Is the Isotopic Composition of Precipitation a Robust Indicator for Reconstructions of Past Tropical Cyclones Frequency? A Case Study on Runion Island From Rain and Water Vapor Isotopic Observations

Based on a 6-year long record (2014-2020) of the isotopic composition of rain (delta 18Op) at Reunion Island (55 degrees E, 22 degrees S), in the South-West Indian Ocean, this study shows that the annual isotopic composition of precipitation in this region is strongly controlled by the number of cyclones, the number of best-track days, and the proportion of cyclonic rain during the year. Our results support the use of delta 18Op in annual-resolved tropical climate archives as a reliable proxy of past cyclone frequency. The influence of the proportion of cyclonic rain on the annual isotopic composition arises from the systematically more depleted precipitation and water vapor during cyclonic events than during less organized convective systems. The analysis of the daily to hourly isotopic composition of water vapor (delta 18Ov) during low-pressure systems and the reproduction of daily delta 18Ov observations by AGCMs with a global medium to coarse resolution (LMDZ-iso and ECHAM6-wiso) suggest that during cyclonic periods the stronger depletion mainly arises from both enhanced large-scale precipitation and water vapor-rain interactions under humid conditions. Water molecules have different forms: 2 atoms of hydrogen and 1 atom of oxygen 16 (majority form), or 1 atom of hydrogen, 1 atom of deuterium (2H) and 1 atom of oxygen 16, or 2 atoms of hydrogen and 1 atom of oxygen 18. The relative proportion of these different molecules in precipitation and water vapor is called isotopic composition. We have analyzed the isotopic composition of rain and water vapor at Reunion Island (55 degrees E, 22 degrees S), in the South-West Indian Ocean, for 6 years (2014-2020). We show that the annual isotopic composition of precipitation in this region is a reliable indicator of the number of tropical cyclones (TCs) during the year. This opens the possibility to use annual-resolved tropical climate archives of the isotopic composition of precipitation, like speleothems, to study how cyclone frequency in the Indian Ocean has varied in the past under different mean climates (warmer or colder). This study also seeks to understand why the isotopic composition of precipitation behaves in this way during TCs. We show that this is due to increased precipitation at the regional scale and enhanced exchanges of water molecules between rain and water vapor in the vicinity of TCs. The annual isotopic composition of precipitation on Reunion Island is controlled by the number of cyclones per year Both precipitation and water vapor are systematically more depleted during cyclonic periods More large-scale precipitation and rain-vapor interactions are responsible for the stronger isotopic depletion during tropical cyclones (TCs)