The fresh water equation of state exhibits a fundamentally important nonlinearity; the temperature at which it attains its maximum density occurs at roughly $4^{\circ }\mathrm{C}$ as opposed to at $0^{\circ }\mathrm{C}$. The form of the equation of state in this regime is quadratic and has the well-known implication that two fluid parcels may mix and produce a child parcel with a higher density; a process known as cabbeling. In this work we report on direct numerical simulations of the Rayleigh-Taylor instability in the cabbeling regime. We demonstrate that the mature, three-dimensional state of the instability leads to cabbeling, which, in turn, serves to sustain an instability that would otherwise saturate outside of the cabbeling regime. We present a new quantitative measure of strong cabbeling potential and use it, along with the mechanical energy, to demonstrate that the Rayleigh-Taylor instability is arrested in two ways that are unique to the cabbeling regime. Our results have implications for radiatively driven, under-ice flows in late winter during which ice prevents mechanical driving by the wind. While our focus is on the freshwater setting, we extend the derivation of the strong cabbeling potential to heat-salt systems, thereby extending its usefulness to the broader community.

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• Flow instability
• Rayleigh-Taylor instability
• Stratified geophysical flows