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 ΔT _d in the atmosphere, is put forward firstly and the relationships between the δ¹⁸O in precipitation and ΔT _d are analyzed for the Ürümqi and Kunming stations, which have completely different climatic characteristics. Although the seasonal variations in δ¹⁸O and ΔT _d 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 ΔT _d, 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 ΔT _d, 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 ΔT _d 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 ΔT _d 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.