Isotherm

An isotherm is a line or surface of constant temperature. In the upper ocean the depth of a chosen isotherm, often the 20 C surface, tracks the thermocline and is a standard index of upper-ocean heat content and ENSO state, since it shoals during El Nino in the eastern Pacific. Tight isotherm spacing marks the thermocline, the layer of strong vertical temperature gradient that caps the mixed layer. On temperature-salinity diagrams isotherms run with isohalines and isopycnals to identify water masses by their joint properties.