Ocean stratification

Ocean stratification is the natural separation of an ocean's water into horizontal layers by density. This is generally stable stratification, because warm water floats on top of cold water, and heating is mostly from the sun, which reinforces that arrangement. Stratification is reduced by wind-forced mechanical mixing, but reinforced by convection. Stratification occurs in all ocean basins and also in other water bodies. Stratified layers are a barrier to the mixing of water, which impacts the exchange of heat, carbon, oxygen and other nutrients. The surface mixed layer is the uppermost layer in the ocean and is well mixed by mechanical and thermal effects. Climate change is causing the upper ocean stratification to increase.
Due to upwelling and downwelling, which are both wind-driven, mixing of different layers can occur through the rise of cold nutrient-rich and sinking of warm water, respectively. Generally, layers are based on water density: heavier, and hence denser, water is below the lighter water, representing a stable stratification. For example, the pycnocline is the layer in the ocean where the change in density is largest compared to that of other layers in the ocean. The thickness of the thermocline is not constant everywhere and depends on a variety of variables.
Between 1960 and 2018, upper ocean stratification increased between 0.7 and 1.2% per decade due to climate change. This means that the differences in density of the layers in the oceans increase, leading to larger mixing barriers and other effects. In the last few decades, stratification in all ocean basins has increased due to effects of climate change on oceans. Global upper-ocean stratification has continued its increasing trend in 2022. The southern oceans experienced the strongest rate of stratification since 1960, followed by the Pacific, Atlantic, and the Indian Oceans. Increasing stratification is predominantly affected by changes in ocean temperature; salinity only plays a role locally.

Density of water in the oceans

The density of water in the ocean, which is defined as mass per unit of volume, has a complicated dependence on temperature, salinity and pressure, which in turn is a function of the density and depth of the overlying water, and is denoted as. The dependence on pressure is not significant, since seawater is almost perfectly incompressible. A change in the temperature of the water impacts on the distance between water parcels directly. When the temperature of the water increases, the distance between water parcels will increase and hence the density will decrease. Salinity is a measure of the mass of dissolved solids, which consist mainly of salt. Increasing the salinity will increase the density. Just like 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 the halocline. Since the density depends on both the temperature and the salinity, the pycno-, thermo-, and haloclines have similar shapes. The difference is that the density increases with depth, whereas the salinity and temperature decrease with depth.
In the ocean, a specific range of temperature and salinity occurs. Using the GODAS Data, a temperature-salinity plot can show the possibilities and occurrences of the different combinations of salinity and potential temperature.

The density of ocean water is described by the UNESCO formula as: The terms in this formula, density when the pressure is zero,, and a term involving the compressibility of water,, are both heavily dependent on the temperature and less dependent on the salinity:with:andIn these formulas, all of the small letters, and are constants that are defined in Appendix A of a book on Internal Gravity Waves, published in 2015.
The density depends more on the temperature than on the salinity, as can be deduced from the exact formula and can be shown in plots using the GODAS Data. In the plots regarding surface temperature, salinity and density, 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 contributes mostly to the density in the oceans. A specific example is the Arabian Sea.

Quantification

Ocean stratification can be defined and quantified by the change in density with depth. The Buoyancy frequency, also called the Brunt-Väisälä frequency, can be used as direct representation of stratification in combination with observations on temperature and salinity.
The Buoyancy frequency,, represents the intrinsic frequency of internal gravity waves. This means that water that is vertically displaced tends to bounce up and down with that frequency.
The Buoyancy frequency is defined as follows:Here, is the gravitational constant, is a reference density and is the potential density depending on temperature and salinity as discussed earlier. Water is considered to have a stable stratification for, leading to a real value of. The ocean is typically stable and the corresponding -values in the ocean lie between approximately in the abyssal ocean and in the upper parts of the ocean. The Buoyancy period is defined as. 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.
If the stratification in a water column increases, implying an increase of the value , turbulent mixing and hence the eddy viscosity will decrease. Furthermore, an increase of, implies an increase of , meaning that the difference in densities in this water column increase as well. Throughout the year, the oceanic stratification is not constant, since the stratification depends on density, and therefore on temperature and salinity. The interannual fluctuations in tropical Pacific Ocean stratification are dominated by El Niño, which can be linked with the strong variations in the thermocline depth in the eastern equatorial Pacific.
Furthermore, tropical storms are sensitive to the conditions on the stratification and hence on its change. On the other hand, mixing from tropical storms also tends to reduce stratification differences among layers.

Observations on increasing stratification

Temperature and salinity changes due to global warming and climate change alter the ocean density and lead to changes in vertical stratification. The stratified configuration of the ocean can act as a barrier to water mixing, which impacts the efficiency of vertical exchanges of heat, carbon, oxygen, and other constituents. Thus, stratification is a central element of Earth's climate system. Global upper-ocean stratification continued its increasing trend in 2022 and was among the top seven on record.
In the last few decades, the stratification in all of the ocean basins has increased. Furthermore, the southern oceans experienced the strongest rate of stratification since 1960, followed by the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. When the upper ocean becomes more stratified, the mixed layer of surface water with homogeneous temperature may get shallower, but projected changes to mixed-layer depth by the end of the 21st century remain contested. The regions with the currently deepest mixed layers are associated with the greatest mixed layer shoaling, particularly the North Atlantic and Southern Ocean basin.
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 2021. Since a change in stratification is mostly visible in the upper 500 meters of the ocean, very specific data is necessary in order to see this in a plot. The resulting plots from the GODAS Data might indicate that there is also a decrease in stratification looking at the differences of the stratification between the years 1980, 2000 and 2020. It is possible to see that the change in stratification is indeed the biggest in the first 500 meters of the ocean. From approximately 1000 meters into the ocean, the stratification converges toward a stable value and the change in stratification becomes almost non-existent.
In many scientific articles, magazines and blogs, it is claimed that the stratification has increased in all of the ocean basins. In the figure below, the trends of the change in stratification in all of the ocean basins have been plotted. This data shows that over the years the stratification has drastically increased. The changes in stratification are greatest in the Southern Ocean, followed by the Pacific Ocean. In the Pacific Ocean, the increase of stratification in the eastern equatorial has found to be greater than in the western equatorial. This is likely to be linked to the weakening of the trade winds and reduced upwelling in the eastern Pacific, which can be explained by the weakening of the Walker circulation.

Causes and consequences

Temperature and mixing

The change in temperature dominates the increasing stratification, while salinity only plays a role locally. The ocean has an extraordinary ability of storing and transporting large amounts of heat, carbon and fresh water. Even though approximately 70% of the Earth's surface consists of water, more than 75% of the water exchange between the Earth's surface and the atmosphere occurs over the oceans. The ocean absorbs part of the energy from sunlight as heat and is initially absorbed by the surface. Eventually a part of this heat also spreads to deeper water. Greenhouse gases absorb extra energy from the sun, which is again absorbed by the oceans, leading to an increase in the amount of heat stored by the oceans. The increase of temperature of the oceans goes rather slow, compared to the atmosphere.
However, the ocean heat uptake has doubled since 1993 and oceans have absorbed over 90% of the extra heat of the Earth since 1955. The temperature in the ocean, up to approximately 700 meters deep into the ocean, has been rising almost all over the globe. The increased warming in the upper ocean reduces the density of the upper ≈500 m of water, while deeper water does not experience as much warming and as great a decrease in density. Thus, the stratification in the upper layers will change more than in the lower layers, and these strengthening vertical density gradients act as barriers limiting mixing between the upper layers and deep-water.
There is limited evidence that seasonal differences in stratification have grown larger over the years.