Abrupt excursion in water vapor isotopic variability during cold fronts at the Pointe Benedicte observatory in Amsterdam Island

Abstract. In order to complement the picture of the atmospheric water cycle in the Southern Ocean, we have continuously monitored water vapor isotopes since January 2020 in Amsterdam Island (37.7983 °S, 77.5378 °E) in the Indian Ocean. We present here the first 2-year-long water vapor isotopic record monitored on this site. We show that the vapor isotopic composition largely follows the vapor mixing ratio, as expected in marine boundary layers. However, we evidence 11 cold front periods of a few days where there is a strong loss of correlation between water vapor δ¹⁸O and mixing ratio. These periods are associated with abrupt negative excursions of water vapor δ¹⁸Ο, often occurring toward the end of precipitation events. Six of these events show a decrease in gaseous elemental mercury suggesting subsidence of air from higher altitude.

Accurately representing the water isotopic signal during these cold fronts is a real challenge for the atmospheric components of Earth System models equipped with water isotopes. While the ECHAM6-wiso model was able to reproduce most of the sharp negative water vapor δ¹⁸O excursions, the LMDZ-iso model at 2° (3°) resolution was only able to reproduce 7 (1) of the negative excursions. Based on a detail model-data comparison, we conclude that the most plausible explanations for such isotopic excursions are rain-vapor interactions associated with subsidence at the rear of a precipitation event.