Measurements of the upper ocean thermal structure show that on the continental shelves the thermocline depth may shoal or deepen generally depending on the bottom topography. Thermocline shoaling and deepening cause changes in the phase speeds of internal waves as described by linear wave theories. On the other hand, the ocean area where internal waves have variable phase speeds may be treated as a dynamically inhomogeneous medium. In this case, theories of nonlinear dispersive wave propagation in inhomogeneous media developed by Tappert and Zabusky <1971> may stand. We used these theories to analyze the evolution of ocean internal solitary waves passing over a seamount in the Gulf of Aden. The results indicate that a surprisingly sharp recess of an internal solitary wave packet, imaged by the space shuttle Discovery, is a signature of spatial phase delay caused by thermocline shoaling over the seamount. Soliton fission due to thermocline shoaling was also observed in the imagery. The observed number of transmitted solitons over the seamount agrees with theoretical predictions. Relative soliton amplitudes measured from the imagery also agree qualitatively with predictions. ¿ 2001 American Geophysical Union