The isotopic composition of near‐surface water vapor at the Maïdo observatory (Reunion Island, southwestern Indian Ocean) documents the controls of the humidity of the subtropical troposphere

We present a 1 year long record of the isotopic composition of near-surface water vapor (δ18Ov) at the Maido atmospheric observatory (Reunion Island, Indian Ocean, 22°S, 55°E) from 1 November 2014 to 31 October 2015, using wavelength-scanned cavity ring down spectroscopy. Except during cyclone periods where δ18Ov is highly depleted (−20.5‰), a significant diurnal variability can be seen on both δ18Ov and qv with enriched (depleted) water vapor (mean δ18Ov is −13.4‰ (−16.6‰)) and moist (dry) conditions (mean qv is 9.7 g/kg (6.4 g/kg)) during daytime (nighttime). We show that δ18Ov diurnal cycle arises from mixing processes for 65% of cases with two distinct sources of water vapor. We suggest that δ18Ov diurnal cycle is controlled by an interplay of thermally driven land-sea breezes and upslope-downslope flows, bringing maritime air to the observatory during daytime, whereas at night, the observatory is above the atmospheric boundary layer and samples free tropospheric air. Interestingly, δ18Ov record also shows that some nights (15%) are extremely depleted (mean δ18Ov is −21.4‰). They are among the driest of the record (mean qv is 2.9 g/kg). Based on different modeling studies, we suggest that extreme nocturnal isotopic depletions are caused by large-scale atmospheric transport and subsidence of dry air masses from the upper troposphere to the surface, induced by the subtropical westerly jet.