Yao Feng

Understanding the drying mechanism is critical for formulating targeted mitigation strategies to combat drought impacts. This study aimed to reveal divergent drying mechanisms in humid and non-humid regions across China from the multidimensional perspectives of climate, vegetation, and energy balance. During the period 1982–2012, the Standardized Precipitation Evapotranspiration Index (SPEI) revealed non-significant drying trends across China. Simultaneously, temperature and precipitation indicated a warming and drying pattern in the humid regions, contrasted with a warming and moistening pattern in the non-humid areas. The coupling effects of declined precipitation, increased vegetation coverage, and elevated temperature exacerbated dryness in the humid regions, while pronounced warming dominantly caused dryness in the non-humid regions. The inverse correlations between the actual evapotranspiration (ET) with precipitation and potential ET (PET) highlighted the principal role of moisture availability in divergent drying mechanisms over humid and non-humid regions. Random Forest models recognized precipitation and PET as the primary factors influencing SPEI in the humid and non-humid regions, respectively. Ongoing warming from 2013 to 2022 mitigated dryness in the humid regions due to the increased latent heat at the expense of sensible heat. Conversely, warming, amplified by the heightened sensible heat, exacerbated drought in the non-humid regions. By identifying the contrasting responses of humid and non-humid regions to warming and moisture availability, this study provides crucial insights for policymakers to mitigate drought impacts and enhance resilience in vulnerable non-humid areas.

Hot-dry winds (HDWs), characterized by simultaneous high temperature, low humidity, and strong wind speed, represent prevalent agrometeorological hazards in northern China. To date, our comprehension of HDWs has been constrained to regional scales due to the dependence upon in situ meteorological observations and the lack of a universal definition of HDWs. In this study, we leveraged a gridded dataset to elucidate the spatiotemporal dynamics of HDWs in northern China from 1961 to 2022. We also introduced a standardized HDW index (sHDWI) to investigate HDW severity. The results indicated more HDW occurrences in eastern Northwest China (NW) and western Inner Mongolia (IM). While there were declining trends in HDW frequency, extent, and severity on average in northern China from 1961 to 1993, these trends were reversed by an overall increase afterward. Intriguingly, regions spanning from NW to IM exhibited significant upward trends in HDW severity according to sHDWI. The spatial extents of light, moderate, and heavy HDWs consistently decreased in North China (NC), with notable expansions over IM, NW, and Northeast China (NE) from 2001 to 2022. Importantly, the increased frequency, expanded extent, and intensified severity of HDWs, particularly, the heavy ones, underscored the escalation of HDW events in recent decades over the arid IM and hyperarid NW, known for their fragile ecological environment. The findings from this research carry significant implications for agricultural production and water management in northern China influenced by the changing patterns of HDWs in the context of climate change. Significance Statement This study aims to enhance our understanding of the large-scale spatiotemporal dynamics of hot-dry winds (HDWs) in northern China using a 0.25° × 0.25° gridded dataset spanning the period from 1961 to 2022. The adoption of the gridded dataset enables a multidimensional exploration of HDWs, encompassing their frequency, extent, and severity which cannot be fully captured by in situ observations. Meanwhile, to quantify changes in HDW severity, we introduce a standardized HDW index that surpasses existing HDW definitions relying on absolute thresholds or relative percentiles of maximum air temperature, relative humidity, and wind speed. Our results reveal a consistent reversal in HDW frequency, extent, and severity around 1993 in northern China and highlight the intensification of heavy HDWs, particularly in the arid and hyperarid regions within the study area.

Assessing vegetation changes in alpine arid and fragile ecosystems is imperative for informed ecological restoration initiatives and adaptive ecosystem management. Previous studies primarily employed the Normalized Difference Vegetation Index (NDVI) to reveal vegetation dynamics, ignoring the spatial heterogeneity alterations caused by bare soil. In this study, we used a comprehensive analysis of NDVI and its spatial heterogeneity to examine the vegetation changes across the Three-River Headwaters Region (TRHR) over the past two decades. A random forest model was used to elucidate the underlying causes of these changes. We found that between 2000 and 2022, 9.4% of the regions exhibited significant changes in both NDVI and its spatial heterogeneity. These regions were categorized into six distinct types of vegetation change: improving conditions (62.1%), regrowing conditions (11.0%), slight degradation (16.2%), medium degradation (8.4%), severe degradation (2.0%), and desertification (0.3%). In comparison with steppe regions, meadows showed a greater proportion of improved conditions and medium degradation, whereas steppes had more instances of regrowth and slight degradation. Climate variables are the dominant factors that caused vegetation changes, with contributions to NDVI and spatial heterogeneity reaching 68.9% and 73.2%, respectively. Temperature is the primary driver of vegetation dynamics across the different types of change, with a more pronounced impact in meadows. In severely degraded steppe and meadow regions, grazing intensity emerged as the predominant driver of NDVI change, with an importance value exceeding 0.50. Notably, as degradation progressed from slight to severe, the significance of this factor correspondingly increased. Our findings can provide effective information for guiding the implementation of ecological restoration projects and the sustainable management of alpine arid ecosystems.

Hot-dry winds (HDWs), characterized by simultaneous high temperature, low humidity, and strong wind speed, represent prevalent agrometeorological hazards in northern China. To date, our comprehension of HDWs has been constrained to regional scales due to the dependence upon in situ meteorological observations and the lack of a universal definition of HDWs. In this study, we leveraged a gridded dataset to elucidate the spatiotemporal dynamics of HDWs in northern China from 1961 to 2022. We also introduced a standardized HDW index (sHDWI) to investigate HDW severity. The results indicated more HDW occurrences in eastern Northwest China (NW) and western Inner Mongolia (IM). While there were declining trends in HDW frequency, extent, and severity on average in northern China from 1961 to 1993, these trends were reversed by an overall increase afterward. Intriguingly, regions spanning from NW to IM exhibited significant upward trends in HDW severity according to sHDWI. The spatial extents of light, moderate, and heavy HDWs consistently decreased in North China (NC), with notable expansions over IM, NW, and Northeast China (NE) from 2001 to 2022. Importantly, the increased frequency, expanded extent, and intensified severity of HDWs, particularly, the heavy ones, underscored the escalation of HDW events in recent decades over the arid IM and hyperarid NW, known for their fragile ecological environment. The findings from this research carry significant implications for agricultural production and water management in northern China influenced by the changing patterns of HDWs in the context of climate change. Significance Statement: This study aims to enhance our understanding of the large-scale spatiotemporal dynamics of hot-dry winds (HDWs) in northern China using a 0.25° × 0.25° gridded dataset spanning the period from 1961 to 2022. The adoption of the gridded dataset enables a multidimensional exploration of HDWs, encompassing their frequency, extent, and severity which cannot be fully captured by in situ observations. Meanwhile, to quantify changes in HDW severity, we introduce a standardized HDW index that surpasses existing HDW definitions relying on absolute thresholds or relative percentiles of maximum air temperature, relative humidity, and wind speed. Our results reveal a consistent reversal in HDW frequency, extent, and severity around 1993 in northern China and highlight the intensification of heavy HDWs, particularly in the arid and hyperarid regions within the study area.

The simultaneous occurrence of extreme weather events such as high temperature, low humidity and strong wind speed can cause devastating impacts on the fragile environment and vulnerable ecosystem in arid Northwest China (NW). However, few studies investigate the multivariate compound hot, dry and windy extremes in NW. In this study, we investigate the temporal and spatial variations of the warm‐season trivariate hot, dry and windy (HDW) and bivariate hot and dry (HD), dry and windy (DW), hot and windy (HW) extremes in NW. From 1960 to 2019, we observed more compound HD, DW, HW and HDW days in the hyper‐arid Xinjiang and west Gansu in NW. Simultaneously, DW days, HW days and HDW days consistently declined at rates of −0.48, −0.88 and −0.42 day·10a⁻¹, except for a discernible increasing rate of 0.56 day·10a⁻¹ in HD days. Notably, trends in four multivariate compound extremes all experienced pronounced reversals from a declining to increasing trend in the 1990s. The higher probability of compound HD, DW and HW extremes contributed to a higher probability of HDW in Xinjiang province. Ultimately driven by the significantly rising temperature (0.21°C·10a⁻¹, p < 0.01), four compound extremes suggested increased probability with the largest increase in compound HW probability from 1960–1989 to 1990–2019. Both the rising trends in the occurrence and the increased probability of four compound extremes highlight intense risks of compound extreme weather over NW in recent decades.

In the context of global warming, the increasing frequency of compound dry–hot extremes pose varying degrees of threats to the natural environment and human society. However, due to the lack of long‐term and high‐accuracy meteorological observations, the characteristics and drivers of compound dry–hot extremes over a century‐long period is still poorly investigated in China. Here we characterize the spatiotemporal variations of growing‐season (May–September) compound dry–hot events (CDHEs) over China using Standardized Dry and Hot Index (SDHI) calculated by monthly temperature and precipitation during 1901–2017. The influences of precipitation and temperature extremes, as well as large‐scale climate indices (i.e., El Niño–Southern Oscillation Index [ENSO], Atlantic Multidecadal Oscillation [AMO], North Atlantic Oscillation [NAO], Pacific Decadal Oscillation [PDO] and North Pacific [NP]) on CDHEs were then explored using the relative contribution analysis, Pearson correlation analysis and cross‐wavelet spectrum analysis. Our findings reveal a significant increase in the occurrence of different severity levels of CDHEs in China with their spatial extent gradually increase, practically since the end of the 20th century. Due to the exacerbation of drought conditions in China since the mid‐20th century, the regions where the Standardized Precipitation Index (SPI) has a more prominent contribution to the SDHI have gradually expanded, predominantly in the eastern, northern and northwestern parts of China. The significant influence of large‐scale climate indices, particularly ENSO and AMO, on the occurrence of compound dry–hot extremes in China is noteworthy, as these indices control the variability of precipitation and temperature, respectively. The study findings demonstrate that AMO exhibits a more significant correlation with SDHI in all subregions of China, while ENSO exhibits marginally stronger periodic coherence with SDHI than the other indices. This study offers valuable insights and references for unravelling long‐term development process of and the underlying influence mechanisms of significant climate patterns on the growing‐season CDHEs in China.

Elucidating the impact of reservoir operation on hydrological signatures is crucial for the effective management of large rivers under the changing climate. This study first revised the reservoir operation scheme in the Soil and Water Assessment Tool (SWAT) model to improve its description of actual operation laws of reservoirs in the upper Yangtze River basin (UYRB). Then, we identified the reservoir-induced hydrological alteration through a hydrological index method driven by observed and simulated daily streamflow from 1960 to 2017. The results revealed the superiority of the revised reservoir algorithm in the SWAT model in simulating streamflow and floods at Cuntan and Yichang stations with the Nash-Sutcliffe efficiency (NSE) coefficient and the Kling-Gupta efficiency (KGE) coefficient improved from 0.01 to 0.08 and 0.01 to 0.05, respectively. Relative to the baseline period (1960–2002), the hydrological signatures in the impact period (2003–2017) changed substantially after 2003. Reservoirs induced a remarkable increase of 27.76% and 55.97% in streamflow from January to March, accompanied by a notable decrease of 6.95% and 20.92% in streamflow from September to October after 2003 at Cuntan and Yichang stations, respectively. Meanwhile, the annual streamflow range contracted, and the flow became more stable with a reduced variation in daily streamflow, extremely low flow spell duration, and extremely high flow spell duration. Consequently, our results improved the quantitative understanding of reservoir-induced alteration and informed the management and planning of reservoir construction in the UYRB under climate change.