Conceptual Analysis of Vortex Contributions to Rogue Wave Formation in the Agulhas Current

Harmonic summation and amplification by winds blowing contrary to currents are known contributions to rogue waves in the region of the Agulhas current, but the causes of the observed wave steepness, asymmetric form, and non-breaking are poorly understood. The potential effect of bathymetric and meteorological features has not been addressed. Vortex theory was applied to develop a theory of wave formation, based on conceptual reasoning. Rogue wave formation is attributed to the following: (1) wind lee vortices causing steepening of a wave’s leeward face, and suppressing wave breaking; (2) boundary layer vortices from the meteorological cold front transferring energy to the wind lee vortices thereby sharpening the wave; (3) Agulhas current boundary layer vortices interacting with water lee vortices to accelerate a jet of water between them, thereby steepening the wave and enhancing the preceding trough; (4) bathymetric topology, especially a canyon on the continental slope, generating a vortex in the Agulhas current. This vortex is detached from the canyon by prising of the coastal downwelling current (induced by the meteorological cold front) and combines with the water lee vortex to heighten the wave, and (5) jetting, which arises when the canyon vortex and the Agulhas current boundary layer vortices pass each other, thereby accentuating wave height, steepness, and asymmetry.