Abstract

Abstract. Polynyas facilitate airโ€“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 (OWPs), and most underestimate their area. As in satellite observations, three models show episodes of high OWP activity separated by decades of no OWP, while other models unrealistically create OWPs nearly every year. In contrast, the coastal polynya area is overestimated in most models, with the least accurate representations occurring in the models with the coarsest horizontal resolution. We show that the presence or absence of OWPs is 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 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 should be addressed. However, emerging iceberg discharge schemes, more adequate vertical grid type or overflow parameterisation are anticipated to improve polynya representations and associated climate prediction in the future.

Highlights

  • Polynyas are areas of open water surrounded by sea ice

  • The decreased open-water polynyas (OWPs) activity we find for climate model (CM) in our Climate Model Intercomparison Project Phase 6 (CMIP6) data set with ongoing global warming is consistent with the results of De Lavergne et al (2014), in which OWPs in the Weddell Sea eventually stop at the end of the extended CMIP5 climate change runs, as the CM models show a stronger warming response than the Earth system model (ESM) versions Dong et al (2020)

  • For the CMIP6 models we found that the maximum strength of the zonally averaged wind stress curl correlates with its latitude (Pearson correlation coefficient of โˆ’0.76, computed again using values from Beadling et al, 2020; see Fig. 11b)

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Summary

Introduction

Polynyas are areas of open water surrounded by sea ice. They are common features within the Southern Ocean winter sea ice and are often classified into two different categories: coastal polynyas and open-water polynyas (OWPs). The heat source that keeps OWPs from freezing over is the presence of comparatively warm and salty Circumpolar Deep Water (CDW), which is usually located just below the base of the upper mixed layer (Santoso et al, 2006). These warmer waters transport heat from the depth to the surface by free or forced convection, caused by surface cooling, brine rejection, or wind and shear stresses respectively (Williams et al, 2007). While open-ocean deep convection is closely linked to OWPs (Cheon and Gordon, 2019), deep convection is not a sufficient condition for OWPs to occur (Dufour et al, 2017)

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