Global and 100-Earth-year mean buoyancy frequency N of the ocean (e.g., Eq. (2.224) in Vallis 2017) in three experiments, where g is the gravitational acceleration, ρ is the locally referenced potential density of the ocean, and ρ₀ is the reference density. Above the pycnocline, the surface ocean (above 1000 m) is strongly stratified, while in the deep ocean (below 2000 m) the stratification is very weak. The value of N for the Earth’s ocean ranges from 10⁻³ to 10⁻² s⁻¹ in the upper ocean, where density changes dramatically with depth. The deep and abyssal ocean is less stratified compared to the upper ocean, with N ranging from10⁻³ to 10⁻⁴ s⁻¹ (Talley 2011; King et al. 2012).