Internal solitary waves in the Red Sea : an unfolding mystery

Abstract

The off-shelf region between 16.0° and 16.5°N in the southern Red Sea is identified as a new hotspot for the occurrence of oceanic internal solitary waves. Satellite observations reveal trains of solitons that, surprisingly, appear to propagate from the center of the Red Sea, where it is deepest, toward the continental shelf, but they do not survive as coherent structures over the shelf. These solitons are characterized by coherent crest lengths exceeding 80 km and crest-to-crest distances of more than 2 km, compatible with signatures of large-amplitude solitary waves. Despite the fact that these Red Sea solitons have large amplitudes, they appear to be generated by very weak surface tides. Tidal current velocity is only about 5 cm s–1 over the shelf, much weaker than over other ocean shelves where similar solitary waves have been reported. The appearance of these waves over this particular geographical stretch suggests generation by a locally amplified internal tide on the main pycnocline. We consider three possible explanations for soliton generation in the Red Sea: interfacial tide resonance, local generation by internal tidal beams generated at the shelf breaks, and local generation by internal tidal beams generated at the shelf breaks but first amplified by repeated focusing reflections.