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Evaluation of the Southern Ocean O 2 /Ar‐based NCP estimates in a model framework
Author(s) -
Jonsson Bror F.,
Doney Scott C.,
Dunne John,
Bender Michael
Publication year - 2013
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/jgrg.20032
Subject(s) - mixed layer , supersaturation , flux (metallurgy) , saturation (graph theory) , environmental science , atmospheric sciences , chemistry , geology , thermodynamics , physics , climatology , mathematics , organic chemistry , combinatorics
The sea‐air biological O 2 flux assessed from measurements of surface O 2 supersaturation in excess of Ar supersaturation (“O 2 bioflux”) is increasingly being used to constrain net community production (NCP) in the upper ocean mixed layer. In making these calculations, one generally assumes that NCP is at steady state, mixed layer depth is constant, and there is no O 2 exchange across the base of the mixed layer. The object of this paper is to evaluate the magnitude of errors introduced by violations of these assumptions. Therefore, we examine the differences between the sea‐air biological O 2 flux and NCP in the Southern Ocean mixed layer as calculated using two ocean biogeochemistry general circulation models. In this approach, NCP is considered a known entity in the prognostic model, whereas O 2 bioflux is estimated using the model‐predicted O 2 /Ar ratio to compute the mixed layer biological O 2 saturation and the gas transfer velocity to calculate flux. We find that the simulated biological O 2 flux gives an accurate picture of the regional‐scale patterns and trends in model NCP. However, on local scales, violations of the assumptions behind the O 2 /Ar method lead to significant, non‐uniform differences between model NCP and biological O 2 flux. These errors arise from two main sources. First, venting of biological O 2 to the atmosphere can be misaligned from NCP in both time and space. Second, vertical fluxes of oxygen across the base of the mixed layer complicate the relationship between NCP and the biological O 2 flux. Our calculations show that low values of O 2 bioflux correctly register that NCP is also low (<10 mmol m −2 day −1 ), but fractional errors are large when rates are this low. Values between 10 and 40 mmol m −2 day −1 in areas with intermediate mixed layer depths of 30 to 50 m have the smallest absolute and relative errors. Areas with O 2 bioflux higher than 30 mmol m −2 day −1 and mixed layers deeper than 40 m tend to underestimate NCP by up to 20 mmol m −2 day −1 . Excluding time periods when mixed layer biological O 2 is undersaturated, O 2 bioflux underestimates time‐averaged NCP by 5%–15%. If these time periods are included, O 2 bioflux underestimates mixed layer NCP by 20%–35% in the Southern Ocean. The higher error estimate is relevant if one wants to estimate seasonal NCP since a significant amount of biological production takes place when mixed layer biological O 2 is undersaturated.