Craig–Gordon model validation using stable isotope ratios in water vapor over the Southern Ocean

• Main Authors: S. S. Dar, P. Ghosh, A. Swaraj, A. Kumar
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-10-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/20/11435/2020/acp-20-11435-2020.pdf

<p>The stable oxygen and hydrogen isotopic composition of water vapor over a water body is governed by the isotopic composition of surface water and ambient vapor, exchange and mixing processes at the water–air interface, and the local meteorological conditions. These parameters form inputs to the Craig–Gordon models, used for predicting the isotopic composition of vapor produced from the surface water due to the evaporation process. In this study we present water vapor, surface water isotope ratios and meteorological parameters across latitudinal transects in the Southern Ocean (27.38–69.34 and 21.98–66.8<span class="inline-formula">^∘</span>&thinsp;S) during two austral summers. The performance of Traditional Craig–Gordon (TCG) <span class="cit" id="xref_paren.1">(<a href="#bib1.bibx10">Craig and Gordon</a>, <a href="#bib1.bibx10">1965</a>)</span> and the Unified Craig–Gordon (UCG) <span class="cit" id="xref_paren.2">(<a href="#bib1.bibx16">Gonfiantini et al.</a>, <a href="#bib1.bibx16">2018</a>)</span> models is evaluated to predict the isotopic composition of evaporated water vapor flux in the diverse oceanic settings. The models are run for the molecular diffusivity ratios suggested by <span class="cit" id="xref_text.3"><a href="#bib1.bibx20">Merlivat</a> (<a href="#bib1.bibx20">1978</a>)</span>, <span class="cit" id="xref_text.4"><a href="#bib1.bibx7">Cappa et al.</a> (<a href="#bib1.bibx7">2003</a>)</span> and <span class="cit" id="xref_text.5"><a href="#bib1.bibx24">Pfahl and Wernli</a> (<a href="#bib1.bibx24">2009</a>)</span>, referred to as MJ, CD and PW, respectively, and different turbulent indices (<span class="inline-formula"><i>x</i></span>), i.e., fractional contribution of molecular vs. turbulent diffusion. It is found that the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="normal">UCG</mi><mrow><mi>x</mi><mo>=</mo><mn mathvariant="normal">0.8</mn></mrow><mi mathvariant="normal">MJ</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="48pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="7a72526eb9d05f6bb93fd3c13dc9f8ae"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-11435-2020-ie00001.svg" width="48pt" height="17pt" src="acp-20-11435-2020-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="normal">UCG</mi><mrow><mi>x</mi><mo>=</mo><mn mathvariant="normal">0.6</mn></mrow><mi mathvariant="normal">CD</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="48pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="4c6a6b1d8487ac445cfb65a0bcbbe13d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-11435-2020-ie00002.svg" width="48pt" height="17pt" src="acp-20-11435-2020-ie00002.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="normal">TCG</mi><mrow><mi>x</mi><mo>=</mo><mn mathvariant="normal">0.6</mn></mrow><mi mathvariant="normal">MJ</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="875cc7df767fec50f321ddb394a1ecd8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-11435-2020-ie00003.svg" width="47pt" height="17pt" src="acp-20-11435-2020-ie00003.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="normal">TCG</mi><mrow><mi>x</mi><mo>=</mo><mn mathvariant="normal">0.7</mn></mrow><mi mathvariant="normal">CD</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="e4d80dd5f27d9d9d667e468bcab2161b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-11435-2020-ie00004.svg" width="47pt" height="17pt" src="acp-20-11435-2020-ie00004.png"/></svg:svg></span></span> models predicted the isotopic composition that best matches with the observations. The relative contribution from locally generated and advected moisture is calculated at the water vapor sampling points, along the latitudinal transects, assigning the representative end-member isotopic compositions, and by solving the two-component mixing model. The results suggest a varying contribution of the advected westerly component, with an increasing trend up to 65<span class="inline-formula">^∘</span>&thinsp;S. Beyond 65<span class="inline-formula">^∘</span>&thinsp;S, the proportion of Antarctic moisture was found to be prominent and increasing linearly towards the coast.</p>