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Discussion papers
https://doi.org/10.5194/acp-2019-782
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-782
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 26 Sep 2019

Submitted as: research article | 26 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Meridional and vertical variations of the water vapour isotopic composition in the marine boundary layer over the Atlantic and Southern Ocean

Iris Thurnherr1, Anna Kozachek2, Pascal Graf1, Yongbiao Weng3,4, Dimitri Bolshiyanov2, Sebastian Landwehr5, Stephan Pfahl1,6, Julia Schmale5, Harald Sodemann3,4, Hans Christian Steen-Larsen3,4, Alessandro Toffoli7, Heini Wernli1, and Franziska Aemisegger1 Iris Thurnherr et al.
  • 1Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland
  • 2Climate and Environmental Research Laboratory, Arctic and Antarctic Research Institute, St Petersburg, Russia
  • 3Geophysical Institute, University of Bergen, Bergen, Norway
  • 4Bjerknes Centre for Climate Research, Bergen, Norway
  • 5Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland
  • 6Institute of Meteorology, Freie Universität Berlin, Berlin, Germany
  • 7Department of Infrastructure Engineering, The University of Melbourne, Melbourne, Australia

Abstract. Stable water isotopologues (SWIs) are useful tracers of moist diabatic processes in the atmospheric water cycle. They provide a framework to analyse moist processes on a range of time scales from large-scale moisture transport to cloud formation, precipitation, and small-scale turbulent mixing. Laser spectrometric measurements on research vessels produce high-resolution time series of the variability of the water vapour isotopic composition in the marine boundary layer. In this study, we present a five-month continuous time series of such ship-based measurements of δ2H and δ18O from the Antarctic Circumnavigation Expedition (ACE) in the Atlantic and the Southern Ocean in the time period from November 2016 to April 2017. We analyse the drivers of meridional SWI variations in the marine boundary layer across diverse climate zones in the Atlantic and Southern Ocean using Lagrangian moisture source diagnostics and relate vertical SWI differences to near-surface wind speed and ocean surface state. The median values of δ18O, δ2H and d-excess during ACE decrease continuously from low to high latitudes. These meridional SWI distributions reflect climatic conditions at the measurement and moisture source sites, such as air temperature, specific humidity, and relative humidity with respect to sea surface temperature. The SWI variability at a given latitude is highest in the extratropics and polar regions with decreasing values equatorwards. This meridional distribution of SWI variability is explained by the variability in moisture source locations and its associated environmental conditions as well as transport processes. The westward located moisture sources of water vapour in the extratropics are highly variable in extent and latitude due to the frequent passage of cyclones and thus widen the range of encountered SWI values in the marine boundary layer. Moisture loss during transport further contributes to the high SWI variability in the extratropics. In the subtropics and tropics, persistent anticyclones lead to well-confined narrow easterly moisture source regions, which is reflected in the low SWI variability in these regions. Thus, the expected range of SWI signals at a given latitude strongly depends on the large-scale circulation. Furthermore, the ACE SWI time series recorded at different heights above the ocean surface provide estimates of vertical SWI gradients in the lowermost marine boundary layer. On average, the vertical gradients with height found during ACE are −0.1 ‰ m−1 for δ18O, −0.5 ‰ m−1 for δ2H and 0.3 ‰ m−1 for d-excess. Careful calibration and post-processing of the SWI data and a detailed uncertainty analysis provide a solid basis for the presented gradients. Using sea spray concentrations and sea state conditions, we show that the vertical SWI gradients are particularly large during high wind speed conditions with increased contribution of sea spray evaporation or during low wind speed conditions due to weak vertical turbulent mixing and dominating effects from non-equilibrium fractionation. Although further SWI measurements at a higher vertical resolution are required to validate these findings, the simultaneous SWI measurements at several heights during ACE show the potential of SWIs as tracers for vertical mixing and sea spray evaporation in the lowermost marine boundary layer.

Iris Thurnherr et al.
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Iris Thurnherr et al.
Data sets

Raw stable water isotope measurements in water vapour, made in the austral summer of 2016/2017 during the Antarctic Circumnavigation Expedition I. Thurnherr and F. Aemisegger https://doi.org/10.5281/zenodo.3250692

Iris Thurnherr et al.
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Short summary
Stable water isotopes (SWIs) are tracers of moist atmospheric processes. We analyse the impact of large- to small-scale atmospheric processes and various environmental conditions on the variability of SWIs using ship-based SWI measurement in water vapour from the Atlantic and Southern Ocean. Furthermore, simultaneous measurements of SWIs at two altitudes are used to illustrate the potential of such measurements for future research to estimate sea spray evaporation and turbulent moisture fluxes.
Stable water isotopes (SWIs) are tracers of moist atmospheric processes. We analyse the impact...
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