Importance of boundary layer mixing for the isotopic composition of surface vapor over the subtropical North Atlantic Ocean

During the summer 2012, we carried out continuous measurements of the isotopic composition (δ) of water vapor over the near-surface subtropical North Atlantic Ocean (STRASSE cruise). In this region of excess evaporation, we investigate the control of evaporation and mixing with a lower troposphere-derived, isotopically depleted air mass on the near-surface δ. We use a simple model to simulate the near-surface δ as the result of a two end-member mixing of the evaporative flux with free tropospheric air. The evaporative flux δ was estimated by the Craig and Gordon equation while the δ of the lower troposphere was taken from the LMDZ-iso global atmospheric circulation model. This simulation considers instantaneous mixing of lower tropospheric air with the evaporated flux and neglects lateral advection. Despite these simplifications, the simulations allow to identify the controls on the near-surface δ. The d-excess variability is largely a consequence of varying kinetic effects during evaporation, even during a convection event when the input of tropospheric vapor was strong. Kinetic effects and mixing processes affect simultaneously the near-surface δ and result in the vapor occupying distinct domains in the δ 18 O-δD space. The relative humidity-d-excess relationship shows that the closure assumption overestimates the d-excess variability at short time scales (less than a day). We interpret this as due to an effect of the residence time of the near-surface water vapor on the d-excess. Finally, we highlight the importance of reproducing mixing processes in models simulating isotopes over the subtropical North Atlantic Ocean and propose an extension of the closure assumption for use in initial conditions of distillation calculations.

[1]  Yohei Matsui,et al.  Evidence of deuterium excess in water vapor as an indicator of ocean surface conditions , 2008 .

[2]  J. Horita,et al.  Liquid-vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature , 1994 .

[3]  S. Bony,et al.  Spread in model climate sensitivity traced to atmospheric convective mixing , 2014, Nature.

[4]  L. Araguás‐Araguás,et al.  Deuterium and oxygen‐18 isotope composition of precipitation and atmospheric moisture , 2000 .

[5]  E. Barkan,et al.  High precision measurements of 17O/16O and 18O/16O ratios in H2O. , 2005, Rapid communications in mass spectrometry : RCM.

[6]  Naoyuki Kurita,et al.  Water isotopic variability in response to mesoscale convective system over the tropical ocean , 2013 .

[7]  P. Courtier,et al.  The ECMWF implementation of three‐dimensional variational assimilation (3D‐Var). I: Formulation , 1998 .

[8]  M. Majoube.  Fractionnement en oxygène 18 et en deutérium entre l’eau et sa vapeur , 1971 .

[9]  E. F. Bradley,et al.  Bulk parameterization of air‐sea fluxes for Tropical Ocean‐Global Atmosphere Coupled‐Ocean Atmosphere Response Experiment , 1996 .

[10]  H. Wernli,et al.  Lagrangian simulations of stable isotopes in water vapor: An evaluation of nonequilibrium fractionation in the Craig-Gordon model , 2009 .

[11]  Kenneth P. Bowman,et al.  Stable isotopic composition of water vapor in the tropics , 2004 .

[12]  H. Craig.  Isotopic Variations in Meteoric Waters , 1961, Science.

[13]  J. Jouzel,et al.  A seasonal deuterium excess signal at Law Dome, coastal eastern Antarctica: A southern ocean signature , 2000 .

[14]  P. Ciais,et al.  Deuterium and oxygen 18 in precipitation: Isotopic model, including mixed cloud processes , 1994 .

[15]  B. Albrecht.  Aerosols, Cloud Microphysics, and Fractional Cloudiness , 1989, Science.

[16]  S. Bony,et al.  Understanding the 17O excess glacial‐interglacial variations in Vostok precipitation , 2010 .

[17]  R. Koster,et al.  A reconsideration of the initial conditions used for stable water isotope models , 1996 .

[18]  S. Bony,et al.  The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection , 2006 .

[19]  M. Derrien,et al.  MSG/SEVIRI cloud mask and type from SAFNWC , 2005 .

[20]  B. Stevens,et al.  Using the Sensitivity of Large-Eddy Simulations to Evaluate Atmospheric Boundary Layer Models , 2012 .

[21]  Dorthe Dahl-Jensen,et al.  Oxygen isotope and palaeotemperature records from six Greenland ice‐core stations: Camp Century, Dye‐3, GRIP, GISP2, Renland and NorthGRIP , 2001 .

[22]  Charles D. Keeling,et al.  The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas , 1958 .

[23]  G. Favreau,et al.  A 1‐year longδ18O record of water vapor in Niamey (Niger) reveals insightful atmospheric processes at different timescales , 2012 .

[24]  H. Sodemann,et al.  What controls deuterium excess in global precipitation , 2013 .

[25]  J. Gat.  OXYGEN AND HYDROGEN ISOTOPES IN THE HYDROLOGIC CYCLE , 1996 .

[26]  S. Bony,et al.  Influence of convective processes on the isotopic composition (δ18O and δD) of precipitation and water vapor in the tropics: 2. Physical interpretation of the amount effect , 2008 .

[27]  R. Reynolds,et al.  The NCEP/NCAR 40-Year Reanalysis Project , 1996, Renewable Energy.

[28]  Heini Wernli,et al.  Isotope composition of air moisture over the Mediterranean Sea: an index of the air-sea interaction pattern , 2003 .

[29]  Jean Jouzel,et al.  A 420,000 year deuterium excess record from East Antarctica: Information on past changes in the origin of precipitation at Vostok , 2001 .

[30]  Hans-Christian Steen-Larsen,et al.  Climatic controls on water vapor deuterium excess in the marine boundary layer of the North Atlantic based on 500 days of in situ, continuous measurements , 2014 .

[31]  H. Oeschger,et al.  North Atlantic climatic oscillations revealed by deep Greenland ice cores , 2013 .

[32]  G. Schmidt,et al.  Intraseasonal isotopic variation associated with the Madden-Julian Oscillation , 2011 .

[33]  J. Jouzel,et al.  Global Climatic Interpretation of the Deuterium-Oxygen 18 Relationship , 1979 .

[34]  Jiuhua Feng,et al.  Numerical Simulations of Airflow and Cloud Distributions over the Windward Side of the Island of Hawaii. Part I: The Effects of Trade Wind Inversion* , 2001 .

[35]  K. Yoshimura,et al.  Influence of synoptic weather events on the isotopic composition of atmospheric moisture in a coastal city of the western United States , 2013 .

[36]  W. Dansgaard.  Stable isotopes in precipitation , 1964 .

[37]  Jonathon S. Wright,et al.  Properties of air mass mixing and humidity in the subtropics from measurements of the D/H isotope ratio of water vapor at the Mauna Loa Observatory , 2011 .

[38]  Sandrine Bony,et al.  Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records , 2010 .

[39]  R. Bradley,et al.  Stable isotopes in precipitation in the Asian monsoon region , 2005 .

[40]  H. Craig,et al.  Deuterium and oxygen 18 variations in the ocean and marine atmosphere , 1965 .

[41]  C. Brenninkmeijer,et al.  The impact of the chemical production of methyl nitrate from the NO + CH 3 O 2 reaction on the global distributions of alkyl nitrates, nitrogen oxides and tropospheric ozone: a global modelling study , 2013 .

[42]  Harald Sodemann,et al.  Continuous monitoring of summer surface water vapor isotopic composition above the Greenland Ice Sheet , 2013 .

[43]  Hans-Christian Steen-Larsen,et al.  Deuterium excess in marine water vapor: Dependency on relative humidity and surface wind speed during evaporation , 2014 .