The Cumulative Effect of Wintertime Weather Systems on the Ocean Mixed‐Layer Stable Isotope Composition in the Iceland and Greenland Seas

Abstract The Iceland and Greenland Seas are characterized by strong heat fluxes from the ocean to the atmosphere during wintertime. Here we characterize the atmospheric signal of this strong evaporation in terms of water vapor isotopes and investigate if such a signal can have a cumulative imprint on the ocean mixed‐layer. Observations include continuous water vapor isotope measurements, event‐based precipitation samples, and sea‐water samples taken at various depths from the research vessel Alliance during the Iceland‐Greenland Seas Project cruise in February and March 2018. In conjunction with a simulation from a regional, isotope‐enabled atmospheric model, we find that the predominant atmospheric isotope signature during predominant marine cold‐air outbreak conditions is −129.8 ± 16.6‰ for δ 2 H and −18.10 ± 2.87‰ for δ 18 O, with a d‐excess of 15.1 ± 7.9‰, indicating enhanced non‐equilibrium fractionation compared to the global average. During events of warm‐air intrusion from mid‐latitudes, near‐surface vapor becomes saturated and the vapor d‐excess approaches equilibrium or becomes negative. Similarly, precipitation d‐excess is lower and thus closer to equilibrium conditions during warm‐air intrusions. There are indications that an evaporation signal of waters exiting the Nordic Seas through Denmark Strait could be locally enhanced over seasons to years, as supported by simple model calculations. Our findings thus suggest that evaporation signals could be transferred into the ocean isotope composition in this region, potentially enabling mass‐balance constraints in isotope‐enabled coupled ocean‐atmosphere models.