Comment on egusphere-2025-5306

Abstract. Rapid Arctic warming and sea ice retreat have increased atmospheric humidity, yet the relative contributions of local evaporation and advected lower-latitude moisture remain poorly quantified. Here, we present high-resolution, ship-based in-situ measurements of near-surface water vapor isotopes across diverse Arctic sea ice regimes. By integrating isotope fractionation models with multi-source meteorological data, we show that sea ice changes act as a key modulator of Arctic water vapor isotopic variations. Under ice-covered conditions, water vapor isotopes are controlled by Rayleigh distillation, producing depleted δ18O with a strong temperature dependence and elevated d-excess from ice-phase processes. As sea ice retreats, kinetic fractionation from local evaporation becomes increasingly important, particularly at temperatures above ~ 5 °C, generating enriched δ18O, elevated d-excess, and a characteristic "anti-temperature" effect. A Bayesian isotope mixing model quantifies the resulting moisture source shift, showing local evaporation contributions rise from 9.3 % in ice-covered regions to 22.7 % in melt regions, despite advected moisture remaining predominant. These findings establish a process-based isotope framework for the Arctic hydrological cycle, complementing conventional meteorological diagnostics and offering a robust benchmark for interpreting paleo-isotope archives.