Abrupt excursion in water vapor isotopic variability during cold fronts at 1 the Pointe Benedicte observatory in Amsterdam Island 2

31 In order to complement the picture of the atmospheric water cycle in the Southern Ocean, we 32 have continuously monitored water vapor isotopes since January 2020 in Amsterdam Island 33 (37.7983 °S, 77.5378 °E) in the Indian Ocean. We present here the first 2-year-long water 34 vapor isotopic record monitored on this site. We show that the vapor isotopic composition 35 largely follows the vapor mixing ratio, as expected in marine boundary layers. However, we 36 evidence 11 cold front periods of a few days where there is a strong loss of correlation 37 between water vapor δ 18 O and mixing ratio. These periods are associated with abrupt negative 38 excursions of water vapor δ 18 Ο, often occurring toward the end of precipitation events. Six of 39 these events show a decrease in gaseous elemental mercury suggesting subsidence of air from 40 higher altitude. 41 Accurately representing the water isotopic signal during these cold fronts is a real challenge 42 for the atmospheric components of Earth System models equipped with water isotopes. While 43 the ECHAM6-wiso model was able to reproduce most of the sharp negative water vapor  18 O 44 excursions, the LMDZ-iso model at 2° (3°) resolution was only able to reproduce 7 (1) of the 45 negative excursions. Based on a detail model-data comparison, we conclude that the most 46 plausible explanations for such isotopic excursions are rain-vapor interactions associated with 47 subsidence at the rear of a precipitation event. 48