In this talk, I will discuss two different mechanisms of internal wave generation in the upper ocean. The first scenario concerns the gravity current propagating at the surface of a stratified medium. This scenario is commonly seen when a river joins the coastal ocean, resulting in the formation of a gravity current due to the density difference between the river water and ocean water. Recent observations near the Columbia river mouth show large amplitude internal waves comparable to the internal waves formed due to tide-topography interaction. We performed laboratory scale computer simulations to understand the conditions under which internal waves are released from the density front. We found that for super-critical regimes, flow is dominated by shear instabilities, Kelvin-Helmholtz billows, and an internal solitary wave is observed propagating at the head of the front. For sub-critical conditions, the solitary wave feature gradually disappears and high mode internal waves are formed and are seen propagating into the interior. From three-dimensional simulations, we quantified the turbulent kinetic energy budget for both supercritical and subcritical conditions and found that the dominant balance is between the shear production and dissipation rate.

The second scenario concerns the evolution of near-inertial waves(NIWs) on a β-plane. We performed large scale simulations by imposing initial zonal velocity in the mixed layer and let it evolve with time. We considered various background stratification values and initial velocity magnitudes, to investigate their effect on the wave characteristics and the decay rate of mixed layer kinetic energy. Increasing the interior stratification strength leads to increased energy content in higher vertical modes and faster decay of mixed layer kinetic energy. The faster decay of mixed layer kinetic energy at higher stratifications is due to the increased downward energy flux at the base of the mixed layer. A strong double-inertial frequency signal is observed in the FFT spectrum, especially at higher stratifications, which could be a result of nonlinear interactions between various modes of near-inertial frequency. As we increase the initial velocity magnitude, the FFT spectrum shows higher harmonics (3f,4f), apart from the f and 2f signals. Also, regions of strong shear 〖Ri〗_g