It is certain that globally the troposphere has warmed since the industrial revolution because of the greenhouse gas emissions. The water vapour content of air increases as the climate warms. However, there is nonsignificant long‐term trend in global‐scale precipitation. This study aims to investigate the sources of the uncertainty of the long‐term precipitation trend under global warming through the water cycle. The data of the historical simulations (1850–2005) from 30 CMIP5 models are used in the estimate. The p‐value is utilized to indicate the significance of the long‐term trend of the water‐vapour‐related variable in the water cycle as well as the air temperature near surface and at the condensation level. For each variable, the p‐value is calculated for each of the grid points. The overall spatial distribution patterns of the p‐values are compared between two variables that linked directly, and a point‐to‐point comparison is also performed to the two fields. Results show that over the globe, the near‐surface specific humidity increases significantly at most of the grid points while the increase of the specific humidity at the condensation level is significant at some, and, however, the long‐term trend of the relative humidity at the condensation level is majorly nonsignificant. This suggests that as assessed from the global grid points, the uncertainty of the long‐term trend of the water‐vapour‐related variable enlarges substantially in the process of condensation, and modestly in the process of the transport of water vapour. Therefore, the uncertainty of the long‐term precipitation trend under global warming is mainly attributed to the condensation and slightly due to the transport of water vapour. The results obtained from the 30 models are consistent with each other, which supports the reliability of the conclusion.

Climate science involves various functional relations and needs to investigate the dominance or relative importance of the variables in their relation. In our previous studies, we examined several problems in which causal relations are established, showing how the dependent quantity is affected by two or more independent variables. With linear fitting, the relative contributions of the variables to the variation of the quantity are compared. In this study, we examine constraint relation, which is a simultaneous multivariate relation, with all variables in the relation being equal in position. The relation can generally be nonlinear. To be convenient for examining the dominance, plane equation fitting can be used to linearize the relation. The equation of state for ideal dry air is investigated as a simple case of the relation. For this special case, a linearized relation can be obtained from both the fitting and the derivation. The scale analysis tool used in dynamic meteorology is applied here for the dominance analysis. Through comparing the scales of the terms, we can simplify the equation. The simplified relations correspond, respectively, to Charle’s law, Boyle’s law, and Gay-Lussac’s law. The geographical preferences of the different dominance patterns are exhibited. In addition, when considering the change of the variable that is smallest in scale, we can identify which factor is the dominator. The ideal gas law is intentionally chosen as the example, since the relation is simple in form, and the results of dominance can be deduced analytically. A comparison demonstrates that the methods used here for the dominance analysis are reliable.

In the context of global warming, the impact of summer high temperature events is increasing. The accumulated summer high temperature is often used to reflect the overall hotness of summer. The internal variation of the accumulated temperature can be affected by both the frequency and intensity. In this study, by using the daily data during summers of 1960–2018, we examine the relative importance of the two factors with a multiple linear regression method. It is demonstrated that that the dominant result of summer accumulated temperature is sensitive to the change of threshold. As the threshold increases, the importance of frequency gradually increases, while the importance of the intensity decreases. In addition, it is found that when the threshold changes, the sensitivity of the dominant results is different over regions. This can provide a basis for the selection of regional thresholds and further improve the representation of accumulated temperature for high summer temperatures.

Seasonal prediction of summer rainfall over the middle and lower reaches of the Yangtze River Basin (MLYRB) remains challenging because the linkage between preceding climate signals and regional precipitation anomalies involves complex dynamic and thermodynamic processes. This study examines March precipitation over the eastern Mediterranean–West Asia region, hereafter referred to as PE, as an upstream spring signal associated with July rainfall anomalies over the MLYRB. The central objective is to determine, through a relative humidity decomposition framework, whether the humidification accompanying PE-related July rainfall anomalies is dominated by moisture changes or by temperature-related saturation effects. The results indicate that high-PE years are associated with a significant increase in July rainfall over the MLYRB. This rainfall enhancement is accompanied by anomalous circulation patterns favourable for moisture transport and convergence over eastern China. Meanwhile, positive relative humidity anomalies extend from the lower to the upper troposphere, with the 400 and 300 hPa levels showing a particularly close spatial correspondence with the significant rainfall anomalies over the rainfall region. Although absolute water vapour content decreases with height, the coherent upper-tropospheric relative humidity response indicates the presence of a deep moist layer, which is favourable for sustained condensation, reduced dry-air entrainment, and persistent monsoon rainfall. A further moisture–temperature decomposition shows that the PE-related relative humidity response is jointly controlled by changes in atmospheric moisture content and saturation vapour pressure. Over the MLYRB, the increase in relative humidity is primarily associated with enhanced moisture content, whereas temperature-induced changes in saturation conditions are more evident in regions with more coherent temperature anomalies. These findings suggest that the PE-related July rainfall anomaly is supported by a combination of dynamic moisture supply and thermodynamic humidification of the atmospheric column. The study provides a physically consistent explanation for the potential precursor relevance of the PE signal and emphasizes the importance of vertical humidity structure in understanding and predicting summer rainfall anomalies over eastern China.

As an important grain-producing region of China, Anhui Province suffered its most severe drought in 70 years in 1978, particularly in summer. During the drought, the west–northward shifted western Pacific subtropical high (WPSH) induced local high temperatures, while a weakened monsoon suppressed south-to-north moisture transport into the region. These two factors jointly triggered the extreme drought. This study aims to quantitatively examine the respective contributions of water vapor and air temperature anomalies to the drought. Based on the tight relationship between precipitation and relative humidity as well as the definition expression of relative humidity, two measures are constructed to separate moisture and temperature effects, with three supplementary indicators for assessing their relative importance. The results show that water vapor anomalies dominate this drought, contributing approximately 73% of the total effect, while temperature anomalies account for 27%. At the condensation levels, the contribution of less-than-normal water vapor to the drought is roughly 4.6 times that of warmer-than-normal temperature. Distinguishing the dominance of moisture and air temperature in the drought and quantifying their respective contribution fractions provides physical support for drought classification and prediction.

Drought is typically divided into meteorological, agricultural, hydrological, and socioeconomic categories. Generally, the transition from meteorological drought to other types of droughts is known as drought propagation. When drought propagation occurs, different types of droughts may still exist simultaneously or successively. In this study, compound droughts are divided into three categories: hydrological meteorological compound drought (HMD), meteorological hydrological compound drought (MHD), and simultaneous compound drought (SD). ERA5 and CMIP6 data are used for analysis under historical and future scenarios. Different types of compound droughts have emerged in extreme centers in different basins. Our analysis indicates a significant upward trend in the duration of these three compound droughts from 1979 to 2022. Additionally, our projections under SSP5-8.5 and SSP2-4.5 suggest a substantial increase in the occurrence of various compound droughts. HMD, MHD and SD all show a consistent upward trend under the future scenario above the moderate-drought level. MHDs are projected to experience the most significant increase compared to the historical period in the far-future period (2066–2099) under SSP5-8.5.