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 (warm water rising, cold water sinking). 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.^[1] The surface mixed layer is the uppermost layer in the ocean and is well mixed by mechanical (wind) and thermal (convection) effects. Climate change is causing the upper ocean stratification to increase.^{[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. 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.^{[clarification needed]}

Between 1960 and 2018, upper ocean stratification increased between 0.7-1.2% 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.^{[clarification needed]} In the last few decades,^[when?] 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.^[2] The southern oceans (south of 30°S) experienced the strongest rate of stratification since 1960, followed by the Pacific, Atlantic, and the Indian Oceans.^[1] Increasing stratification is predominantly affected by changes in ocean temperature; salinity only plays a role locally.^[1]