Changes of atmospheric water vapour isotopes in the Arctic at the interface with sea ice and open ocean

Evaporation from the increasingly ice-free Arctic ocean causes moistening of the atmosphere and serves as an unprecedent water source for the Northern Hemisphere. Atmospheric transport of moisture and its interaction with the other Arctic hydrological compartments can be tracked by primary and secondary water isotope parameters. We present observations of atmospheric humidity, δ18O, δD and d-excess, obtained from a cavity-ring-down spectrometer installed on RV Polarstern and operated continuously during the MOSAiC expedition. The dataset reveals a clear seasonal cycle of the atmospheric water vapour; positive correlation is found both with local specific humidity and air temperature. The comparison of synoptical events, characterized by abrupt isotopic fluctuations, with simultaneous observations from land-based Arctic stations indicates a strong influence of sea ice coverage on the isotopic signal. For an in-depth understanding of the isotopic changes, the observations are compared to results of an isotope-enhanced ECHAM6 atmosphere simulation. The model-data comparison assesses the capability of this state-of-the-art AGCM to capture the first-order evaporation/condensation processes and their seasonal evolution. However, a systematic overestimation of winter values and overall decreased variability of modeled values is found. Investigation of such discrepancies may help to identify deficits in the representation of the fine-scale exchange processes characterizing the central-Arctic water cycle

Atmospheric water vapour isotopes in the Arctic at the interface with sea ice and open ocean

Due to the recent and severe downtrend in sea ice coverage, Arctic-derived moisture serves as new, increasingly important, water source for the northern hemisphere. Feedback and exchange processes between the different hydrological compartments of the Arctic might be tracked by stable water isotopologues (H216O, H218O, HD16O). This is possible as evaporative sources, phase changes and transport history have a specific imprint on the isotopic compositions. The MOSAiC drift experiment offered the unique possibility to tackle the main hydrological processes occurring in the Central Arctic, covering a complete seasonal cycle, including the understudied Arctic winter. A Cavity Ring Down Spectrometer (CRDS) was installed on board of RV Polarstern and atmospheric humidity, δ18O, δD and d-excess were observed continuously from October 2019 to October 2020. Simultaneously, isotopic changes of water vapour have been measured by international partners at several land-based Arctic stations. A first analysis of the Polarstern isotopic vapour dataset reveals a range of 30‰ (min=-48.4; max=-11.4; mean=-32.4) variations in δ18Ο of atmospheric water vapour. A clear seasonal cycle with the most depleted values occurring in the dry and cold winter months and increasingly enriched values in spring, peaking in August is noticed. Strong, positive correlation is found with both local specific humidity (r2 = 0.87) and air temperature (r2=0.81). Several short-term events on synoptical time scales with abrupt fluctuations in the isotopic composition are detected throughout the entire dataset, especially during the freeze up phase (Oct-Nov) and the transition from frozen conditions to summer melt (Apr-Jun). Preliminary comparison of the Polarstern data with measurements from different Arctic stations indicates a strong influence of sea ice coverage on the isotopic signal. For an in-depth understanding of the observed isotopic changes, we quantitatively compare the measured isotopic signatures with model results from an ECHAM6 atmosphere simulation, which includes explicit water isotope diagnostics. For this simulation, pressure and temperature fields have been nudged to ERA5 data. The model-data comparison assesses the capability of this state-of-the-art AGCM to capture the first-order evaporation/condensation processes and their seasonal evolution. However, both a systematic overestimation of winter values and overall decreased variability of modeled isotope values as compared to the observation is found. Investigation of such discrepancies may help to identify deficits in the representation of the fine-scale exchange processes characterizing the central-Arctic water cycle

Isotopic traits of the Arctic water cycle

The Arctic hydrological cycle undergoes rapid and pronounced changes, including marine and terrestrial ice loss, increased atmospheric humidity, shifting ocean circulation regimes, and changes in the magnitude and frequency of extreme weather events. Stable water isotopes (δ18O, δ2H) and the secondary parameter d-excess can be used to trace the processes within this new evaporative system including the potential feedback of them into the global climate system. However, characteristics of δ18O, δ2H 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 Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition provided a unique opportunity to collect, analyze, and synthesize discrete samples of the different hydrological compartments in the central Arctic, covering a complete seasonal cycle over the course one year. These observations can lead to a new insight into coupled climate processes operating in the Arctic. Here, we present the isotopic traits of more than 1,900 discrete samples (i.e., seawater, sea ice, snow, brine, frost flower, lead ice, ridge ice). We found that: (i) average seawater δ18O of -1.7‰ conforms to observed and modelled isotopic traits of the Arctic Ocean with more depleted seawater closer to the north pole in winter and relatively enriched seawater in lower latitudes in spring; (ii) second year ice is relatively depleted compared to first year ice with average δ18O values of -3.1‰ and -0.7‰, respectively. This might be due to post-depositional exchange processes with snow; (iii) snow has the most depleted isotopic signature among all compartments (mean δ18O=-15.1‰) and a gradual enrichment trend in snow profiles from top to bottom might be partially due to sublimation of deposited snow. Our dataset provides an unprecedented description of the present-day isotopic composition of the Arctic water covering a complete seasonal cycle. We try to assess the relative contribution of snow, sea ice, leads, and melt ponds spatially and temporally on regional and local moisture in the Arctic. This will ultimately contribute to resolve the linkages between sea ice, ocean, and atmosphere during critical transitions from frozen ocean to open water conditions

The isotopic composition of water vapour in the Central Arctic during the MOSAiC campaign: local versus distant-moisture sources.

The Arctic atmosphere has undergone a process of moistening during the past decades. The loss of sea ice has led to enhanced transfer of heat and moisture from the ocean to the lower atmosphere, while strengthening of cyclonic events has enhanced the poleward transport of moisture from lower latitudes. Eventually, the increased humidity of the Arctic air masses serves today as a new, increasingly important source of moisture for the northern hemisphere. Still, to date, the relative contributions of local evaporation versus distant-moisture sources remains uncertain, as well as the processes responsible for exchanges within and between the hydrological compartments of the Arctic. Such uncertainties limit our ability to understand the importance of the Arctic water cycle to global climate change and to project its future. In this study we use atmospheric water vapour isotopes to investigate the origin of the Arctic moisture and assess whether and which relevant changes occur within the coupled ocean-sea ice-atmosphere system (i.e., sea ice, sea water, snow, melt ponds). Stable isotopologues of water (HDO, H218O) have different saturation vapour pressures and molecular diffusivity coefficients in air. These differences lead to isotopic fractionation during each phase change of water, making water isotopes a powerful tracer of the Arctic hydrological cycle. Water vapour humidity, delta-18O, and delta-D have been measured continuously by a Picarro L2140i Cavity Ringdown Spectrometer installed onboard research vessel Polarstern during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition, which took place in the Central Arctic Ocean from October 2019 to September 2020. Our measurements depict a clear seasonal cycle and a strong and significant covariance of delta-18O and delta-D with air temperature and specific humidity. At the synoptic time scale the dataset is characterized by the occurrence of events associated with humidity peaks and abrupt isotopic excursions. We use statistical analysis and backwards trajectories to i) identify the origin of the air masses and the relative contributions of distant vs. locally sourced moisture, and ii) illustrate the isotopic fingerprint of these two distinct moisture contributors and discuss on the source-to-sink processes leading to their differences. Further, the MOSAiC observations are compared to an ECHAM6 simulation, nudged to ERA5 reanalysis data and enabled for water isotope diagnostics. The model-data comparison makes it possible to explore the spatial representativeness of our observations and assess whether the model can correctly simulate the observed isotopic changes. In the future, our observations may serve as a benchmark to test the parametrization of under(mis-)represented fractionation processes such as snow sublimation, evaporation from leads and melt ponds. Our study provides the very first isotopic characterization of the Central Arctic moisture throughout an entire year and contributes to disentangling the influence of local evaporative processes versus large-scale vapour transport on the Arctic moistening

Isotope measurements of the Arctic water cycle and exchange processes between seawater, sea ice, and snow during MOSAiC

For the past two decades, the Arctic water cycle changed rapidly due to surface air temperatures (SATs) increasing at twice the global rate. Terrestrial ice (i.e. Greenland Ice Sheet) and marine sea-ice loss, alterations of ocean circulation patterns, and shifting atmospheric moisture sources and transport are some of the most pronounced changes caused by the Arctic amplification, fostering increased humidity levels. Stable water isotopes (δ18O, δ2H) and the secondary parameter d-excess are valuable tracers for hydrological changes, including how these shifts may affect the global climate system. However, it is only recently that we are using precipitation and water vapor networks to resolve water isotope patterns and processes in the Arctic. However, a fully coordinated study of the entire water cycle attributes year-long including sea ice, ocean water, vapor, and precipitation has until recently has been absent. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition provided a unique opportunity to collect, analyze, and synthesize discrete samples of the different hydrological compartments in the central Arctic, covering a complete one-year seasonal cycle using a combination of ship-based, the pan-Arctic Water Isotope Network (PAPIN). These observations can lead to new insights into coupled ocean-atmosphere climate processes operating in the Arctic, especially during extreme events, sea ice formation, sea ice retreat, and during a dichotomy of synoptic weather patterns over the MOSAiC-year. We present the isotopic traits of more than 2,200 discrete samples (i.e., seawater, sea ice, snow, brines, frost flowers, lead ice, ridge ice, and precipitation) collected during MOSAiC. Snow has the most depleted δ18O values (-16.3 ± 9.1‰; the number of samples N=306), whereas seawater is the most enriched δ18O compartment (-1.5 ± 0.9‰; N=302) of the Arctic water cycle. Precipitation throughout the Arctic Basin varied from -10‰ to -35‰. Snow profiles are gradually enriched in δ18O from top to bottom by ~20‰ partially due to sublimation of deposited snow, as well as snow metamorphism and its effects on the water isotopes. Second-year ice (SYI) is isotopically relatively depleted in δ18O (-4.2 ± 2.6‰; N=200) compared to first-year ice (FYI) (-0.7 ± 2.1‰; N=635) and insulated FYI (i.e. FYI grown at the bottom of SYI) (-1.7 ± 2.4‰; N=214). The latter is likely caused by post-depositional exchange processes with snow. Open water leads (-1.6 ± 2.4‰; N=137) and melt ponds (-2.1 ± 2.7‰; N=109) on the surface of sea ice contribute to the moistening of the atmosphere in the Arctic on a regional scale. Our dataset provides an unprecedented snapshot of the present-day isotopic composition of the Arctic water cycle during an entire year. The coupling of these discrete samples data with the continuous measurements of atmospheric water vapor may shed light on the relative contribution of snow, sea ice, seawater, open water leads, and melt ponds both spatially and temporally to regional and local moisture levels in the Arctic. Stable water isotopes will ultimately contribute to resolving the linkages between sea ice, ocean, and atmosphere during the critical transition from frozen ocean to open water conditions

Isotopic traits of the Arctic water cycle

The Arctic hydrological cycle undergoes rapid and pronounced changes, including alterations in oceanic and atmospheric circulations, and precipitation patterns. Stable water isotopes (δ18O, δ2H, d-excess) can be used to trace these processes including their potential to feedback into the global climate system. The MOSAiC expedition provided a unique opportunity to collect, analyze, and synthesize discrete samples of the different hydrological compartments in the central Arctic, comprising sea ice, seawater, snow, and melt ponds. Here, we present spatio-temporal variations in the isotopic signatures of more than 1,000 water samples. We found that (i) average seawater δ18O of -1.7‰ conforms to observed and modelled isotopic traits of the Arctic Ocean; (ii) second year ice is relatively depleted compared to first year ice with average δ18O values of -3.1‰ and -0.7‰, respectively. This might be due to post-depositional exchange processes with snow, which has the most depleted isotopic signature among all compartments (mean δ18O=-15.1‰). Our dataset provides an unprecedented description of the present-day isotopic composition of the Arctic water covering a complete seasonal cycle. This will ultimately contribute to resolve the linkages between sea ice, ocean, and atmosphere during critical transitions from frozen ocean to open water conditions

Meltwater layer dynamics in a central Arctic lead: Effects of lead width, re-freezing, and mixing during late summer

17 pages, 9 figures, 1 table.-- Data accessibility statement: The data analyzed in this study were mainly retrieved from links below: RINKO profiler-derived variables: https://doi.pangaea.de/10.1594/PANGAEA.945337, water sampling derived variables: https://doi.pangaea.de/10.1594/PANGAEA.945285, meteorological variables: https://doi.org/10.1594/PANGAEA.935267, and MSS profiler-derived variables: https://doi.org/10.1594/PANGAEA.939816. The oxygen isotope data stems from the ISOLAB Facility at AWI in PotsdamLeads play an important role in the exchange of heat, gases, vapour, and particles between seawater and the atmosphere in ice-covered polar oceans. In summer, these processes can be modified significantly by the formation of a meltwater layer at the surface, yet we know little about the dynamics of meltwater layer formation and persistence. During the drift campaign of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), we examined how variation in lead width, re-freezing, and mixing events affected the vertical structure of lead waters during late summer in the central Arctic. At the beginning of the 4-week survey period, a meltwater layer occupied the surface 0.8 m of the lead, and temperature and salinity showed strong vertical gradients. Stable oxygen isotopes indicate that the meltwater consisted mainly of sea ice meltwater rather than snow meltwater. During the first half of the survey period (before freezing), the meltwater layer thickness decreased rapidly as lead width increased and stretched the layer horizontally. During the latter half of the survey period (after freezing of the lead surface), stratification weakened and the meltwater layer became thinner before disappearing completely due to surface ice formation and mixing processes. Removal of meltwater during surface ice formation explained about 43% of the reduction in thickness of the meltwater layer. The remaining approximate 57% could be explained by mixing within the water column initiated by disturbance of the lower boundary of the meltwater layer through wind-induced ice floe drift. These results indicate that rapid, dynamic changes to lead water structure can have potentially significant effects on the exchange of physical and biogeochemical components throughout the atmosphere–lead–underlying seawater systemThis study was supported by the Japan Society for the Promotion of Science (grant numbers: JP18H03745; JP18KK0292; JP17KK0083; JP17H04715; JP20H04345) and by a grant from the Joint Research Program of the Japan Arctic Research NetworkCenter. MM and HM are supported through the German Federal Ministry of Education and Research (grant number 03FO869A). ALW and KS were funded through the UK Natural Environment Research Council (NERC) (Grants No NE/S002596/1 and NE/S002502/1, respectively). ESD was supported by NERC through the EnvEast Doctoral Training Partnership (NE/L002582/1), as well as NERC and the Department for Business, Energy & Industrial Strategy (BEIS) through the UK Arctic Office. EJC was supported by the National Science Foundation (USA) NSF OPP 1821911 and NSF Graduate Research Fellowship. CG was funded through the Spanish funding Agency (AEI) though the grant PCI 2019-111844-2. MMS was funded through NSF OPP-1724467, OPP-1724748, and OPP-2138787. DB was funded through the German funding Agency (DFG) through grant BA1689/4-1With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe

Hydroclimatic Controls on the Isotopic (δ18 O, δ2 H, d-excess) Traits of Pan-Arctic Summer Rainfall Events

Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (delta O-18, delta H-2, 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 delta H-2 = 7.6.delta O-18-1.8 (r(2) = 0.96, p 0.75 parts per thousand/degrees 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 delta O-18 values. Yet 32% of precipitation events, characterized by lower delta O-18 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

Continuous near-surface atmospheric water vapour isotopic composition from Polarstern cruise PS122-4 (MOSAiC)

We present the isotopic dataset of near-surface water vapour, continuously surveyed from the Polarstern research vessel during the Leg 4 of the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC). The isotopic measurements were acquired continuously by a Cavity Ring-Down Spectrometer (CRDS) installed onboard the RV Polarstern; the ambient inlet was located at 29m asl. A custom-designed calibration module, including four water standards, was operated in parallel to the CRDS. The data is quality controlled and the measurements are calibrated against humidity dependency and deviation from the VSMOW standard values. Knowingly affected or erroneous data were removed

Continuous near-surface atmospheric water vapour isotopic composition from Polarstern cruise PS122-5 (MOSAiC)

We present the isotopic dataset of near-surface water vapour, continuously surveyed from the Polarstern research vessel during the Leg 5 of the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC). The isotopic measurements were acquired continuously by a Cavity Ring-Down Spectrometer (CRDS) installed onboard the RV Polarstern; the ambient inlet was located at 29m asl. A custom-designed calibration module, including four water standards, was operated in parallel to the CRDS. The data is quality controlled and the measurements are calibrated against humidity dependency and deviation from the VSMOW standard values. Knowingly affected or erroneous data were removed