Contrasting Seasonal Isotopic Signatures of Near-Surface Atmospheric Water Vapor in the Central Arctic During the MOSAiC Campaign

The Arctic is experiencing unprecedented moistening which is expected to have far-reaching impact on global climate and weather patterns. However, it remains unclear whether this newly sourced moisture originates locally from ice-free ocean regions or is advected from lower latitudes. In this study, we use water vapor isotope measurements in combination with trajectory-based diagnostics and an isotope-enabled atmosphere general circulation model, to assess seasonal shifts in moisture sources and transport pathways in the Arctic. Continuous measurements of near-surface vapor, delta 18O, and delta D were performed onboard RV Polarstern during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition from October 2019 to September 2020. Combining this isotope data set with meteorological observations reveals that the spatiotemporal evolution of delta 18O mimics changes in local temperature and humidity at synoptic to seasonal time scales, while corresponding d-excess changes suggest a seasonal shift in the origin of moisture. Simulation results from the particle dispersion model FLEXPART support these findings, indicating that summer moisture originates from nearby open ocean, while winter moisture comes from more remote sources with longer residence time over sea-ice. Results from a nudged ECHAM6-wiso simulation also indicate that evaporative processes from the ocean surface reproduce summer isotope values, but are insufficient to explain measured winter isotope values. Our study provides the first isotopic characterization of Central Arctic moisture over the course of an entire year, helping to differentiate the influence of local processes versus large-scale vapor transport on Arctic moistening. Future process-based investigations should focus on assessing the non-equilibrium isotopic fractionation during airmass transformation over sea-ice. The Arctic is warming twice as fast as the world's average and its water cycle is undergoing large changes. The Arctic atmosphere is becoming more humid; however, it is unclear whether the moisture is sourced locally from ice-free ocean regions or if the vapor is transported into the Arctic from lower latitudes. To assess the origin of the moisture, we use the isotopic composition of water vapor obtained onboard a research icebreaker drifting across the Central Arctic ocean. Our observations indicate that the spatial-temporal evolution of delta 18O follows the changes in local temperature and humidity. The corresponding deuterium excess signal, which is used as a diagnostic of moisture source conditions, suggests a seasonal shift in the origin of the air masses. Results from a backward trajectory analyses and from an atmosphere general circulation model indeed show that summer moisture originated from the ocean surface near the sampling site, while winter moisture had a longer residence time over the sea-ice making it more prone to experience isotopic fractionation along the transport. Future model simulations should assess the importance of in-cloud microphysics and surface moisture exchanges in winter, while additional field observations are needed to reduce the uncertainties on the isotopic data. We present a year of atmospheric water vapor isotopes measurements obtained on a drift cruise across the frozen Central Arctic oceanSummer moisture mostly originates from the open ocean and its isotopic composition reflects source region's evaporative conditionsECHAM6-wiso cannot reproduce winter moisture's isotopic composition, suggesting non-equilibrium exchanges during transport over sea-ice