Using dissolved oxygen concentrations to determine mixed layer depths in the Bellingshausen Sea

Abstract. Concentrations of oxygen (O₂) and other dissolved gases in the oceanic mixed layer are often used to calculate air-sea gas exchange fluxes. The mixed layer depth (z_mix) 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 z_mix defined using oxygen may be different than z_mix defined using temperature or density. Here, we propose to define an O₂-based mixed layer depth, z_mix(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 analysis of O₂ profiles from the Bellingshausen Sea (west of the Antarctic Peninsula) and corroborated by visual inspection. Comparisons of z_mix(O₂) with z_mix based on potential temperature differences, i.e., z_mix(0.2 °C) and z_mix(0.5 °C), and potential density differences, i.e., z_mix(0.03 kg m⁻³) and z_mix(0.125 kg m⁻³), showed that z_mix(O₂) closely follows z_mix(0.03 kg m⁻³). Further comparisons with published z_mix climatologies and z_mix derived from World Ocean Atlas 2005 data were also performed. To establish z_mix for use with biological production estimates in the absence of O₂ profiles, we suggest using z_mix(0.03 kg m⁻³), which is also the basis for the climatology by de Boyer Montégut et al. (2004).