Ocean Sci. Discuss., 8, 1505-1533, 2011
www.ocean-sci-discuss.net/8/1505/2011/
doi:10.5194/osd-8-1505-2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Ocean Science (OS). Please refer to the corresponding final paper in OS.
Using dissolved oxygen concentrations to determine mixed layer depths in the Bellingshausen Sea
K. Castro-Morales1,2 and J. Kaiser1
1University of East Anglia, Norwich, UK
2Alfred-Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Abstract. Concentrations of oxygen (O2) and other dissolved gases in the oceanic mixed layer are often used to calculate air-sea gas exchange fluxes; for example, in the context of net and gross biological production estimates. The mixed layer depth (zmix) may be defined using criteria based on temperature or density differences to a reference depth near the ocean surface. However, temperature criteria fail in regions with strong haloclines such as the Southern Ocean where heat, freshwater and momentum fluxes interact to establish mixed layers. Moreover, the time scales of air-sea exchange differ for gases and heat, so that zmix defined using O2 may be different to zmix defined using temperature or density. Here, we propose to define an O2-based mixed layer depth, zmix(O2), as the depth where the relative difference between the O2 concentration and a reference value at a depth equivalent to 10 dbar equals 0.5 %. This definition was established by numerical analysis of O2 profiles in coastal areas of the Southern Ocean and corroborated by visual inspection. Comparisons of zmix(O2) with zmix based on potential temperature differences, i.e. zmixθ = 0.2 °C) and zmixθ = 0.5 °C), and potential density differences, i.e. zmixσθ = 0.03 kg m−3) and zmixσθ = 0.125 kg m−3), showed that zmix(O2) closely follows zmixσθ = 0.03 kg m−3). Further comparisons with published zmix climatologies and zmix derived from World Ocean Atlas 2005 data were also performed. To establish zmix for use with biological production estimates in the absence of O2 profiles, we suggest using zmixσθ = 0.03 kg m−3), which is also the basis for the climatology by de Boyer Montégut et al. (2004).

Citation: Castro-Morales, K. and Kaiser, J.: Using dissolved oxygen concentrations to determine mixed layer depths in the Bellingshausen Sea, Ocean Sci. Discuss., 8, 1505-1533, doi:10.5194/osd-8-1505-2011, 2011.
 
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