The Formation of Double-Diffusive Layers in a Weakly Turbulent Environment

Double-diffusive stratification in the ocean is characterized by staircase structures consisting of mixed layers separated by high-gradient interfaces in temperature and salinity. These double-diffusive layers, which flux heat vertically, are observed over a vast region of the Arctic Ocean at the top boundary of the relatively warm and salty Atlantic water layer. In one formalism for the origin of double-diffusive layers, staircase formation arises when a heat source is applied at the base of water that is stably stratified in salinity. This framework is extended to consider the effect of intermittent shear-driven turbulence on diffusive-convective staircase formation. One-dimensional numerical model results indicate that there is a critical level of intermittent turbulence above which a staircase cannot form. This is framed in terms of a critical diffusivity ratio (ratio of effective salinity diffusivity to effective thermal diffusivity) that cannot be exceeded for a staircase to persist. This critical ratio is not a universal constant but rather differs for each staircase. Model results further indicate that layer thicknesses decrease with height in a staircase, with the variation in thickness over a staircase being more pronounced in the presence of intermittent turbulence. Finally, results suggest that increased diffusivity ratios lead to decreased heat fluxes across interfaces; if a staircase is subject to intermittent turbulence levels (below the critical level), vertical heat fluxes will be smaller than in the absence of shear-driven turbulence. Findings are related to double-diffusive staircases, and associated heat fluxes, in the weakly turbulent Arctic Ocean. Plain Language Summary Double diffusion is a type of convective mixing process that may arise in the oceans where temperature and salinity determine density gradients. Active double diffusion manifests as stacked well-mixed water layers, forming a staircase structure. The Arctic Ocean exhibits a notable double-diffusive staircase which indicates how deep-ocean heat is mixed vertically toward the sea ice. In this study, we examine how this double-diffusive heat transport may be influenced by mechanical mixing, or turbulence, such as that driven by winds and waves. We find that below a threshold level of turbulence, the vertical transport of heat through the staircase may be reduced as turbulence increases. However, we find that above this threshold level of turbulence, double diffusion can no longer operate to generate a well-formed staircase. Results contribute to understanding how turbulence affects vertical heat transport in a changing Arctic Ocean that may experience higher wind-driven mixing as sea ice continues to retreat.