From AMS Glossary
A term for water of exceptionally uniform properties over an extensive depth range, caused in most instances by convection.
Mode waters represent regions of water mass formation; they are not necessarily water masses in their own right but contribute significant volumes of water to other water masses. Because they represent regions of deep sinking of surface water, mode water formation regions are atmospheric heat sources. Subantarctic Mode Water is formed during winter in the subantarctic zone just north of the subantarctic front and contributes to the lower temperature range of central water; only in the extreme eastern Pacific Ocean does it obtain a temperature low enough to contribute to Antarctic Intermediate Water. Subtropical Mode Water is mostly formed through enhanced subduction at selected locations of the subtropics and contributes to the upper temperature range of central water. Examples of Subtropical Mode Water are the 18°C water formed in the Sargasso Sea, Madeira Mode Water formed at the same temperature but in the vicinity of Madeira, and 13°C water formed not by surface processes but through mixing in Agulhas Current eddies as they enter the Benguela Current.