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Abstract The mixed layer of polynyas is vital for local climate as it determines the exchange of properties and energy between ocean, sea ice, and atmosphere. However, its evolution is poorly understood, as it is controlled by complex interactions among these components, yet highly undersampled, especially outside summer. Here, we present a 2-month, high vertical-resolution, full-depth hydrographic dataset from the southeastern Amundsen Sea polynya in austral autumn (from mid-February to mid-April 2014) collected by a recovered seal tag. This novel dataset quantifies the changes in upper-ocean temperature and salinity stratification in this previously unobserved season. Our seal-tag measurements reveal that the mixed layer experiences deepening, salinification, and intense heat loss through surface fluxes. Heat and salt budgets suggest a sea ice formation rate of ∼3 cm per day. We use a one-dimensional model to reproduce the mixed layer evolution and further identify key controls on its characteristics. Our experiments with a range of reduced or amplified air–sea fluxes show that heat loss to the atmosphere and related sea ice formation are the principal determinants of stratification evolution. Additionally, our modeling demonstrates that horizontal advection is required to fully explain the mixed layer evolution, underlining the importance of the ice-covered neighboring region for determining sea ice formation rates in the Amundsen Sea polynya. Our findings suggest that the potential overestimation of sea ice production by satellite-based methods, due to the absence of oceanic heat flux, could be offset by horizontal advection inhibiting mixed layer deepening and sustaining sea ice formation.
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Homogenization for convection-enhanced thermal transport in sea ice
Sea ice regulates heat exchange between the ocean and atmosphere in Earth’s polar regions. The thermal conductivity of sea ice governs this exchange, and is a key parameter in climate modelling. However, it is challenging to measure and predict due to its sensitive dependence on temperature, salinity and brine microstructure. Moreover, as temperature increases, sea ice becomes permeable, and fluid can flow through the porous microstructure. While models for thermal diffusion through sea ice have been obtained, advective contributions to transport have not been considered theoretically. Here, we homogenize a multiscale advection–diffusion equation that models thermal transport through porous sea ice when fluid flow is present. We consider two-dimensional models of convective flow and use an integral representation to derive bounds on the thermal conductivity as a function of the Péclet number. These bounds guarantee enhancement in the thermal conductivity due to the added flow. Further, we relate the Péclet number to temperature, making these bounds useful for global climate models. Our analytic approach offers a mathematical theory which can not only improve predictions of atmosphere–ice–ocean heat exchanges in climate models, but can provide a theoretical framework for a range of problems involving advection–diffusion processes in various fields of application.
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- PAR ID:
- 10592779
- Publisher / Repository:
- The Royal Society Publishing
- Date Published:
- Journal Name:
- Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
- Volume:
- 480
- Issue:
- 2296
- ISSN:
- 1364-5021
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1098/rspa.2023.0747
