Thermohaline staircase

Thermohaline staircases are patterns that form in oceans and other bodies of salt water, characterised by step-like structures observed in vertical temperature and salinity profiles; the patterns are formed and maintained by double diffusion of heat and salt. The ocean phenomenon consists of well-mixed layers of ocean water stacked on top of each other. The well-mixed layers are separated by high-gradient interfaces, which can be several meters thick. The total thickness of staircases ranges typically from tens to hundreds of meters. Two types of staircases are distinguished. Salt-fingering staircases can be found at locations where relatively warm, salty water overlies relatively colder, fresher water. Here, large-scale temperature and salinity both increase upward, making the mixing process of salt fingering possible. Locations hosting these types of staircases include beneath the Mediterranean outflow, in the Tyrrhenian Sea, and northeast Caribbean. Diffusive staircases can be found at locations where both temperature and salinity increase downward, such as in the Arctic Ocean and in the Weddell Sea. An important feature of thermohaline staircases is their great stability in space and time. They can persist several years or more and can extend for hundreds of kilometers. The interest in thermohaline staircases is partly due to the fact that the staircases represent mixing hot spots in the main thermocline.

Extensive definition and detection To determine the presence of thermohaline staircases, the following steps can be taken according to the algorithm designed by Van der Boog. The first step of the algorithm is to identify the mixed layers by locating weak vertical density gradients in conservative temperature and absolute salinity. To do so, the threshold gradient method is used with a threshold of ∂ σ 1 / ∂ p m a x = 0.0005 kg m − 3 pbar − 1 {\displaystyle \partial \sigma _{1}/\partial p_{max}=0.0005\,{\text{kg m}}^{-3}{\text{pbar}}^{-1}} , with p {\textstyle p} the pressure and σ 1 {\displaystyle \sigma _{1}} (100 bar) the reference pressure. The vertical conservative temperature, absolute salinity, and potential density gradients are all below the threshold value by meeting these three conditions:

| α ρ 0 ∂ T ∂ p | ≤ 0.005 kg m 3 bar , {\displaystyle \left|\alpha \rho _{0}{\frac {\partial T}{\partial p}}\right|\,\leq 0.005\,{\frac {\text{kg}}{{\text{m}}^{3}{\text{bar}}}},}

| β ρ 0 ∂ S ∂ p | ≤ 0.005 kg m 3 bar , {\displaystyle \left|\beta \rho _{0}{\frac {\partial S}{\partial p}}\right|\,\leq 0.005\,{\frac {\text{kg}}{{\text{m}}^{3}{\text{bar}}}},}

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