Background

What is \(H_s\)?

Significant wave height, or \(H_s\), is a statistical quantity that describes the mean amplitude of the highest \(\frac{1}{3}\) of recorded waves. It describes how tall the waves are on average, specifically making a good estimate of the wave height you would notice when looking out at the ocean.

Different processes that can modify \(H_s\):

Enhanced winds at the surface (effect from storms): Storms produce strong winds which create waves through an exchange of momentum between the atmosphere and ocean (Holthuijsen 2010). These waves are known as wind-sea. Wind-sea waves typically have shorter periods and amplitudes resulting in less wave energy. Their larger counterparts, known as swell, have longer periods and much higher amplitudes. Swell do not typically feel the affect of winds because of their already high wave energy. In contrast to the spatial scale we are focusing on (<30 km), this process occurs at larger scales (>30 km).

Current induced refraction (non-local effect): Refraction leads to convergences and divergences of wave action through changes in wave direction. These changes in wave directions are caused by spatial gradients in current velocity. When a gradient in velocity is present, there is vorticity that induces a change in wave direction.

Enhanced winds at the surface (local effect): When crossing from the cold side to the warm side of a temperature front, there is a decrease in the vertical stability of the atmosphere which intensifies the turbulence in the atmospheric boundary layer. Because of this, near-surface wind shear is amplified and increases near-surface winds (Frenger et al. 2013). These near-surface winds create wind-sea. While they are present in a physical sense, when analyzing \(H_s\) signatures wind-sea is not visible because of the statistical nature of this wave variable. This process is currently only backed by theory and is not well established. There is still much to be answered about this phenomenon.

Concertina effect (local effect): This effect can work both ways. If current and wave directions are aligned and moving in the same direction, wave height can decrease from currents stretching the waves and influencing a longer period. If current and wave directions are aligned and moving in opposing directions, wave height can increase from currents pushing the into waves and influencing a shorter period. The impact of this effect is stronger on wind-sea waves because of their lower energy.