Jiawei Hao

Water vapor (WV) is a vital basis of water and energy cycles and varies with space and time. When researching the variations of moisture in the atmosphere, it is intuitive to think about the total WV of the atmosphere column, precipitable water (PW). It is an element that needs high-altitude observations. A surface quantity, surface WV pressure (SVP), has a close relationship to PW because of the internal physical linkage between them. The stability of their linkage at climatic scales is verified using monthly mean data from 1979 to 2021, while studies before mainly focused on daily and annual cycles in local areas. The consistency of their variations is checked with three reanalysis datasets from three angles, the interannual variations, the long-term trends, and the empirical orthogonal function (EOF) modes. Results show that the interannual correlation of SVP and PW can reach a level that is quite high and are significant in most areas, and the weak correlation mainly exists over low-latitude oceans. The long-term trends, as well as the first EOF modes of these two quantities, also show that their variations are consistent, with spatial correlation coefficients between the long-term trends of two variables that are generally over 0.6, but specific differences appearing in some regions including the Tropical Indian Ocean and Middle Africa. With the correspondence of PW and SVP, the variations of total column WV can be indicated by surface elements. The correspondence is also meaningful for the analysis of the co-variation in total column vapor and temperature. For example, we could research the relations between SVP and air temperature, and they can reflect the co-variance of total column vapor and near-surface air temperature, which can avoid analyzing the relation between column-integrated moisture content and surface air temperature directly.

It has generally been regarded that, in the warming climate, atmospheric water vapor may increase due to the enhancement in surface evaporation, which is expected from the Clausius–Clapeyron (C–C) equation, along with the assumption that relative humidity experiences small changes. If the variation in relative humidity is small, the response of water vapor to temperature will be closely in line with the C–C equation. However, whether relative humidity experiences large or small changes needs be assessed, and the change of relative humidity should be compared with the change in surface–air temperature. In this study, we link surface vapor pressure, which characterizes atmospheric water vapor, to surface-air temperature, and treat both the temperature and relative humidity as influencing factors. A method based on linear regression is applied to compare the interannual variabilities of relative humidity and temperature in the interannual variation in surface vapor pressure. Whether the year-to-year perturbation of relative humidity is important, compared with the perturbation in surface-air temperature, is explored Results show that, at high latitudes of both hemispheres, the variation in vapor pressure is dominated by air temperature, and relative humidity has small positive contributions. Thus, the variation in relative humidity over these regions is comparably small, and the response of water vapor to temperature can well follow the C–C equation. Differently, at mid-low latitudes, especially on land, air temperature plays a negative role in the variation in vapor pressure. Relative humidity offsets the negative contribution and dominates the variation in vapor pressure, suggesting that the variation in the relative humidity over these regions is comparably large. Hence, the response of water vapor to temperature deviate from the C–C equation. Analysis indicates that the different results of the dominance from the two influencing factors are affected by the dual effects of precipitation or wet-air transport over land. Both precipitation and the transport of cold wet air could break the C–C relation between water vapor pressure and temperature.

Northeast China (NEC) is an agricultural base of the country. It suffered a severe drought in 2014. During the drought, the western Pacific subtropical high is stronger than normal and stretches to the west. The warm and moist air in the southwest edge of the high transports northwestwards and thus cannot affect NEC. Over high latitudes, in the regions west and east to NEC, there are large anomalies in geopotential height, which favour meridional exchange and may lead to stronger‐than‐normal cold air intrusions. Because of the lack of warm and moist air supply from the south while the invasion of the cold and dry air from the north, the atmosphere over NEC is finally drier and colder than normal in saturation levels. Based on the physical relation between seasonal precipitation and relative humidity, along with the defined C_q and C_T, indicators for measuring the changes in moisture and temperature, the effects of water vapour and air temperature on the drought can be linearly separated, and thus compared. Results reveal that, at middle and high levels, the dryness of the air, which plays a positive role in the drought, can offset the negative effect of the coldness, and the atmosphere over NEC then maintains a lower‐than‐normal relative humidity. In the near‐surface level, the air is drier and warmer than normal, which facilitates the drought.

Water vapor is a vital basis of water and energy cycles and varies with space and time. When researching the variations of moisture in the atmosphere, it is intuitive to think about the total water vapor of the atmosphere column, precipitable water. It is an element that needs high-altitude observations. A surface quantity, surface water vapor pressure, has a close relationship to precipitable water because of the internal physical linkage between them. The stability of their linkage at climatic scales is verified in this work, while studies before mainly concentrated on daily and annual cycles. The consistency of their variations is checked with three reanalysis datasets from three angles, including the interannual variations, the long-term trends, and the first EOF modes. Results show that the interannual correlation of surface water vapor pressure and precipitable water can reach a quite high level in most areas, and the weak correlation regions mainly exist in low-latitude areas. The long-term trends, as well as the first EOF modes of these two quantities also show that their variations are well consistent, but specific differences appear in some regions, and there are discrepancies among different datasets. With the correspondence of precipitable water and surface water vapor pressure, the variations of total column water vapor can be indicated by the surface quantity, surface water vapor pressure. The correspondence is also meaningful for the analysis of the co-variation of total column vapor and temperature. For example, we could research the relations between surface vapor pressure and air temperature, and it can reflect the co-variance of total column vapor and near-surface air temperature. Linking the column integrated moisture content to surface temperature directly or integrating the column temperature can be avoided.

The thermal forcing of the Tibet Plateau (TP) causes circulation anomalies that may affect the East Asian monsoon. Hence, summer precipitation in China has a close relation to the previous heat source over TP. The saturation of the atmosphere is the precondition of precipitation, so variations of vapour and temperature adjust the generation of precipitation synergistically. The heating of TP exerts impacts on summer precipitation in China, essentially, through modulating vapour and temperature. The goal of this study is to identify whether the variations of temperature or vapour dominate in the end. Besides, according to the heating characteristics over TP, a more accurate area of spring (March–May) heat source is chosen to indicate the midsummer (July) precipitation in China. The configuration of contributions of temperature and vapour can be divided into two cases. One is that vapour and temperature both increase or decrease, so they play inverse roles. It is easy to find the dominant element in this circumstance. The other is that vapour and temperature have concurrent effects. In this case, their significances need to be further compared. The method based on the linkage of the precipitation and relative humidity is used to quantitatively compare the effects of temperature and vapour. The results are robust and can complement the qualitative analysis with circulations and temperature. In years that there is intense heating on west TP, the Yangtze River basin (YZRB) is controlled by more vapour transport from oceans. The downstream of the Yellow River basin (YLRB) is affected by the dry and cold northerly winds. Dominance analysis shows that vapour general plays a more important role in two major river basins. Maybe not exactly coincide with someone's envision, not all the places are dominated by the vapour. Around northeast China, temperature contributes little more than vapour.