halocline

Halocline is a layer in a body of water in which salinity changes rapidly with depth. It forms wherever freshwater input or evaporation creates a vertical salinity gradient, such as in estuaries, coastal seas adjacent to rivers, fjords, and Arctic coastal zones.

Typically the halocline lies between a fresher surface layer and a more saline bottom layer, and its exact depth and slope depend on wind mixing, river discharge, and tides. It can be detected as a sharp change in salinity with depth, measured as a gradient dS/dz in practical salinity units per meter.

Haloclines influence water density and thus vertical stratification, often reducing mixing between layers. This can affect oxygen supply, nutrient transport, and plankton distribution, and can interact with temperature gradients to shape the pycnocline and overall stratification.

In some regions the halocline is transient, strengthening in dry seasons with high evaporation or during pulses of freshwater inflow, and weakening with storms and strong winds that promote mixing. In lakes and seas with substantial river influence, haloclines can persist across seasons.

Etymology: from halos meaning salt and klinein to slope.