Internal tide

Internal tide is a term used to describe oscillations that have the same frequency as the tide, but are produced within the ocean and not associated with surface movements (which is why they are called 'internal').

The internal tide is a yo-yo movement of isopycnals (iso-density lines); the interface between warm and cold water oscillates vertically. The internal tide is produced by the interaction between the external tide (the tide seen on the coast) and the underwater relief (medio-oceanic ridges, continental slope).