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Originalbeitrag (peer-reviewed)

Zeitschrift Journal of Physical Ocenaography
Volume (Issue) 42(6)
Titel der Proceedings Journal of Physical Ocenaography


In this paper we explore the conditions under which a double-di usive interface may become unstable. Focus is placed on the case of a cold, fresh water layer above a warm, salty layer, i.e., the di usive convection (DC) regime. The \di usive interface" between these layers will develop gravitationally unstable boundary layers due to the more rapid di usion of heat (the destabilizing component) relative to salt. Previous studies have assumed that a purely convective-type instability of these boundary layers is what drives convection in this system, and that this may be parameterized by a boundary layer Rayleigh number. We test this theory by conducting both a linear stability analysis, and direct numerical simulations of a di usive interface. Our linear stability analysis reveals that the transition to instability always occurs as an oscillating di usive convection mode, and at boundary layer Rayleigh numbers much smaller than previously thought. However, these ndings are based on making a quasi-steady assumption for the growth of the interfaces by molecular di usion. When di using interfaces are modeled (using direct numerical simulations) we observe that the time dependence is signi cant in determining the instability of the boundary layers, and the breakdown is due to a purely convective-type instability. Our ndings therefore demonstrate that the relevant instability in a DC staircase is purely convective.