Internal solitons on the Black Sea shelf: observation of waves of record amplitudes

When conducting work in the fall of 2015 on the Black Sea northeast shelf, we recorded internal waves, the unusualness of which attracts special attention for the following reasons. For the first time in 40 years of internal waves observations in the Black Sea, such high waves with amplitudes of 14–16 m were measured. The generation of these anomalous waves was connected with a cold atmospheric front passing over the sea. It was the first experimental evidence in the sea of such mechanism for internal waves generation. The observed internal waves had a clear seen character of nonlinear soliton-like waves.

We met the train of internal solitons during a sub-satellite survey conducted in the sea from a motor yacht equipped with ADCP “Rio Grande 600 kHz” in the waters near Cape Tolsty. The train was found at a point of the sea with a depth of 33 m and then was recorded on seven multidirectional tacks oriented normal to the coast. It moved across the shelf to the coast along the bottom thermocline, while the bottom currents accompanying it had a northwestern coastal orientation. The train included four waves of a soliton-like shape with sharpened crests and flattened troughs. Their lengths were 100-110 m, heights up to 14-16 m, vertical velocities in orbital currents reached 0.15-0.20 m/s. Another property of nonlinear waves was also expressed - the amplitude ranking of waves in the train. Traced on successive tacks for 2.5 hours, internal waves had preserved the soliton-like shape and as well the strong vertical component in their orbital currents. Despite the fact that the train was moving along the bottom thermocline, the effect of internal waves was sufficient to appear on satellite radar images of the sea surface of the study area. The performed processing of satellite images confirmed the wave parameters measured by contact methods.  An interesting fact of a long accompaniment of internal solitons by a school of fish was discovered. Fishes were concentrated in areas where internal waves carried the components of fish food supply to the surface from the bottom layers. The work was partially supported by RFBR grant 19- 05-00715.