Ocean internal waves and turbulence: Numerical simulations, observations, and theoretical progress

Abstract:
Internal waves and tides exist in the ocean interior due to differences in fluid densities. Interactions between internal tides generated by tidal flow over bathymetric features and near-inertial waves generated by winds yield a spectrum of internal waves at many frequencies. High-resolution realistic ocean simulations have come a long way and now contain a well-developed spectrum. We present results from some of our most recent works where we suggest improvements to the well-known K-Profile Parameterisation which leads to a better internal wave wavenumber spectrum in regional ocean simulations by comparing it to observational data. Improvement in the vertical wavenumber spectrum of internal waves is crucial for fine-scale parameterization. Breaking of internal waves causes mixing, and it has important effects on the distribution of ocean temperature and nutrients. I will discuss observations of year-long geophysical turbulence in the Bay of Bengal measured with mixing meters called Chipods. We study the role of low-salinity water in modulating subsurface turbulence and elaborate on the seasonal nature and depth penetration of turbulence. This could be of importance to improving our understanding of the role of the Bay in the Indian monsoon. We find a prolonged suppressed phase of turbulence in the post-monsoon season which conditions the Bay for the summer monsoon of the next year. We also provide evidence of an interesting diurnally varying mixing signature in the Bay which, we strongly suspect, is biological in origin. I will briefly describe how the observations of vertical turbulence variability in the Bay inspired our theoretical work on nonlinear optimal perturbations in a viscosity-stratified flow. I will also touch upon an ongoing work where we take up the task of mapping the incoherent tides of the world's oceans using spatiotemporal basis functions.