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

Mamoru Tanaka1, Ichiro Yasuda1, Jiro Yoshida1, Keunjong Lee2, Yasutaka Goto3 and Takahiro Tanaka4, (1)Atmosphere and Ocean Research Institute, University of Tokyo, Tokyo, Japan, (2)Korea Institute of Ocean Science and Technology, Busan, South Korea, (3)Japan Meteorological Agency, Tokyo, Japan, (4)Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Shiogama, Japan
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. KρKθ), 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 χθ was found in the DC layer, where χθ 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 ROT (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.