from two different storms can add and build to an extreme size due to the continuous input of energy. Also, the Benjamin-Feir index, introduced in Chapter 11, measures a phenomenon called the Benjamin-Feir instability. It is defined as the ratio of the mean square slope of the frequency spectrum peak to its normalized width. Under the right conditions, instability causes the wave train to break up into periodic groups. Within each group a further focusing takes place, producing a very large and steep wave having a height roughly three times the initial height of the wave train.5
Finally, other research is directed at seeing whether frequency or amplitude modulation could be responsible for extreme waves. Or is it possible that certain wave periods and frequencies will resonate with a given sea state condition to create extreme waves? The analogy to this is easily demonstrated in a bathtub. A bathtub, or a harbor for that matter, has a series of resonant frequencies. If you take a piece of wood (or possibly just your hand) and get the water sloshing back and forth at just the right period, a large wave will occur.