We describe a mechanism that leads to the spontaneous formation of a thermohaline staircase in the high-latitude oceans. Our analysis of this mechanism is based upon a model in which uniform gradients of temperature and salinity are assumed and is applied to a simplified mean-field model of stratified turbulence. Detailed analysis employs a parametrization of turbulent diapycnal diffusivities (Bouffard & Boegman, Dyn. Atmos. Oceans, vol. 61, 2013, pp. 14–34). This parametrization is apparently unique in that it distinguishes between the diapycnal diffusivities for heat and salt on the basis of their Prandtl (Schmidt) numbers. Our model predicts that the temperature and salinity profiles will be susceptible to linear instability if the buoyancy Reynolds number lies in the range 0.18–91, and a nonlinear mean-field model simulation demonstrates that it evolves into a well-defined thermohaline staircase that matches the characteristics of those found in the high-latitude oceans. The criterion for initial instability is furthermore shown to be consistent with the observed regional variability of staircase occurrence in the Arctic Ocean as determined by the most recent observational datasets.

Millennium time scale Dansgaard-Oeschger oscillations of glacial climate, clear evidence for the occurrence of which was first provided on the basis of oxygen isotopic data from a Greenland ice core 25 years ago, have recently been shown to arise naturally (without explicit freshwater forcing) in a fully coupled modern climate model description of the interactions between the overturning circulation of the oceans, the atmosphere, and sea ice under maximum glacial conditions. The fast transitions from cold stadial to warm interstadial conditions in a typical DO oscillation are characterized by the appearance of an extensive polynya in the stadial sea ice cover of the Irminger Sea south of Denmark Strait that opens due to the onset of intense vertical mixing below the sea ice lid. Through detailed stability analysis of the water column in the region where the polynya first forms, together with analysis of the action of the KPP (Kappa Profile Parameterization) of stratified turbulence employed to represent water column diapycnal diffusivity, the authors show that the opening of the polynya is controlled by this turbulence parameterization. The relative contributions of the different components of the parameterization to polynya opening are investigated in order to better understand the rapid climate change that ensues. The authors furthermore show that the characteristic period of the model predicted Dansgaard-Oeschger oscillation is also controlled by the detailed nature of this parameterization, a characteristic of the oscillation not previously explained.

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