Negative APE dissipation as the fundamental criterion for double diffusive instabilities

TL;DR

This paper introduces the sign of APE dissipation rate as the fundamental criterion for double diffusive instabilities, providing a new theoretical framework that predicts both salt finger and diffusive convection phenomena.

Contribution

It develops a theory for APE dissipation rate in two-component fluids, correcting previous criteria and revealing its critical role in ocean mixing and stratified fluid stability.

Findings

The APE dissipation rate predicts the occurrence of double diffusive instabilities.

The theory accounts for enhancement, suppression, or reversal of APE dissipation in different regimes.

Negative APE dissipation can occur in stable stratifications, challenging existing assumptions.

Abstract

The background potential energy (BPE) is the only reservoir that double diffusive instabilities can tap their energy from when developing from an unforced motionless state with no available potential energy (APE). Recently, Middleton and Taylor linked the extraction of BPE into APE to the sign of the diapycnal component of the buoyancy flux, but their criterion can only predict diffusive convection instability, not salt finger instability. Here, we show that the problem can be corrected if the sign of the APE dissipation rate is used instead, making it emerge as the most fundamental criterion for double diffusive instabilities. A theory for the APE dissipation rate for a two-component fluid relative to its single-component counterpart is developed as a function of three parameters: the diffusivity ratio, the density ratio, and a spiciness parameter. The theory correctly predicts the occurrence of both salt finger and diffusive convection instabilities in the laminar unforced regime, while more generally predicting that the APE dissipation rate for a two-component fluid can be enhanced, suppressed, or even have the opposite sign compared to that for a single-component fluid, with important implications for the study of ocean mixing. Because negative APE dissipation can also occur in stably stratified single-component and doubly stable two-component stratified fluids, we speculate that only the thermodynamic theory of exergy can explain its physics; however, this necessitates accepting that APE dissipation is a conversion between APE and the internal energy component of BPE, in contrast to prevailing assumptions.