NAT was in accordance with the increasing mean temperature, decreasing snow cover days in winter and permafrost degradation. The NAT variation had both positive and negative effects on agricultural productions in this highland region.

Key words: negative accumulated temperature (NAT), variation trend, abrupt change of climate, snow cover, permafrost, correlation, agricultural regions of the Yarlung Zangbo River and its two tributaries in Tibet