Out of one, many: creating broadband spectra of oceanic internal waves

Internal gravity waves, which owe their existence to a combination of stratification and the Earth’s rotation, are known to contribute significantly to vertical mixing and the functioning of the global ocean circulation, yet they are far too small and too fast to be resolvable in climate models. This means we rely on theory and simple parameterizations to model their impact on the resolved flow in these models. A persistent theory challenge has been to explain observed broadband wave spectra even though the dominant internal wave sources are known to be nearly monochromatic. Nonlinear processes such as wave--wave or wave--current interactions are therefore essential to explain the observations. In particular, wave turbulence theory based on resonant wave--wave interactions has long been a candidate for this problem, but the practical problems of applying this theory to a non-isotropic, non-canonical fluid dynamics system have been immense. In this talk, I will introduce these issues and then discuss recent advances in wave turbulence theory that greatly simplified its application to internal waves and led to predicted wave spectra that are arguably much closer to the observations than indicated by previous work.

Image credit: Richard R. Schünemann (Unsplash) .