Constructive interference (superposition)
To these we may add several additional possible mechanisms for extreme wave formation that are the subject of current research:
Spatial or temporal focusing effects
Modulation and resonance
The first category, strong currents, has indisputably been the source of extreme waves. When swell or storm waves encounter a fast-moving opposing current such as the Agulhas Current or the Gulf Stream, they tend to “pile up” as their velocity is reduced. Professor Chris Garrett pointed out that a wave with a phase speed of c meters per second can be stopped by an opposing current of only 1/4c.1 When this happens, steep, high waves result, proceeded or followed by deep troughs. Thus, the evidence is clear that the probability of encountering an extreme wave is greater under these conditions and a prudent mariner should avoid this situation if possible.
The increase in wave size as a function of wind velocity, fetch, and wind duration is a well-known phenomenon in storms. There are correlations that provide estimates of the significant wave heights under varying storm conditions, but none that predict the random occurrence of extreme waves. This suggests that some additional mechanism, yet to be fully understood, is at work. It would be useful to know if extreme wave formation is governed by a threshold effect; in other words, do seas have to build to a certain point before extreme waves are produced? Or is it purely a statistical effect?
The evidence seems to indicate the latter possibility, because many mariners (myself included) recall sailing in relatively calm seas where the significant wave height was a few feet, but suddenly a wave two to three times as high struck the vessel.
Likewise, shallow water, bottom effects, and refraction have the effect of slowing waves and causing wave heights to increase. Areas where there is a sharp transition in sea depth are potential danger zones