EGU2020-8015, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-8015
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Reshuffling of Nutrients in the Southern Ocean

Ivy Frenger1, Ivana Cerovecki2, and Matthew Mazloff2
Ivy Frenger et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany (ifrenger@geomar.de)
  • 2Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

Deep waters upwell in the Southern Ocean, replete with nutrients. Some of these nutrients enter lighter mode and intermediate waters (MIW), fueling upper ocean productivity in the otherwise nutrient depleted (sub)tropical waters. However some of the upwelled nutrients are retained in the Southern Ocean or leak into denser bottom waters (AABW), making them unavailable for upper ocean productivity. Despite its fundamental importance for the global ocean productivity, this “reshuffling” of nutrients between Southern Ocean water masses, and its driving forces and temporal variability, have not been quantified to date.

We analyze the globally major limiting macronutrient, nitrate (NO3), using the results of a data-assimilating coupled ocean-sea-ice and biogeochemistry model, the Biogeochemical Southern Ocean State Estimate (B-SOSE), for the years 2008 – 2017. Using a water mass framework, applied to five day averaged SOSE output south of 30oS, we quantify the processes controlling NO3 inventories and fluxes. The water mass framework enables us to assess the relative importance of physical processes (such as surface buoyancy fluxes and diapycnal mixing) and biogeochemical processes (such as productivity and remineralization) in driving the transfer of NO3 from upwelling deep waters (CDW) to MIW and AABW, and its interannual variability.

Our results show that two thirds of the NO3 supplied to MIW occurs through lightening, or transforming, of CDW waters during the course of the overturning circulation. The other third of the NO3 supplied to MIW occurs through upward mixing of NO3 from NO3-enriched CDW. This means that physical processes determine the mean MIW NO3 content. Biology does not have a net effect on MIW NO3: while biological uptake draws down the MIW concentration of  NO3 near the surface, remineralization of organic matter compensates for this MIW loss below the surface. Also, we find that the productivity in the subtropical waters south of 30oS is fed through both, the canonical upward mixing of NO3 through the thermocline, and through the near surface supply from MIW. Thus, again, water mass transformation is playing a large role in nutrient distributions. 

In ongoing work, we assess the drivers of variability of the reshuffling of NO3 between water masses and their potential sensitivity to climate change.

How to cite: Frenger, I., Cerovecki, I., and Mazloff, M.: Reshuffling of Nutrients in the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8015, https://doi.org/10.5194/egusphere-egu2020-8015, 2020

Comments on the presentation

AC: Author Comment | CC: Community Comment | Report abuse

Presentation version 2 – uploaded on 07 May 2020
Layout updates based on comments to increase clarity.
  • CC1: Strong variations of NO3 in CDW, Alexander Haumann, 15 May 2020

    Thanks for the very nice display, Ivy! Great work!

    There seems to be a strong difference in NO3 between upper and lower CDW. Does this have to do with changes in the biological (remineralization) or physical (upwelling) contributions? Are there substantial differences in NO3 in the source deep waters, i.e. NADW or IPDW?

    • AC2: Reply to CC1, Ivy Frenger, 21 May 2020

      Thanks for your question, Alex!

      Are you referring to the imoprt of NO3 into the Southern Ocean that is weaker in upper and larger in deeper CDW density layers? If so, this is the import at 30S. Biology is not a large source in the SO north of approx 55S (see also your second question). A quick look at import of NO3 from the different basins suggests that the main reason for larger imports with greater densities is the greater volume of the deeper density layers, with some (yet unquantified) contribution of increasing NO3 concentrations with deeper densities. I have looked too little at the contributions of the different basins (definitely sth to do for the future), though, to tell you details on the NO3 contribution of NADW vs IPDW.

      I hope this answers your question :-).

  • CC2: Source of biological NO3 gain in CDW, Alexander Haumann, 15 May 2020

    Another related question to the one above:
    Could you clarify where the 30% gain in NO3 in the CDW layer from biology comes from? Is this mostly from sinking particles from the Antarctic surface waters, i.e. recycled nutrients from upwelled CDW?

    • AC1: Reply to CC2, Ivy Frenger, 21 May 2020

      Thanks also for Q#2 :-).

      Yes, exactly. I call this also nitrate "loop", "retention", or "trapping".

Presentation version 1 – uploaded on 05 May 2020 , no comments