Published November 22, 2021 | Version v0.1.0
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caneill et al. The polar transition from alpha to beta regions set by a surface buoyancy flux inversion.

  • 1. Department of Marine Sciences, University of Gothenburg, Gothenburg, Sweden
  • 2. LOCEAN Laboratory, Sorbonne Université-CNRS-IRD-MNHN, Paris, France
  • 3. Department of Meteorology, Stockholm University, Stockholm, Sweden


NEMO outputs used for the paper Caneill, R., Roquet, F., Madec, G., and Nycander, J. (2022). The polar transition from alpha to beta regions set by a surface buoyancy flux inversion. Journal of Physical Oceanography.


The stratification is primarily controlled by temperature in subtropical regions (alpha-ocean), and by salinity in subpolar regions (beta-ocean). Between these two regions lies a transition zone, often characterized by deep mixed layers in winter and responsible for the ventilation of intermediate or deep layers. While of primary interest, no consensus on what controls its position exists yet. Amongst the potential candidates, we find the wind distribution, air-sea fluxes or the nonlinear cabbeling effect. Using an ocean general circulation model in an idealized basin configuration, a sensitivity analysis is performed testing different equations of state. More precisely, the thermal expansion coefficient (TEC) temperature dependence is explored, changing the impact of heat fluxes on buoyancy fluxes in a series of experiments. The transition zone is found to be located at the position where the sign of the surface buoyancy flux reverses to become positive, in the subpolar region, while wind or cabbeling are found of secondary importance. This inversion becomes possible because the TEC is reducing at low temperature, enhancing in return the relative impact of freshwater fluxes on the buoyancy forcing at high latitudes. When the TEC is made artificially larger at low temperature, the freshwater flux required to produce a positive buoyancy flux increases and the transition zone moves poleward. These experiments demonstrate the important role of competing heat and freshwater fluxes in setting the position of the transition zone. This competition is primarily influenced by the spatial variations of the TEC linked to meridional variations of the surface temperature.


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