Ocean stratification

Stratified layers are a barrier to the mixing of water, which impacts the exchange of heat, carbon, oxygen and other nutrients.

[1][clarification needed] 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.

[clarification needed] Between 1960 and 2018, upper ocean stratification increased between 0.7 and 1.2% per decade due to climate change.

[1] This means that the differences in density of the layers in the oceans increase, leading to larger mixing barriers and other effects.

[1] Increasing stratification is predominantly affected by changes in ocean temperature; salinity only plays a role locally.

The density of water in the ocean, which is defined as mass per unit of volume, has a complicated dependence on temperature (

Since the density depends on both the temperature and the salinity, the pycno-, thermo-, and haloclines have similar shapes.

Using the GODAS Data,[4] a temperature-salinity plot can show the possibilities and occurrences of the different combinations of salinity and potential temperature.

[5][clarification needed] 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.

[2] 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.

[1] 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.

[9] By looking at the GODAS Data[4] 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.

In many scientific articles, magazines and blogs, it is claimed that the stratification has increased in all of the ocean basins (e.g. in Ecomagazine.com[10] and NCAR & UCAR News [11]).

[1] The change in temperature dominates the increasing stratification, while salinity only plays a role locally.

[1] The ocean has an extraordinary ability of storing and transporting large amounts of heat, carbon and fresh water.

There is limited evidence that seasonal differences in stratification have grown larger over the years.

Hence, it can be stated that salinity plays a more local role in the increase of stratification, even though it is less present compared to the influence of the temperature.

[1][14] In the Arctic, the decrease of salinity, and hence density, can be explained by the input of freshwater from melting glaciers and ice sheets.

[15] Since oxygen plays a direct and important role in the cycles of carbon, nitrogen and many other elements such as phosphorus, iron and magnesium, de-oxygenation will have large consequences.

[1] To illustrate, in the period between 1970 and 1990, approximately 15% of the de-oxygenation can be explained by an increase of temperature and the rest by reduced transport due to stratification.

[12] This has led to an increase of hypoxic zones, which can lead to a change in behaviour of the aquatic flora and fauna.

In some cases, the expanded surface warm layer supports life moving to greater depths[16], while in others the differing densities of stratified waters act to separate species from one another[17].

Stratification of water limits the distribution of nutrients required for life, resulting in oligotrophic regions spreading as the surface thermocline strengthens[18].

As water stratification increases, the amount of dissolved oxygen made available to organisms further from the surface decreases[21].

[22] Heat stored in the mixed layer in the tropical western Pacific plays a vital role in El Nino development.

The depth of the mixed layer is associated with physical, chemical and biological systems and is one of the most important quantities in the upper ocean.

An exact relation between an increase in stratification and a change in the mixed layer depth has not yet been determined and remains uncertain.

[28] It has been found that the mixed layer in the extension of the Kuroshio Current, at the west side of the North Pacific, has decreased more than 30 meters.

[9] Although it is virtually certain that upper ocean stratification will increase through the 21st century, scientists express low confidence in how the mixed-layer depth will evolve.

The halo-, thermo-, and pycnocline at 10E, 30S. For this image the annual means of the year 2000 from the GODAS Data [ 4 ] has been used.
Potential temperature - salinity plot. This plot was generated using the GODAS Data [ 4 ] of 2020.
Occurrences of combinations of potential temperature and salinity in the ocean. This plot was generated using the GODAS Data [ 4 ] of 2020.
The surface temperature, surface salinity and surface potential density calculated and plotted using the annual mean over the year 2000 of the GODAS Data. [ 4 ]
Annual and latitudinal means of for different ocean basins. This plot was generated using the GODAS Data [ 4 ] of 1980, 2000 and 2020.
Change in annual and latitudinal means of for different ocean basins. This plot was generated using the GODAS Data [ 4 ] of 1980, 2000 and 2020.
This figure shows the global change in stratification since the year 1960 until 2018 from 0 to 2000 meters. [ 1 ] (a) Global, (b) Pacific Ocean, (c) Atlantic Ocean, (d) Indian Ocean and (e) Southern oceans. The thin grey lines indicate the interannual variations. The small plot in (a) shows the rates of for the global case and for the basins. This is calculated by centered differences of the smooth time series (Glb: Global, Pac: Pacific, Atl: Atlantic, So: Southern, Ind: Indian). The trends have been plotted for various datasets, indicated by the different lines.
Annual means and change in annual means of the mixed layer depth of 1980, 2000 and 2020. This plot was generated using the GODAS Data. [ 4 ]
Identification of regions with increase and decrease of annual means of the mixed layer depth of 1980, 2000 and 2020. This plot was generated using the GODAS Data. [ 4 ]