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

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 δ2H and −18.10 ± 2.87‰ for δ18O, 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. Plain Language Summary The sea area between Iceland, Greenland, and Fram Strait experiences strongly variable weather conditions during wintertime. Often, cold air sweeps over open waters, leading to intense extraction of heat and water from the ocean. Also opposite conditions can occur, where air from warmer and more humid mid‐latitudes extends northward, reaches saturation, and loses heat to the underlying surface. During the Iceland‐Greenland Seas Project cruise in February and March 2018, we measured the stable water isotope composition in water vapor, snow, rain, and the ocean water column at different depths. Using these measurements and a regional weather prediction model capable of simulating the isotopic composition, we find indications for an imprint of the atmospheric evaporation that is transferred into the mixed layer of the ocean over longe