circulation

…a special kind of water circulation that traps plant nutrients and stimulates primary production. Fresh water, being lighter than salt water, tends to form a distinct layer that floats at the surface of the estuary. At the boundary between fresh and salt water, there is a certain amount of mixing…

The proof of Thomson’s theorem depends on the concept of circulation, which Thomson introduced. This quantity is defined for a closed loop which is embedded in, and moves with, the fluid; denoted by K, it is the integral around the loop of…

…an event known as an overturn (a mixing of the water layers). Since then the trend has been toward restoring the formerly stratified water layers, but with more instances of overturning.

Water circulates within but not between the layers, more vigorously within the epilimnion. The boundary between these layers is the metalimnion, a zone of rapid temperature change. With the onset of autumn, the epilimnion cools and the water becomes denser, sinking and mixing with the hypolimnion.…

…results in what is termed circulation, or overturn, of lake waters. Lakes that cool to below 4 °C in winter experience two turnover periods, as just described, and are called dimictic lakes. Most lakes in temperate regions fall into this category. Lakes that do not cool to below 4 °C…

…surface water sinks, and convective overturn proceeds as the density of the surface water increases with the decreasing temperature. By the time the surface water reaches 4 °C (39.2 °F), the temperature of maximum density for fresh water, the density-driven convective overturn has reached the bottom of the lake, and…