Large-amplitude internal waves at the Mascarene Ridge in the Indian Ocean

Observations of intense internal waves near the underwater sill on the Mascarene Ridge are described and analysed. Large thermocline disturbances are generated with tidal periodicity on both sides of the sill. The lee-side disturbance at the western edge of the sill has a strong second mode component. It quickly evolves into a train of short, soliton-like internal waves while moving to the east against a slackening combined (steady + tidal) current. The upstream-side disturbance arising near the eastern edge of the sill has the character of a large depression. Moving to the east, towards the open ocean, it evolves into a sequence of huge internal soliton-like waves with up to 90 m amplitudes (in the upper thermocline) and phase speed of 3 m/s. The solitons first appear at a certain distance (about 100 km) from the sill and manifest themselves on the sea surface by alternating bands of rough and smooth sea. It can be explained in terms of ray propagation of the internal tide emanated from the sill, as it was first proposed by New and Pingree for the Bay of Biscay.