Surface processes alter the water stable isotope signal of the surface snow after deposition. However, it remains an open question to which extent surface post-depositional processes should be considered when inferring past climate information from ice core records. Here, we present simulations for the Greenland Ice Sheet, combining outputs from two climate models with an isotope-enabled snowpack model. We show that surface vapor exchange and associated fractionation imprint a climate signal into the firn, resulting in an increase in the annual mean value of delta 18O by +2.3 parts per thousand and a reduction in d-excess by -6.3 parts per thousand. Further, implementing isotopic fractionation during surface vapor exchange improves the representation of the observed seasonal amplitude in delta 18O from 65.0% to 100.2%. Our results stress that surface vapor exchange is important in the climate proxy signal formation and needs consideration when interpreting ice core climate records.|The climate information contained in falling snow is modified by exchange processes with the atmosphere after the snow has fallen to the surface. It is important to understand how this modification affects the interpretation of past climate information from ice core isotope records. In this study, we combined outputs from two climate models to simulate the climate signal in a snow core on the Greenland Ice Sheet. We evaluate the snow core model using snow observations from the Greenland Ice Sheet. By simulating snow cores with and without the modification at the surface, we find a considerable impact of the surface modification on the climate signal in the snow core. Further, considering the surface modification causes an improved representation of the seasonal changes compared to observations. Our findings highlight the importance of surface processes in forming climate information contained in ice cores and underscore the need to include these processes in the ice core interpretation.|Water isotopic fractionation during vapor exchange significantly affects the simulated annual and seasonal isotope climate signal in ice coresThe simulated seasonal amplitude of the delta 18O signal in the snowpack improves when including surface vapor exchange induced fractionationA phase shift in the simulated seasonal maximum in d-excess toward early autumn is induced by vapor exchange, consistent with observations