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Origin and dynamics of thermohaline staircases in the ocean.

One of the most spectacular and intriguing effects realized in multicomponent fluids is the spontaneous formation of a series of relatively well-mixed layers separated by sharp interfaces, known as thermohaline staircases.  The interest of the oceanographic community in the dynamics and transport characteristics of thermohaline staircases is not limited to mere intellectual curiosity.  Staircases are often associated with elevated rates of vertical mixing, which in turn can influence the ocean’s ability to transport heat, salt, nutrients, pollutants, and carbon dioxide.  It is generally accepted that staircases in the ocean are produced by double-diffusive convection, broadly defined as a combination of processes driven by dissimilar diffusivities of density components.  However, the specific mechanisms of staircase formation have not been fully explained after more than half a century of observation.  This presentation reviews recent analytical and numerical staircase models, which suggest that spontaneous layering is linked to parametric instabilities of the equilibria with uniform stratification.  Most previous theoretical investigations of thermohaline layering were based on the flux-gradient model, which assumes that the vertical transport of density components is uniquely determined by their local background gradients. The key deficiency of this approach is that layering instabilities predicted by the flux-gradient model have unbounded growth rates at high wavenumbers.  This limitation is addressed by developing an alternative asymptotic multiscale model, which leads to well-posed evolutionary equations for layering modes.