## 4 Discussion  
### Temporal Climatic Variations and Possible Connections with Atmospheric and Oceanic Circulations  
Hubei Province has experienced a significant increase in air temperature of 0.017 \({}^{\circ}\)C year\({}^{-1}\) (\(p<0.01\)) during 1961-2015, which is slightly less than the increasing trend in global land surface air temperature of 0.02 \({}^{\circ}\)C year\({}^{-1}\) (\(p<0.01\)) for the same period [59]. As a result of this significant warming, all warm-related indices present increasing trends, especially for SU25 (0.65 days year\({}^{-1}\) with \(p<0.01\)) during 1982-2015. Increasing trends in warm-related indices have also been documented in previous studies that focused on Northwestern China [60], the Yangtze River Basin [17], and mainland China [4; 14], which are in line with the global warming. However, increasing trends for many indices in the study area are higher than those for the whole China. For instance, WSDI increases at a rate of 0.22 days year\({}^{-1}\) in Hubei Province, much higher than the 0.075 days year\({}^{-1}\) trend for the whole of China for the same period 1961-2015. Owing to the diverse climate, terrains and land cover changes within China, the different trends in temperature indices between the whole China and subregions are generally understandable. Based on multiple datasets from models, satellite images and thermal cameras, [PERSON] et al. [61] implied that variability of the air temperature is related to spatial details and scales. In details, the rapid urbanization and land cover changes in the Yangtze River Basin and Central China (e.g., Wuhan) likely lead to a prominent increase in temperature because of surface urban heat islands [62] and changes in surface energy budget [63]. In addition, the trends of all the seven warm-related indices for the period 1982-2015 are much higher than the trends for 1961-2015 (Figure 4), indicating accelerated increases in the temperature extremes and risks in the study area. Based on percentile temperature indices, [PERSON] et al. [14] also found that trends of warm and hot days increased more rapidly after the early 1980s than before the 1980s in most regions in China. Moreover, previous studies demonstrated that indices for nighttime temperature changed much faster than those for the daytime temperature based on global studies [2; 18], which was also valid for Hubei Province. Many significant trends at the stations are seen in indices for nighttime temperature (i.e., TN90p and TN10p) but are scarce in daytime temperature indices (i.e., TX90p and TX10p). As for the precipitation indices, there are no significant trends at the 0.1 significance level for Hubei Province as a whole, except for CWD. This result is consistent with the investigation from neighboring Province (i.e., Hunan Province), where the trends in precipitation extremes at most sites are also insignificant [18]. In addition, our results indicate accelerated changes in precipitation indices, which slightly decrease for extreme precipitation and increase for CDD if comparing trends between 1961-2015 and 1982-2015 (Figure 4). Global warming is expected to increase the moisture holding capacity of the atmosphere, and consequently increase the potential risks from extreme rainfall. In fact, changes in extreme rainfall have spatial variations that extreme rainfall increases typically in coastal regions and might decrease in inland regions (e.g., Hubei Province) [15]. This difference is likely caused by the well-recognized decline in wind speed [55; 56]. Nevertheless, the extreme precipitation is relatively high in the study area and accounts for up to 40% of the total annual precipitation in many years, which is expressed as the ratio between R95p and PRCPTOT. Furthermore, based on the CMIP5 models, [PERSON] et al. [64] proposed that all categories of precipitation are projected to increase in frequency over the coming decades. The temporal variations in these indices are likely influenced by large-scale oceanic and atmospheric circulation oscillations [33].