Density inversions observed in double-diffusive layer from the northern North Pacific

Polar and sub-polar regions are favorable for diffusive convection (hereafter DC), where relatively fresh, cold water overlies warmer, saltier water. There, general assumptions associated with turbulent mixing do not hold (e.g. K_ρ ≠ K_θ), and hence, turbulent processes and subsequent fluxes over DC layers are elusive. Here, we present a potential contribution of DC to mechanical turbulence. Microstructure profiles were acquired around the Aleutian Islands. Turner angle exhibited local minimum values of -80° to -60° in 100 to 300 m depth, suggesting DC layer formation there. Substantial increase in temperature variance dissipation rate χ_θ was found in the DC layer, where χ_θ was substantially larger than TKE dissipation rate ε. Density inversions were frequently found in weakly stratified layers with low levels of gradient Richardson number Ri, typical in the ocean. However, density inversions were also found in DC layers with moderate levels of Ri (>1), which were much higher than the typical threshold values of Ri = 0.25. Interestingly, we found that, in DC layers, the density inversions occurred with low levels of R_OT (ratio of Ozmidov scale to Thorpe scale), low levels of ε, and low levels of buoyancy Reynolds number. Those are the features of recently induced, “young” inversions. Our findings are consistent with Radko & Stern (2011), who numerically simulated shear flows on salt sheets and argued that double diffusive convection provides conditions favorable for successive generations of density inversions.