Wave physics in the Agulhas Current using near-exact computations of wave-wave interactions

Abstract

Strong ocean currents such as the Agulhas Current near South-Africa are alleged to generate freak waves due to the enhancement of wave energy under counter-current conditions. It has been suggested that such enhancement is due to current-induced refraction but it is actually the result of a balance between many processes. To understand this balance and its possible effect on the occurrence of freak waves, the relative importance of all processes of wave propagation, generation and dissipation in the Agulhas Current are shown with high-resolution (1/32°) maps of the intensity of these processes (including the transfer of energy between waves and currents). These computations are carried out for three representations of the Agulhas Current: an analytical approximation of the coastal jet, an analytical approximation of a ring and one realistic case over a 2000 x 2000 km2 area. Since an increased directionality of the waves seems to greatly enhance the probability of generating a freak wave, we do not use the conventional technique of approximating quadruplet wave-wave interactions which over-estimates that directionality. Instead we use a near-exact approach which is deemed to be more accurate. An inspection of the computational results shows that the currents do enhance the wave heights and the directionality somewhat but the degree to which this occurs is hardly enough to seriously affect the probability of freak wave occurrence. The intensity of all processes is of the same order of magnitude except that of the transfer of energy between the waves and the currents, which is one order of magnitude smaller than the other intensities.