The instability of diffusive convection and its implication for the thermohaline staircases in the deep Arctic Ocean

Abstract

Abstract. In the present study, the classical description of diffusive convection is updated to interpret the instability of diffusive interfaces and the dynamical evolution of the bottom layer in the deep Arctic Ocean. In the new consideration of convective instability, both the background salinity stratification and rotation are involved. The critical Rayleigh number of diffusive convection is found to vary from 10 3 to 10 11 in the deep Arctic Ocean as well as in other oceans and lakes. In such a wide range of conditions, the interfaceinduced thermal Rayleigh number is shown to be consistent with the critical Rayleigh number of diffusive convection. In most regions, background salinity stratification is found to be the main hindrance to the occurrence of convecting layers. With the new parameterization, it is predicted that the maximum thickness of the bottom layer is 1051 m in the deep Arctic Ocean, which is close to the observed value of 929 m. The evolution time of the bottom layer is predicted to be ∼ 100 yr, which is on the same order as that based on 14 C isolation age estimation.

New layer thickness parameterization of diffusive convection in the ocean

“…As discussed in Turner (1973), Ra c in the diffusive convection is different from the typical value (∼1000) in the thermal convection. This is confirmed in the recent work, where Ra c can reach 10 11 depending on the local environment conditions (Zhou et al, 2014). In some sense, the new model (Eq.…”

Near‐Inertial Wave Propagation in the Deep Canadian Basin: Turning Depths and the Homogeneous Deep Layer

“…If we consider only the unstable layers (Figures 3 and 4), the thickness of the homogeneous layer is ∼600 − 700 m. However, assuming that the top of the homogeneous layer is at ∼3,000 m, just above the peak in $${N}_{\mathit{bin}}^{2}$$ (Figure 3), the thickness increases up to 1,000 m, which is consistent with Zhou et al. (2014) and therefore with the convective instability hypothesis.…”

“…A considerable fraction of this heat remains in the homogeneous bottom layer. Approximately F H ∼ 0.1 − 45 mW m −2 escape through density staircases above the bottom layer (Carmack et al., 2012; Timmermans et al., 2003; Zhou et al., 2014; Zhou & Lu, 2013).…”

“…The locally unstable values of $${N}_{\mathit{bin}}^{2}$$ might be related to fluctuations around $${N}_{\mathit{bin}}^{2}=0$$, so that the layer does have neutral buoyancy but is unstable, suggesting periods of convective mixing. In addition, the homogeneous layer may have a theoretical thickness of 1,051 m (Zhou et al., 2014) based on a comparison of the deep Canadian homogeneous layer thickness to experiments conducted with salt‐stratified fluids heated from below. If we consider only the unstable layers (Figures 3 and 4), the thickness of the homogeneous layer is ∼600 − 700 m. However, assuming that the top of the homogeneous layer is at ∼3,000 m, just above the peak in $${N}_{\mathit{bin}}^{2}$$ (Figure 3), the thickness increases up to 1,000 m, which is consistent with Zhou et al.…”

Temporal Variability in Bottom Water Structures of the Continental Slope in the Northern South China Sea

“…Below the ocean depth of 200 m, both the potential temperature, θ , and absolute salinity, S , decreased as the depth approached the ocean bottom, which offered a background condition for the double diffusive instability (S.‐Q. Zhou et al., 2014). The diagram of potential temperature, θ , and salinity, S , also clearly exhibits this feature in the deep water.…”