Preprints
https://doi.org/10.5194/tc-2021-23
https://doi.org/10.5194/tc-2021-23

  04 Feb 2021

04 Feb 2021

Review status: a revised version of this preprint is currently under review for the journal TC.

Southern Ocean polynyas in CMIP6 models

Martin Mohrmann1, Céline Heuzé2, and Sebastiaan Swart1,3 Martin Mohrmann et al.
  • 1Department of Marine Sciences, University of Gothenburg, Gothenburg, Sweden
  • 2Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden
  • 3Department of Oceanography, University of Cape Town, Rondebosch, South Africa

Abstract. Polynyas facilitate air-sea fluxes, impacting climate-relevant properties such as sea ice formation and deep water production. Despite their importance, polynyas have been poorly represented in past generations of climate models. Here we present a method to track the presence, frequency and spatial distribution of polynyas in the Southern Ocean in 27 models participating in the Climate Model Intercomparison Project phase 6 (CMIP6) and two satellite based sea ice products. Only half of the 27 models form open water polynyas (OWP), and most underestimate their area. As in satellite observations, three models show episodes of high OWP activity separated by decades of no OWPs, while other models unrealistically create OWPs nearly every year. The coastal polynya area in contrast is often overestimated, with the least accurate representations occurring in the models with the coarsest horizontal resolution. We show that the presence or absence of OWPs are linked to changes in the regional hydrography, specifically the linkages between polynya activity with deep water convection and/or the shoaling of the upper water column thermocline. Models with an accurate Antarctic Circumpolar Current (ACC) transport and wind stress curl have too frequent OWPs. Biases in polynya representation continue to exist in climate models, which has an impact on the regional ocean circulation and ventilation that require to be addressed. However, emerging iceberg discharge schemes, vertical discretisation or overflow parameterisation, are anticipated to improve polynya representations and associated climate prediction in the future.

Martin Mohrmann et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2021-23', Anonymous Referee #1, 13 Mar 2021
    • AC1: 'Reply on RC1', Martin Mohrmann, 18 Jun 2021
  • RC2: 'Comment on tc-2021-23', Carolina Dufour, 21 May 2021
    • AC2: 'Reply on RC2', Martin Mohrmann, 18 Jun 2021

Martin Mohrmann et al.

Martin Mohrmann et al.

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Short summary
Polynyas are large open water areas within the sea ice. We developed a method to estimate their area, distribution, and frequency for the Southern Ocean in climate models and observations. All models have polynyas along the coast but few do so in the open ocean, in contrast to observations. We examine potential atmospheric and oceanic drivers of open water polynyas and discuss recently implemented schemes that may have improved some models’ polynya representation.