Humidity effect and its influence on the seasonal distribution of precipitation δ18O in monsoon regions

Abstract

The humidity effect, namely the markedly positive correlation between the stable isotopic ratio in precipitation and the dew-point deficit ΔTd in the atmosphere, is put forward firstly and the relationships between the δ18O in precipitation and ΔTd are analyzed for the Urumqi and Kunming stations, which have completely different climatic characteristics. Although the seasonal variations in δ18O and ΔTd exhibit differences between the two stations, their humidity effect is notable. The correlation coefficient and its confidence level of the humidity effect are higher than those of the amount effect at Kunming, showing the marked influence of the humidity conditions in the atmosphere on stable isotopes in precipitation. Using a kinetic model for stable isotopic fractionation, and according to the seasonal distribution of mean monthly temperature at 500 hPa at Kunming, the variations of the δ18O in condensate in cloud are simulated. A very good agreement between the seasonal variations of the simulated mean δ18O and the mean monthly temperature at 500 hPa is obtained, showing that the oxygen stable isotope in condensate of cloud experiences a temperature effect. Such a result is markedly different from the amount effect at the ground. Based on the simulations of seasonal variations of δ18O in falling raindrops, it can be found that, in the dry season from November to April, the increasing trend with falling distance of δ18O in falling raindrops corresponds remarkably to the great ΔTd, showing a strong evaporation enrichment function in falling raindrops; however, in the wet season from May to October, the δ18O in falling raindrops displays an unapparent increase corresponding to the small ΔTd, except in May. By comparing the simulated mean δ18O at the ground with the actual monthly δ18O in precipitation, we see distinctly that the two monthly δ18O variations agree very well. On average, the δ18O values are relatively lower because of the highly moist air, heavy rainfall, small ΔTd and weak evaporation enrichment function of stable isotopes in the falling raindrops, under the influence of vapor from the oceans; but they are relatively higher because of the dry air, light rainfall, great ΔTd and strong evaporation enrichment function in falling raindrops, under the control of the continental air mass. Therefore, the δ18O in precipitation at Kunming can be used to indicate the humidity situation in the atmosphere to a certain degree, and thus indicate the intensity of the precipitation and the strength of the monsoon indirectly. The humidity effect changes not only the magnitude of the stable isotopic ratio in precipitation but also its seasonal distribution due to its influence on the strength of the evaporation enrichment of stable isotopes in falling raindrops and the direction of the net mass transfer of stable isotopes between the atmosphere and the raindrops. Consequently, it is inferred that the humidity effect is probably one of the foremost causes generating the amount effect.