Isotopic composition (δ18O, δ2H) of the Indian Summer Monsoon in Southern Oman: A model-aided process interpretation

Highlights • Monsoon precipitation was sampled at six sites along an elevation gradient in S-Oman. • Precipitation is isotopically lighter at higher elevations. • A 3-step model shows that the pattern is not caused by the classic elevation effect. • The system is governed by sub-cloud evaporation, causing a pseudo-elevation effect. • This effect may occur in other (semi-arid) areas too. Abstract Study region This study deals with the isotopic composition of monsoon precipitation in Southern Oman, specifically in the Salalah coastal plain and the adjacent Dhofar Mountains. Study focus Monsoon variability on the southern Arabian Peninsula has been described based on the oxygen isotope composition of various terrestrial archives, often on the millennial scale. However, the factors influencing the spatio-temporal variability of the oxygen isotope composition of today's monsoon precipitation have not yet been described for this region. Here, we present stable isotope ratios of oxygen and hydrogen for the Indian Summer Monsoon in the greater Salalah area. New hydrological insights for the region The precipitation amount-weighted mean isotope values along an elevation transect show lighter signatures at higher elevation, but generally low deuterium excess values (d<10 ‰). To trace the origin and (short-term) history of the precipitation system, we applied a 3-step model incorporating evaporation of seawater, condensation, and sub-cloud evaporation. Our modelling exercise indicates that the classic elevation effect is quasi-negligible, and that sub-cloud evaporation is likely to be the main driver behind the encountered isotope pattern. This pseudo-elevation effect implies that the precipitation is not depleted in heavy isotopes with increasing elevation, but rather enriched with decreasing elevation. This effect may prevail in other (semi-arid) areas as well.