Double-Diffusive Convection in Ocean Systems

Double-diffusive convection arises when two scalar fields, typically temperature and salinity, diffuse at distinct rates in stratified seawater. In the ocean, this process manifests chiefly as salt fingering, where warm, salty water overlies cooler, fresher water, and as diffusive layering, where the inverse stratification occurs. These instabilities generate fine-scale thermohaline staircases, enhance vertical fluxes of heat and salt and modulate large-scale circulation. Salt fingers transport heat downward and salt upward, accelerating diapycnal mixing beyond molecular diffusion, while diffusive layering fosters step-like density profiles that influence buoyancy fluxes. The distribution of double-diffusive regimes is governed by background stratification, shear and turbulent mixing, with occurrences clustered in subtropical gyres, high-latitude haloclines and seasonal thermoclines. Understanding these processes is vital for quantifying ocean heat uptake, assessing impacts on marine ecosystems and improving climate models.

Research from Nature Portfolio

Recent observational profiles in the Arabian Sea have revealed active salt fingering within the diurnal thermocline during calm conditions, confirmed by staircase-like microstructure and enhanced dissipation ratios, emphasising the role of daytime entrainment and detrainment in generating salinity maxima. Global analyses of Argo and ice-tethered profiler data have shown that around 14 percent of profiles contain thermohaline staircases, with spatial clustering over features such as the Tasman Leakage. Although the combined energy contribution of double-diffusive fluxes is small relative to the mechanical energy budget of the global ocean, regional enhancements in diapycnal heat transport may substantially influence local stratification and sea-ice interactions. Broad-area acoustic surveys in polar seas have imaged fine-scale staircases across hundreds of kilometres, elucidating staircase evolution, vertical flux scaling and interactions with haline stratification beneath sea ice.

Double-Diffusive Convection in Ocean Systems publication trend

The graph below shows the total number of articles in double-diffusive convection in ocean systems across all publications each year (not limited to Nature Index journals).

Technical terms

Double-diffusive convection: Convective motions driven by differing molecular diffusivities of heat and salt in stratified fluids.

Salt fingering: A salt-driven instability in which warm, saline water overlies cooler, fresher water, forming elongated convective cells.

Thermohaline staircase: Layered profiles of alternating well-mixed and strongly stratified layers resulting from double-diffusive processes.

Turner angle: A diagnostic index quantifying the relative contributions of temperature and salinity gradients to density stability.

Diapycnal mixing: Vertical mixing that occurs across surfaces of constant density, enhancing transport of heat and salt.

References

1. Observational evidence of salt finger in the diurnal thermocline. Scientific Reports (2023).
2. Double-diffusive mixing makes a small contribution to the global ocean circulation. Communications Earth & Environment (2021).
3. Acoustic Mapping of Thermohaline Staircases in the Arctic Ocean. Scientific Reports (2017).
4. Frequency-Domain Reverse-Time Migration with Analytic Green’s Function for the Seismic Imaging of Shallow Water Column Structures in the Arctic Ocean. Sensors (2023).
5. A general criterion for the release of background potential energy through double diffusion. Journal of Fluid Mechanics (2020).
6. Estimating Dissipation Rates Associated With Double Diffusion. Geophysical Research Letters (2021).