Cabbeling and Mixing at a Compensated Thermohaline Front

Idealized direct numerical simulations are used to explore the development and dynamical consequences of cabbeling at a thermohaline front. Cabbeling is a process, due to nonlinearity in the equation of state, by which water masses with different temperatures and salinities become denser when mixed along isopycnals. Focus on the significance of this mechanism is motivated by recent observations of an extremely sharp density-compensated front at the North Wall of the Gulf Stream, susceptible to cabbeling and that coincides with a sheet of strong cyclonic vorticity. We study the submesoscale mixing processes responsible for cabbeling with a simple physical model including a strong cyclonic mixing layer. These simulations explore the evolution of a nearly barotropic flow subjected to strong stratification and moderate rotational effects. Cabbeling is initially driven by barotropic instability, manifesting in the highly strained braid regions of the vertical vorticity field. These regions are sites of modification of density and small-scale overturning instabilities. These features are absent in simulations with a linear equation of state indicating that the instabilities are a consequence of cabbeling. Dependence on the strength of the thermohaline front is explored, showing the potential for strong vertical mixing owing to cabbeling-driven instability for sufficiently intense fronts. Vorticity dynamics and energetics are used to explain the emergence and robustness of the cabbeling-driven instability.