Abstract
Context: Harmonic summation and amplification by winds blowing contrary to currents are known contributions to rogue waves, 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.
Method: Vortex theory was applied qualitatively to the weather and ocean conditions of the Agulhas region.
Results: Rogue wave formation is attributed to: (1) Wind lee vortices cause steepening of wave leeward face, and suppresses wave breaking. (2) Boundary layer vortices from the meteorological cold front transfer energy to the wind lee vortices thereby enhancing their wave sharpening effect. (3) Agulhas current boundary layer vortices interact 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, generates 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. (5) Jetting arises when the canyon vortex and the Agulhas current boundary layer vortices pass each other, thereby accentuating wave height, steepness, and asymmetry.
Conclusions: The novel contribution is the provision of a mechanism for rogue wave formation, using vortex theory, that conceptually integrates wave formation, Agulhas sea currents, bathymetric features including submarine canyons, and meteorological cold front weather systems.