Inter- and intra-annual wind speed change
Using the MASH method, the daily wind speed time series were filtered for all three stations: Tsetang, Lhasa, and Nyêmo. The left column in Fig. 4 is a visual representation of MASH for wind speed with w = 10 days and Y = 20 years. The original data covered the periods from 1960–2015, 1960–2015, and 1974–2015 at each station, so the results contained Nh = 37, 37 and 23 lines, respectively. Blue lines represent the earlier wind speed horizons, and red lines are more recent. At the Tsetang station (Fig. 4a), there was a clear and uneven decreasing trend distributed across the entire time series, which can be roughly divided into three stages: a slight increase from horizon from 1960 to 1979 and 1975 to 1994, then a marked decrease from horizon from 1976 to 1995 and 1987 to 2006, and a weak increase during the recent horizons, as identified in the figure. Although decreasing wind speeds were observed in all months, this image clearly shows that the largest changes in wind speed occurred during the gale period from March to April. Thus, the difference between wind speed maximums and minimums also gradually decreased. A similar wind speed change trend was observed at the Nyêmo station from 1974 to 2015 despite the minor differences observed from July to August (Fig. 4c). For the Lhasa station (Fig. 4b), a non-significant wind speed change from June to August was detected from 1960 to 2015, a departure from the other two stations. The plotted MASH results, however, provided a clear and descriptive illustration of how daily wind speed changed over time.
The right column in Fig. 4 is another way to display the MASH results, emphasizing the duration of change for different months rather than the changes in wind speed magnitude. The colour scale represents the smoothed wind speed. Wind reductions were clearly visible in almost all months at the Tsetang, Lhasa, and Nyêmo stations, especially in the spring months (the red parts in Fig. 4b,d,f), starting from the 1st to the 20th (1960–2003), the 1st to the 25th (1960–2008), and the 1st to the 10th (1974–1997) moving horizons at the three respective stations.
To better understand the changes across different periods in the time series, the MMK method was used to estimate the smoothed data trends. Figure 5 shows daily wind speed changes at Tsetang (blue line), Lhasa (red line), and Nyêmo (green line) from 1960 to 2015, 1960 to 2015, and 1974 to 2015, respectively. Most months exhibited significant decreasing trends (negative values in the graph). The results for the Tsetang and Nyêmo stations were similar, with a statistically significant decrease for almost every day throughout the year. Only in December at Tsetang and in parts of July, September, November, and December at Nyêmo were the daily wind speed trends non-significant. In contrast, at the Lhasa station, non-significant trends were detected from late May until August for most of September and early October. The largest daily decadal changes at the Tsetang and Nyêmo stations occurred in April (− 0.7 m/s) and March (− 0.69 m/s), respectively, while at the Lhasa station, they occurred in February (− 0.25 m/s).
The original data trends were also conducted in the same way to assess the influence of MASH filtering on trend estimates. The results are presented in Fig. 6. This figure shows that the trends are not statistically significant on most of the days at the three stations, which is probably because there is no filtering of interannual variability, as already discussed when describing Fig. 3b. According to the 21-day smoothing averaged curve, the largest daily decadal changes at three stations occurred in March at both the Tsetang and Nyêmo stations and in February at the Lhasa station, which almost coincided with the MASH results. However, the trends seemed to be much gentler than the MASH filtered data.
To estimate the accuracy of the MASH method results, the widely used MK test was applied. Simultaneously, the least squares linear regression (LSLR) was chosen as evidence for the outcomes of the MK test. Both tests were performed to analyse mean wind speed trends at annual, seasonal, and monthly temporal scales (Table 2). Mean annual and monthly wind speeds were obtained by averaging daily data for the corresponding years and months, respectively.
The MK test results showed statistically significant decreasing trends in mean annual wind speeds at the Tsetang and Lhasa stations from 1960 to 2015, with decadal rates of − 0.286 and − 0.078 m/s, respectively. On the other hand, the results at Nyêmo showed larger decreasing trends from 1974 to 2015 (− 0.38 m/s per decade), which contrasted with the MASH results, where the most distinct trend occurred at the Tsetang station. The monthly 225 mean wind speed declines differed across the three stations. Both the Tsetang and Nyêmo stations exhibited statistically significant decreases for each month, with the highest rates of decline occurring in April (− 0.41 m/s per decade) and March (− 0.59 m/s per decade), respectively. In contrast, a non-significant wind speed increase was detected at the Lhasa station from June to August, and the maximum slope decline occurred in February (− 0.17 m/s per decade). These results corresponded well with the MASH results described above, indicating that using the MASH method is feasible and effective to filter the wind speed data. A comparison of the MK and LSLR results also showed that the trends were detected consistently despite the minor difference observed in some cases. In June at the Lhasa station, MK detected an increasing trend, while the LSLR had a decreasing slope; however, both related p-values were extremely high (see Figure S1 online), indicating low confidence in the results.
Trend analysis and discussion
In summary, with the exception of a non-significant increasing trend in the summer at Lhasa, wind speed declines were detected at the annual, seasonal and monthly scales from 1960 to 2015, especially in the spring at the Tsetang and Nyêmo stations and in the winter at the Lhasa station. These results were detected by both the MASH method and the MK test and were consistent with most other regions over China23,43,44,45. Additionally, three stages of wind speed change in the study area from 1960 to 2015 were detected using the MASH method. To further understand changes across different periods, the annual mean wind speed variability at the three stations and the station-averaged values are shown in Fig. 7. There were two apparent transitions in wind speed at the three stations and their averaged values, occurring in 1975 and 2006. On average, the annual mean increasing trend during the first period (1960–1975: referred to as P1) was 0.044 m/s per year and then weakened during the second period (1976–2006: referred to as P2) with a slope of − 0.046 m/s per year. During the third period (2007–2015: referred to as P3), there was a marked increasing trend in the station-averaged annual mean wind speed (0.105 m/s per year). Similarly, this recent recovery in wind speed after the 2000s has also been reported in some other regions, including the Czech Republic12, Iran46, China (TP27, southwestern23 and nationwide24), Spain and Portugal47, and South Korea48. Globally, however, reversed terrestrial stilling was detected in approximately 2010 based on updated data (1978–2017), and the recent global mean annual wind speed increasing rate of 0.24 m/s per decade is three-fold the former decreasing rate49. Our results supported the latest scientific findings. It is worth noting that the turning point in our study is approximately 2–3 years earlier than the global turning point, demonstrating that the wind speed changes in the Tibetan Plateau may be a sensitive indicator of global climate change. In addition, Zha et al.50 also pointed out that the recovery in wind speed over Eastern China could also occur in the next two decades. The predicted tendency, if evidenced to be true at the global scale, could elucidate the causes of global terrestrial stilling and is worthy of continuous attention.
In addition, the different trends between stations showed spatial differentiation in the study area. Of the three stations, the Tsetang station had the most pronounced reduction in wind speed, but at the lowest elevation, which was inconsistent with the findings of Guo et al.45 that higher-elevation environments have experienced greater reductions in wind speed. This finding indicated that elevation-dependent reductions in wind speed may not be applicable for the regional-scale decline in our study.
Overall, combined with statistical tests, such as the MMK and Sen’s method, the MASH method was able to simultaneously handle both seasonal and interannual variabilities, and the results were effectively visualized in different patterns, making seasonal process analysis and trend detection easier.
Possible causes of the observed changes
Changes in wind speed may be the result of a combination of several natural and anthropogenic processes operating on local, regional, and global scales. Previous studies have explored the possible forces that affect wind speed, which can be roughly divided into two parts based on the aerodynamic equation19: the driving force, such as large-scale atmospheric circulation patterns23,43,51 and air temperatures8,14,23, and the drag force, including surface roughness due to land use/land cover changes52 and urbanization8,44 and local air pollution15,53. However, the specific causes of wind speed changes differ regionally. In this study, we investigated the possible main driving force and drag force of wind speed changes in the MYR by examining the effects of climatic changes and human activities on wind speeds.
Effect of large-scale atmospheric circulation
Changes in large-scale air circulation patterns are one of the key factors affecting surface wind speeds. Vautard et al.52 noted that wind speed decreases in the Northern Hemisphere were partly (10%–50%) related to changes in atmospheric circulation. Yang et al. also showed that changes in longitudinal (meridional) and latitudinal (zonal) wind speeds at 500 hPa were related to wind speed changes in China23.
To quantify changes in large-scale atmospheric circulation from 1960 to 2015, the monthly zonal component (u) and meridional component (v) of the 10 m wind at 500 hPa during 1960–2015 (P1, P2 and P3) were derived over the domain of 28°–30°N and 90°–92°E from the NCEP/NCAR Reanalysis Project (available at https://www.cdc.noaa.gov/) with a spatial resolution of 2.5° × 2.5°54. Regional average trends were calculated according to amalgamate grid points.
The annual and seasonal zonal (u) and meridional (v) component variabilities for different periods are shown in Fig. 8, and the LSLR was used to estimate the trends. Zonal wind speeds (u) blow in a west–east direction, where a positive (negative) value indicates westerly (easterly) wind, and meridional winds (v) blow in the north–south direction, where a positive (negative) value indicates a southerly (northerly) wind. Correlation coefficients between u (v) and near-surface wind speed at three stations using Spearman’s correlation coefficient (SCC) are shown in Table 3.
As Fig. 8 shows, a statistically significant (α = 0.01) decreasing trend was observed for u at both annual (− 0.23 m/s per decade) and seasonal scales across the entire period (1960–2015), with the largest decadal declines occurring in spring (− 0.32 m/s) and autumn (− 0.31 m/s). Contrary to u, there was a slight decline in v (− 0.06 m/s per decade) from 1960 to 2015, and the seasonal variability was not consistent, with the largest decadal increases and decreases occurring in spring (0.11 m/s) and autumn (− 0.32 m/s), respectively. These results indicated that the wind speed decline in the study area may have been mainly influenced by the decreased u, which may reflect the decreased strength of the westerlies, and this finding can also be confirmed by the statistical correlation results between the wind speed and decreased u (Table 3). As mentioned earlier, westerly winds are dominant winds, and a decrease in zonal winds can explain the downward trends in this region. Similar findings have also been reported in China23,55 and Turkey14.
Meanwhile, u and v variabilities were also detected during different periods (P1, P2, and P3): a gradually strengthening, decreasing trend was observed in u from P1 (− 0.01 m/s per decade) to P3 (− 0.71 m/s per decade), while for v, there was a sharp increase during P1 (0.47 m/s per decade) and then a gradual decrease from P2 (− 0.02 m/s per decade) to P3 (− 0.67 m/s per decade). The high positive correlation coefficients between averaged station wind speed and v during P1 (0.61) shown in Table 3 indicated that increased wind speed from 1960 to 1975 was mainly associated with a stronger monsoon circulation; during P2, on the other hand, significant positive correlation coefficients with u (0.39) demonstrated that decreased westerlies had notable effects on wind speed reductions in the region.
Moreover, the effects of the u(v) components on wind speed varied seasonally at different stations. Notably, during P2, wind speeds at both Tsetang and Nyêmo were significantly positively correlated with decreasing u in autumn and winter and negatively correlated with increasing v in spring, whereas there were no evident relationships between the u and v components and wind speed at Lhasa in all seasons. In addition, the correlations between wind speed at Lhasa and the u components in summer were larger than v, which was the opposite of the results at the other two stations. These obvious differences may be the reason why the wind speed in Lhasa declined non-significantly from June to September. Although there were no evident relationships between the u(v) components and wind speed in most seasons during P3, it is worth noting that the winter wind speed at Tsetang had a high positive correlation with increased v (0.75). This result demonstrated that the enhanced winter monsoonal circulation may have led to wind speed recovery in winter during 2007–2015.
Overall, the significant high correlations between wind speed and u(v) occurred when u(v) changed significantly, confirming that wind speed changes in the MYR could be related to changes in westerlies and monsoonal circulation.
Except for the westerlies and Asia monsoonal circulation, the wind speed variability in the study area can also be affected by the Tibetan Plateau monsoon (TPM)23. The Tibetan Plateau monsoon index (TPMI), first proposed by Zhao et al.56, is used to describe the magnitude of the TPM and is the cumulative value of the geopotential height difference between each grid and 500 geopotential metres in a given region. TPMI contains two indices based on different latitude and longitude ranges: index A represents 25°–35°N, 80°–100°E, and index B represents 30°–40°N, 75°–105°E. In this study, index A data were selected according to the geographical location of the study area. The monthly TPMI data were provided by the National Climate Center of China (online at https://cmdp.ncc-cma.net/).
Based on the LSLR test and SCC results from Table 4, statistically significant increasing trends for annual TPMI were observed from 1960 to 2015 at a rate of 0.107 hPa/a (α = 0.05). Increases also occurred in all four seasons, with the highest rate of 0.139 hPa/a in autumn. More importantly, the correlation coefficients showed that there were strong negative correlations between wind speed and TPM, especially during P2, with correlation coefficients ranging from − 0.54 to − 0.78, demonstrating that the enhanced TPM had a significant effect on wind speed decline in the MYR by changing the regional atmospheric circulation mode. Previous studies have documented that the Tibetan Plateau influences global and regional atmospheric circulation systems through strong thermodynamic and dynamic effects. A significant increase in the TPMI could affect the regional monsoon circulation and westerly circulation, thus leading to a decline in wind speed25. However, the specific mechanisms behind these relationships require further study. Additionally, the non-significant SCC results during P3 indicated that TPM may not have been the main reason for the short-term recovery in wind speed over recent years.
Effect of air temperature and pressure changes
Numerous researchers have found that rising temperatures may be a possible cause of wind speed change because air temperature changes can affect surface pressure gradients and wind speed in turn8,14,19,23.
In this study, a significant (α = 0.01) increasing temperature trend was detected from 1960 to 2015, with the highest increases in winter (0.031 °C/a) (Table 5). The strongest significant (α = 0.01) negative correlations between air temperature and wind speed were observed in autumn (− 0.67). However, air temperatures in the MYR did not rise consistently. A sharp decreasing trend was detected from 2007 to 2015 at a rate of − 0.102 °C/a. The high negative correlation (− 0.73) during P3 suggested a possible link between the recent recovery in wind speed and air temperature reductions, especially in winter (− 0.87). These results demonstrated that there could be a connection between wind speed and air temperature in the MYR.
Similar to the average wind speed, dramatic increasing trends in annual and seasonal temperatures were observed at all three stations. The strongest trends occurred in winter and ranged from 0.035 to 0.06 °C/a (Table 5). Inter- and intra-annual changes in the relationship between daily wind speed and temperature at the three stations are also shown in Fig. 9. The intensity of each horizon indicates the degree to which the wind speed or temperature rose or fell. The images in the left column of Fig. 9 show that there were three clear stages of variability in the relationship between air temperature and wind speed from 1960 to 2015, corresponding to the three stages of wind speed change mentioned above (the left column of Fig. 9). As seen in the right column of Fig. 9, both minimum air temperatures and wind speed levels at the three stations were observed in December and January, whereas maximum air temperatures occurred from June to August, which was two months later than the maximum wind speeds. While higher air temperature increased in winter (Table 5), a stronger decrease in wind speed occurred in spring (Table 2). Higher surface temperatures in winter may reduce surface pressure, thereby weakening the temperature and pressure gradient between land and the adjacent oceans, ultimately leading to declines in wind speed51.
Previously, the horizontal pressure gradient force (PGF) was the main driving force for horizontal motion, which can be affected by the horizontal temperature gradient. For instance, asymmetric air temperature increases at different latitudes may decrease the horizontal temperature gradient, which leads to a decrease in the horizontal PGF, and in turn, the regional wind speed declines23. To further investigate the internal relationship between the horizontal temperature gradient changes and wind speeds, three latitude regions were defined within the study area (85°–95° E) according to Yang et al.23: low latitude (LL: 15°–25° N), middle latitude (ML: 35°–45° N), and high latitude (HL: 55°–65° N), and monthly surface temperature and pressure data at 500 hPa obtained from the NCEP reanalysis online (www.cdc.noaa.gov/) were used to estimate surface air temperature and pressure gradients between the three latitudinal regions. Warming rates from 1960 to 2015 showed significant increases at 0.006, 0.013 and 0.045 °C/a for the low, middle and high latitudinal zones, respectively, with the strongest warming occurring in HL regions (Fig. 10a–c). This asymmetric warming would decrease latitudinal temperature gradients23,27, and the different interannual variability between the surface temperature gradients of HL-ML and LL-ML also illustrates these changes (Fig. 11a,b). Together with surface pressure, the maximum increasing trend in the ML region and a weak decreasing trend in the HL region also represented these asymmetric changes over the last five decades (Fig. 10d–f).
Moreover, long-term decreases were also observed in the annual mean surface pressure gradient between the HL and ML and between the LL and ML, except for a significant increase in the LL-ML surface pressure gradient during P3 (Fig. 11c,d). Such variations were roughly consistent with the wind speed changes in the study area, and it was evident that the LL-ML (correlation coefficient with wind speed: 0.66) surface pressure gradient may be the main factor affecting the wind speed changes compared with the HL-ML (0.4). Obviously, the interannual variability in the two surface temperature gradients was roughly consistent with the surface pressure gradients, especially in the LL-ML, with a significant negative correlation coefficient that reached − 0.51. All of the above results demonstrated that the surface pressure gradient variability between the middle- and low-latitudes in the study region may have been a primary contributor to wind speed changes under asymmetric warming from 1960 to 2015 in the study area.
Effect of several climate indices
Additionally, some climate indices were used to describe the atmospheric circulation variability. Because changes in circulation patterns differ regionally, the influence of circulation patterns on wind speed also has regional differences. Previous studies have demonstrated that Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) variability influence surface wind speeds in the Northern Hemisphere, such as Europe47,57, the US58, and China59. The Pacific Decadal Oscillation (PDO) is the leading principal component of monthly sea surface temperature anomalies in the North Pacific Ocean60. It can modulate the impacts of the El Niño-Southern Oscillation (ENSO) on East Asian atmospheric circulation61, and is also associated with wind speed changes62. However, Lin et al. argued that AO/NAO/PDO trends were not in phase with the changes in wind speeds over China from 1960 to 200924. Therefore, it is worth discussing whether wind speed variability is related to these indices in the study area. The AO and NAO indices were downloaded from NOAA (www.cpc.ncep.noaa.gov), and PDO was provided by JISAO (Joint Institute for the Study of the Atmosphere and Ocean) of University of Washington (online at https://research.jisao.washington.edu/pdo/PDO.latest).
Figure 12 shows the annual anomalies of the AO, NAO, and PDO indices from 1960 to 2015. For the AO/NAO (Fig. 12a, b), a statistically significant increasing trend for the AO (0.07 per decade) and a non-significant trend for the NAO (0.014 per decade) were detected (Table 6). Both the AO and NAO indices exhibited gently increasing trends before/after 1990 and then decreased from 1990 to 2006. A sharp increasing trend was observed after 2007, which was consistent with wind speed trends during P3. In addition, statistically significant (α = 0.05) increasing trends were detected in winter for both the AO and NAO. Previous studies have shown that the AO/NAO are the most important modes for atmospheric circulation in the mid-high latitudes of the Northern Hemisphere, with significant impacts on many climatic factors near the surface in winter27,30,63. However, the SCC results (Table 6) showed that the correlations between the AO/NAO and average wind speed were not significant across all seasons in the study area. Therefore, wind speed changes in the MYR cannot simply be linked with oscillation indices such as the AO/NAO, as Lin et al. concluded24. As shown in Fig. 12c, the PDO index experienced three periods during 1960–2015: a negative/positive phase before/after 1975 and a sharp upward step after 2006. Although there were no significant correlations between PDO and wind speed at the annual and seasonal scales (Table 6), the wind speed transitions corresponded well with the positive/negative phases of PDO. Because changes in the positive/negative or warm/cool phases of the PDO are related to sea surface temperature (SST)/sea level pressure (SLP) patterns, it is possible that they could affect the wind speed in the study area, which has a typical East Asia monsoon climate62. Thus, the PDO was regarded as an indicator of wind speed variability.
Effects of surface roughness
As mentioned above, the effects of regional-scale factors on wind speed, including urbanization/land use and land cover change and anthropogenic activities, cannot be ignored. Previous studies have shown that the difference between the trends at urban and rural stations in China is quite small23,43,51. In this study, we selected the urbanization rate as an indicator to describe urbanization in the MYR wide valley regions, which is defined as the ratio of the population in cities to the total population of this region64,65,66,67. Urbanization rate data corresponding to the stations were from the 1960–2015 statistical yearbooks of the Tibet Autonomous Region, including the cities of Lhasa and Shannan. As shown in Fig. 13a, the urbanization rate increased by 1.3 times from 1960 to 2015, especially since the 2000s, with a larger increase rate of 4.59% per decade, and a significantly (p < 0.01) negative correlation between the urbanization rate and wind speed was detected (− 0.51). However, the wind speed in this region underwent three stages, which was inconsistent with the increasing urbanization rate. This disagreement demonstrated that urbanization should not be a main cause of surface wind speed changes in the MYR.
Furthermore, increasing land use change is considered to be an important factor for 25–60% of wind speed reductions in the Northern Hemisphere52. Some studies showed that LUCC had distinct effects on the slowdown in surface wind speed over Eastern China65,68,69 and caused a decline in wind speed of 0.12–0.17 m s−1 decade−1 from the 1980s to 2010s32,67. Compared with these developed areas, the MYR Basin remains a natural watershed formed by climate and hydrology, and only 1–2.02% of land cover in this region changed during 1980–201570,71. In addition, the dominant land cover types in the MYR were bare soils and short grasses due to the semiarid plateau monsoon climate, and the changes in surface roughness were highly non-significant. NDVI trends in the MYR from 1982 to 2010 were very gentle, as demonstrated by Li et al.72, with a decadal trend of 0.007 (Fig. 13b). Therefore, it is doubtful that changes in surface roughness would have such a significant impact on surface wind speeds. In other words, despite some land use changes and industrialization in the Tibetan Plateau73, the direct human impact was relatively small in this study area. These findings coincide with the latest literature’s conclusion that the increased surface roughness was probably not the main reason for the near-surface wind variability but the internal decadal ocean–atmosphere oscillations49.
Identification of the main factors affecting wind changes
The effects of climatic factors (such as u, v, TPM, T/ΔT/ΔP and AO/NAO/PDO) and human activities (urbanization rate and NDVI) on wind variation were discussed above. These variables may have causal relationships with each other, and the significant correlation between some of these factors demonstrates these relationships (see Table S2 online). The principal component analysis (PCA) is a technique for reducing the number of parameters to extract the most important factors in the analysis of meteorological data74,75. Thus, PCA was conducted to determine the contributions of climate change and human activities to wind speed changes using Social Sciences software (Version 24.0).
The PCA reduced the total variance in 13 variables to 3 uncorrelated principal components, with the cumulative contributions reaching 75.13% (Tables 7 and 8).
The first component was strongly correlated (loadings of more than 0.7) with TPM and ΔTHL-ML, with the factor score coefficients of TPM being the highest, which may be termed the atmospheric circulation factor. The second component brought together ΔTLL-ML and ΔPHL-ML, with factor score coefficients greater than 0.85. Therefore, the second component had a close relationship with the surface pressure/temperature gradient, which can be termed the dynamic factor. The third component strongly correlates with the urbanization rate (loadings of more than 0.85) and may be termed the anthropogenic activities factor. The three components explained 38.22%, 22.83% and 14.07% of the variances in the original 13 variables, but the former two components, which reflect climatic variation, were more important. This result supports the conclusion discussed in “Effects of surface roughness” section that the wind speed variation in recent decades has mainly accounted for climate change rather than human activities.