An experimental study of weakly nonlinear waves atop a vertically sheared current

We carry out a laboratory study of weakly nonlinear waves atop a following, vertically sheared current, relevant for a wide range of coastal and ocean locations. The experimental observations are compared with existing theories for the shear-current modified dispersion relation and amplitudes of second-harmonic waves, where a good agreement has been shown. Both the experiments and theoretical calculations show that the findings for wave groups and regular waves are consistently affected by the presence of the following current. For regular (carrier) waves with a fixed absolute frequency, the additional presence of the following current is found to decrease the amplitude of the second-harmonic waves when the linear amplitude is fixed, yet increase the second-harmonic waves when the steepness is fixed owing to the additional shear-current modified effects on the dispersion relation of linear waves. These nuanced and apparently contrasting findings address the significance of resolving the combined wave–current field effectively to ensure a correct interpretation of wave properties in field measurements.