In a fluid whose density changes with the height, either abruptly at a sharp surface of discontinuity or gradually, it is possible to have internal wave motions whose amplitude is greatest at the density discontinuity or, in the case of a gradual density change, somewhere in the interior of the fluid and not at the free upper surface where the surface waves have their maximum amplitude....
Haurwitz, Bernhard. 1948. "The effect of ocean current on internal waves." Journal of Marine Research 7, (3). https://elischolar.library.yale.edu/journal_of_marine_research/662