Ocean stratification

Stratification of water occurs in many different ways. Two main types of stratification can be defined; uniform and layered stratification. In all of the ocean basins, layered stratification occurs. The stratified layers act as a barrier to the mixing of water, which can impact the exchange of heat, carbon, oxygen and other nutrients . By means of upwelling and downwelling, which are both wind-driven, allow mixing of different layers by means of the rise of cold nutrient-rich and warm water respectively. Generally, the layers are based on the weight of the water. As quantification of this weight, density is used and it is not counterintuitive that heavier, and hence denser, water is located below the lighter water. An example of a layer in the ocean is the pycnocline, which is defined as a layer in the ocean where the change in density is relatively huge compared to the other layers in the ocean. The thickness of the thermocline is not constant everywhere, but it depends on a variety of variables. Over the years the stratification of the ocean basins has increased . An increase in stratification means that the differences in density of the layers in the oceans increase, leading to larger mixing barriers.

Density of Water in the Oceans

The halo-, thermo-, and pycnocline at 10E, 30S. For this image the annual means of the year 2000 from the GODAS Data has been used.

The surface temperature, surface salinity and surface potential density calculated and plotted using the annual mean over the year 2000 of the GODAS Data

The density of water in the oceans, which is defined as mass per unit of volume, has a complicated dependence on temperature ($T$), salinity ($S$) and pressure ($p$) (or equivalently depth) and is denoted $\rho (S,T,p)$. The dependence on pressure is not significant, since seawater is not perfectly incompressible . A change in the temperature of the water has an influence on the distance between water parcels. When the temperature of the water increases, this distance will increase and hence the density will decrease. A change in salinity induces an opposite reaction. Salinity is a measure of the mass of dissolved solids, which consist mainly of salt, in the water and hence increasing the salinity will increase the density. Just as the pycnocline defines the layer with a fast change in density, similar layers can be defined for a fast change in temperature and salinity; the thermocline and halocline respectively. Since the density depends on both the temperature and the salinity, the pycno-, thermo-, and halocline have a similar shape, except that the density increases with depth, whereas the salinity and temperature decrease with depth. According to the UNESCO formula, the density of ocean water can be defined as follows, where $K(T,S,p)$ is a term involving the compressibility of the water ;

$$\rho ={\frac {\rho (S,T,p)}{1-{\frac {p}{K(S,T,p)}}}}$$

In this formula, $\rho (S,T,0)$ and $K(S,T,p)$ are both heavily dependent on the temperature and less dependent on the salinity;

${\textstyle \rho (S,T,0)=\rho _{SMOW}+B_{1}S+C_{1}S^{1.5}+d_{0}S^{2}}$ with:

$${\begin{aligned}{}&\rho _{SMOW}=a_{0}+a_{1}T+a_{2}T^{2}+a_{3}T^{3}+a_{4}T^{4}+a_{5}T^{5}\\{}&B_{1}=b_{0}+b_{1}T+b_{2}T^{2}+b_{3}T^{3}+b_{4}T^{4}\\{}&C_{1}=c_{0}+c_{1}T+c_{2}T^{2}\\\end{aligned}}$$

and ${\textstyle K(S,T,p)=K(S,T,0)+A_{1}p+B_{2}p^{2}}$ with

$${\begin{aligned}{}&K(S,T,0)=K_{w}+F_{1}S+G_{1}S^{1.5}\\{}&K_{w}=e_{0}+e_{1}T+e_{2}T^{2}+e_{3}T^{3}+e^{4}T^{4}\\{}&F_{1}=f_{0}+f_{1}T+f_{1}T+f_{2}T^{2}+f_{3}T^{3}\\{}&G_{1}=g_{0}+g_{1}T+g_{2}T^{2}\\{}&A_{1}=A_{w}+(i_{0}+i_{1}T+i_{2}T^{2})S+j_{0}S^{1.5}\\{}&A_{w}=h_{0}+h_{1}T+h_{2}T^{2}+h_{3}T^{3}\\{}&B_{2}=B_{w}+(m_{0}+m_{1}T+m_{2}T^{2})S)\\{}&B_{w}=k_{0}+k_{1}T+k_{2}T^{2}\end{aligned}}$$

Here, all of the small letters, $a_{i},b_{i},c_{i},d_{0},e_{i},f_{i},g_{i},i_{i},j_{0},h_{i},m_{i}$ and ${\textstyle k_{i}}$ are constants defined in . That the density depends more on the temperature than on the salinity can be seen from plots using the GODAS Data. In those plots, it can be seen that locations with the coldest water, at the poles, are also the locations with the highest densities. The regions with the highest salinity, on the other hand, are not the regions with the highest density, meaning that temperature mostly determines the density in the oceans.

Ocean Stratification

Ocean stratification can be defined and quantified by the change in density with depth. It is not uncommon to use the Buoyancy frequency, or sometimes called the Brunt-Väisälä frequency as direct representation of stratification in combination with observations on temperature and salinity. The Buoyancy frequency $N$ is defined as follows;

$$N^{2}={\frac {-g}{\rho _{0}}}{\frac {\partial \rho }{\partial z}}$$

Here $g$ is the gravitational constant, $\rho _{0}$ is the density of pure water and $\rho$ is the potential density depending on temperature and salinity as discussed earlier. Water is considered to have a stable stratification for $\partial \rho /\partial z<0$, leading to a real value of $N$. The ocean is typically stable and the corresponding $N$-values in the ocean lie between approximately $10^{-4}$, in the abyssal ocean, and $10^{-3}$, in the upper parts of the ocean. The Buoyancy period is defined as $1/N$. Corresponding to the previous values, this period typically takes values between approximately 10 and 100 minutes . In some parts of the ocean unstable stratification appears, leading to convection.

Observations in the Oceans

In the last few decades, the stratification in all of our ocean basins has increased . By looking at the GODAS Data provided by the NOAA/OAR/ESRL PSL, the Buoyancy frequencies can be found from January 1980 up to and including March of 2021. Since the temperature influences the density and hence the stratification, it is necessary to look at the mean of the Buoyancy frequency over an entire year to be able to see the change over the years.

Change in Ocean Stratification

Causes

Temperature rise

Consequences

De-oxygenation

change of mixed layer --> consequences for phytoplankton and light

See also

• Brunt-Väisälä frequency – Measure of fluid stability against vertical displacementPages displaying short descriptions of redirect targets
• Buoyancy frequency – Measure of fluid stability against vertical displacementPages displaying short descriptions of redirect targets
• Downwelling – Process of accumulation and sinking of higher density material beneath lower density material
• Density – Mass per unit volume
• Halocline – Stratification of a body of water due to salinity differences
• Lake stratification – Separation of water in a lake into distinct layers
• Mixed layer – Layer in which active turbulence has homogenized some range of depths
• Pressure – Force distributed over an area
• Salinity – Proportion of salt dissolved in water
• Stable stratification
• Temperature – Physical quantity of hot and cold
• Thermocline – Thermal layer in a body of water
• Upwelling – Replacement by deep water moving upwards of surface water driven offshore by wind