Stable water vapour isotopes as integrated tracers of moisture sources, transport and deposition during warm air intrusions in the Arctic

Air-mass intrusions arriving from the mid-latitudes introduce moisture and heat into the Arctic and perturb cloud properties. These events have a strong impact on the water cycle as their frequency and intensity control the inter-annual variability of mean surface air temperature, humidity and energy budget. Warm air intrusions are all short-lived events related to blocking situations of the large-scale circulation, however, the characteristics of each individual air intrusion depend on the season, the sourcing of the air masses, the characteristics of the boundary layer and the surface conditions during the long-range transport.

In this study, we use atmospheric water vapour isotopes (H₂¹⁶O, H₂¹⁸O, HD¹⁶O) to trace the origin of the moisture and to gain insights into the exchange processes occurring during four distinct warm air intrusion events, recorded during a one-year expedition in the Central Arctic. Stable water isotopes can track feedback loops and exchange processes between the hydrological compartments of the Arctic, because evaporative sources, phase changes and interactions within hydrological compartments all have specific imprints on the isotopic compositions. Continuous observations of near-surface atmospheric vapour were obtained onboard RV Polarstern during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) drifting expedition in 2019-2020. By combining a moisture source diagnostic to the particle dispersion model FLEXPART, we constrain the magnitude and the location of the surface moisture uptake into the air masses.

We found that the moisture transported during the events originated from different locations, namely lower North Atlantic sector (<70°N), upper North Atlantic (>70°N), continental Siberia and sea-ice. The different evaporative conditions over these regions are key to determine the distinct isotopic signature of the sampled air masses. Further, we observe opposite sensitivity of d-excess to local temperature and humidity in the moisture sourced from the sea-ice. D-excess is a second order isotope parameter interpreted as a diagnostic of non-equilibrium fractionation. We further investigate the mechanisms leading to non-equilibrium phase changes and we examine the roles of: (i) mixed-phase cloud formation where water vapour is supersaturated with respect to ice, (ii) evaporation from leads and melt ponds, and (iii) changes in vapour isotopes with respect to snow on sea ice during sublimation/deposition regimes.

With this work we aim at better understanding the transport of mid-latitudes moisture into the Central Arctic region and identifying the moisture exchange processes with the Arctic cryosphere. In view of the projected increase of frequency and duration of warm air intrusions in the Arctic, our study contributes to understanding the mechanistic consequences of such short-lived events on the whole Arctic water cycle.