Large-amplitude internal solitary waves (ISWs) propagating eastward toward the Alboran Sea have long been known in the Strait of Gibraltar. New in-situ data and satellite images evidence northwestward propagating ISWs. These waves are probably the reflection of the well-known eastward propagating wave along the Moroccan shelf. A simple 2D-vertical section, run with the compressible non-hydrostatic Coastal and Regional Ocean COmmunity model, illustrates that the Moroccan slope is conducive to reflection of incident solitary waves of amplitude observed in the Strait of Gibraltar. A clear signature of these waves in the Tarifa high-frequency sea level oscillations is depicted that paves the way to studies of the seasonality of ISWs in the Strait of Gibraltar with the long-time series of sea level at Tarifa. The polarity of the reflected waves, observed with Acoustic Doppler Current Profilers/Conductivity-Temperature-Depth array moorings, presents a slight variability possibly due to the internal tide oscillations. The reflected ISWs arrives at the mooring location in phase with the peak of the internal tide. For the strongest tide, the pynocline approaches a critical point where the polarity of the ISWs might reverse.

Key Points

Northwestward propagating internal solitary waves (ISWs) suspected to be reflections are evidenced in the Strait of Gibraltar

A simple numerical approach illustrates that incident ISWs of amplitude observed in the area can reflect on the Moroccan slope

High-frequency sea level oscillations at Tarifa reveal a signature consistent with the observed ISWs

Plain Language Summary

The Strait of Gibraltar is known as a hotspot for internal solitary waves (ISWs) generation, which propagate eastward toward the Alboran Sea. A new in-situ data set combined with satellite images reveal northwestward propagating ISWs in the Strait of Gibraltar. These waves are likely generated by the reflection of the eastward propagating waves on the Moroccan shelf as shown by simple numerical simulations. The reflected wave train, whose leading wave can be either an elevation or a depression wave, arrives at the observation site in phase with the internal tide trough (maximum sinking of the isopycnals). In strong spring tides, the interface depth approaches the critical point, which allows the possibility of change of wave-polarity, that is, depression versus elevation, thus explaining the double nature of the observed leading waves. Signatures of the reflected waves have been also identified in the high-frequency sea level oscillations at Tarifa. This study paves the way for long-term observations of ISWs in the Strait of Gibraltar.