Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (δ18O, δ2H, d-excess) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 (n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where δ2H = 7.6⋅δ18O–1.8 (r2 = 0.96, p < 0.01). Mean amount-weighted δ18O, δ2H, and d-excess values were −12.3, −93.5, and 4.9‰, respectively, with the lowest summer mean δ18O value observed in northwest Greenland (−19.9‰) and the highest in Iceland (−7.3‰). Southern Alaska recorded the lowest mean d-excess (−8.2%) and northern Russia the highest (9.9‰). We identify a range of δ18O-temperature coefficients from 0.31‰/°C (Alaska) to 0.93‰/°C (Russia). The steepest regression slopes (>0.75‰/°C) were observed at continental sites, while statistically significant temperature relations were generally absent at coastal stations. Model outputs indicate that 68% of the summer precipitating air masses were transported into the Arctic from mid-latitudes and were characterized by relatively high δ18O values. Yet 32% of precipitation events, characterized by lower δ18O and high d-excess values, derived from northerly air masses transported from the Arctic Ocean and/or its marginal seas, highlighting key emergent oceanic moisture sources as sea ice cover declines. Resolving these processes across broader spatial-temporal scales is an ongoing research priority, and will be key to quantifying the past, present, and future feedbacks of an amplified Arctic water cycle on the global climate system.

The Arctic’s winter water cycle is rapidly changing, with implications for snow moisture sources and transport processes. Stable isotope values (δ18O, δ2H, d-excess) of the Arctic snowpack have potential to provide proxy records of these processes, yet it is unclear how well the isotope values of individual snowfall events are preserved within snow profiles. Here, we present water isotope data from multiple taiga and tundra snow profiles sampled in Arctic Alaska and Finland, respectively, during winter 2018–2019. We compare the snowpack isotope stratigraphy with meteoric water isotopes (vapor and precipitation) during snowfall days, and combine our measurements with satellite observations and reanalysis data. Our analyses indicate that synoptic-scale atmospheric circulation and regional sea ice coverage are key drivers of the source, amount, and isotopic composition of Arctic snowpacks. We find that the western Arctic tundra snowpack profiles in Alaska preserved the isotope values for the most recent storm; however, post depositional processes modified the remaining isotope profiles. The overall seasonal evolution in the vapor isotope values were better preserved in taiga snow isotope profiles in the eastern Arctic, where there is significantly less wind-driven redistribution than in the open Alaskan tundra. We demonstrate the potential of the seasonal snowpack to provide a useful proxy for Arctic winter-time moisture sources and propose future analyses.

<p>Stable isotopes of oxygen and hydrogen in precipitation (δ<sup>18</sup>O<sub>P</sub>, δ<sup>2</sup>H<sub>P</sub>, d-excess) are valuable hydrological tracers linked to ocean-atmospheric processes such as moisture source, storm trajectory, and seasonal temperature cycles. However, characteristics of δ<sup>18</sup>O<sub>P</sub>, δ<sup>2</sup>H<sub>P</sub> and d-excess and the processes governing them are yet to be quantified across the Arctic due to a lack of long-term empirical data. The Pan-Arctic Precipitation Isotopes Network (PAPIN) is a new coordinated network of 24 stations aimed at the direct sampling, analysis, and synthesis of precipitation isotope geochemistry in the north. Our ongoing event-based sampling provides a rich spatial dataset during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (“MOSAiC”) expedition and new insight into coupled climate processes operating in the Arctic today. To date, precipitation δ<sup>18</sup>O and δ<sup>2</sup>H data (2018-2019) exhibit pronounced spatial and seasonal variability that broadly conforms to theoretical and observed understanding: (1) decreasing δ<sup>18</sup>O<sub>P</sub>/ δ<sup>2</sup>H<sub>P</sub> with increasing latitude and elevation, (2) decreasing δ<sup>18</sup>O<sub>P</sub>/ δ<sup>2</sup>H<sub>P</sub> with increasing continentality, and (3) increasing δ<sup>18</sup>O<sub>P</sub>/ δ<sup>2</sup>H<sub>P</sub> with increasing SAT. However, event-based sampling reveals remarkable variability among these relationships. For example, our observed Arctic mean summer -latitude slope of -0.3‰/degree of latitude is 50% smaller than the annual latitude effect in the mid-latitudes (-0.6‰/degree). This rate decreases to -0.1‰/degree of latitude in Finland and Russia, while in Alaska and northern Canadian a -0.7‰/degree latitudinal rate is observed. Similarly, we observe marked spatial differences in mean δ<sup>18</sup>O-temperature coefficients. Using back-trajectory analysis, we attribute these nuances to divergent moisture sources and transport pathways into, within, and out of the Arctic, and demonstrate how atmospheric circulation processes drive changes in isotope geochemistry and climate that are linked to sea ice concentration. For example, Alaska moisture derived from the North Pacific Ocean, Sea of Okhotsk, and the Bering Sea remains relatively enriched in <sup>18</sup>O<sub>P</sub>/<sup>2</sup>H due to higher sea surface temperatures, whereas moisture originating from ice-covered seas to the north is characterized by relatively depleted values. This is the first coordinated network to quantify the spatial patterns of isotopes in precipitation, simultaneously, across the entire Arctic. In combination with a Pan-Arctic network of continuous water vapor isotope analyzers, our process-level studies will resolve the patterns and processes governing the δ<sup>18</sup>O, δ<sup>2</sup>H and d-excess values of the Arctic water cycle during the MOSAiC expedition and beyond.</p>

<p>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.</p> <p>In this study, we use atmospheric water vapour isotopes (H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, HD<sup>16</sup>O) 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.</p> <p>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.</p> <p>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. </p>

<p>Stable isotope ratios (δ<sup>18</sup>O and δ<sup>2</sup>H) in precipitation (<sub>P</sub>) and atmospheric water vapor (<sub>V</sub>) can provide mechanistic information about water cycle processes such as moisture evaporation, transport and recycling dynamics. Such insight is valuable in the Arctic where declining sea ice is amplifying atmospheric temperature and humidity, leading to complex seasonal patterns of synoptic climate and atmospheric moisture transport. Here, we present two years of continuous water vapor isotope data from Pallas-Yllästunturi National Park, northern Finland, to investigate moisture source and transport processes in the Barents Region of the Arctic. High-resolution (1-sec) measurements obtained between December 2017 and December 2019 are coupled with on-site automated weather station data – including air temperature, humidity, solar flux, wind speed and direction – as well as event-based precipitation sampling and stable isotope data over the same interval. Over the two-years, mean vapor δ<sup>18</sup>O<sub>V</sub>, δ<sup>2</sup>H<sub>V </sub>and <em>d-excess</em><sub>V</sub> values are -24.50‰, -181.49‰ and 14.49‰, respectively. These values are strongly correlated and define a local vapor line for Pallas where δ<sup>2</sup>H<sub>V </sub>= 7.6 x δ<sup>18</sup>O<sub>V</sub> + 5.9 (R<sup>2</sup>=0.98). We observe a mean offset of 10.9 ‰ between Pallas δ<sup>18</sup>O<sub>V </sub>and δ<sup>18</sup>O<sub>P</sub>, and <em>d-excess</em> is -4.8 ‰ lower in δ<sup>18</sup>O<sub>P</sub>. There is a larger offset between vapor and precipitation <em>d-excess</em> during summer (-8.4‰) compared to winter (0.1‰) that may reflect varying fractionation coefficients between solid and liquid cloud-precipitation phases. The timeseries exhibits strong seasonality characterized by lower δ<sup>18</sup>O<sub>V</sub>/δ<sup>2</sup>H<sub>V </sub>and higher <em>d-excess</em> during winter, and the reverse during summer. In winter these broad patterns are primarily driven by synoptic-scale processes that influence the source and transport pathway of atmospheric moisture, and three dominant oceanic evaporative source regions are identified: the Barents, Norwegian, and Baltic Seas. Yet on diurnal timescales we observe distinct summer diel cycles that correlate with local fluctuations in specific humidity (q). These seasonal relationships are explored in context of spatial-temporal patterns in sea ice and snow cover distribution, as well as evapotranspiration processes across northern Eurasia. Finally, to better understand how current changes in the Arctic hydrologic cycle relate to inherent variability of the polar jet stream and related synoptic-scale weather, our isotope data are examined in context of dynamic circulation modes of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO).</p>