Appears in collection : 2026 - T2 - WS2 - Instabilities and transitions in geophysical flows

In two-component seawater, the thermobaric and cabbeling nonlinearities of the equation of state, combined with the disparate molecular diffusivities of salt and heat, give rise to a diverse range of diffusive instabilities. Beyond standard salt finger and diffusive convection instabilities, these include instabilities associated with densification upon mixing. Unlike conventional turbulent diapycnal mixing, which dissipates available potential energy (APE) into background potential energy (BPE), diffusive instabilities can extract energy from the BPE to energise the fluid and provide a source of turbulent kinetic energy. This suggests that in regimes where diffusive instabilities occur, the BPE contains a "latent" form of APE that remains poorly understood. This work reviews the fundamental nature of these processes and identifies the remaining challenges in developing a comprehensive framework for their understanding, drawing on a variety of test cases and examples.