Stanford University

*** Ph.D. Thesis/ Oral Defense ***

The behaviour of near-inertial waves in submesoscale flows

Jamie Hilditch

Friday, May 9^th , 9:00 AM

Green Earth Sciences – RM 365

Zoom link

Department of Earth System Science

Advisor: Dr. Leif Thomas

Abstract:  The upper ocean can often be found performing a peculiar dance with the surface features executing a circular shuffle — the signature move of near-inertial waves. The stage for this performance is a complicated mess of eddies, fronts and filaments that form the background flow. Spatial variability in this background flow causes the waves to shuffle at different rates in different places and lose synchronicity. Physically, this refraction drastically changes the propagation behaviour of the waves with important consequences for upper ocean mixing. In this talk, we’ll investigate the interactions of near-inertial waves with a class of flows known as the submesoscale. Inspired by observations made in the northern Gulf of Mexico, we’ll look at a series of increasingly complex idealised models to dive into and understand the key physics shaping the wave dynamics and the fate of the wave energy.

The defining features of geophysical fluid dynamics are rotation and density stratification. Restoring forces associated with these two features, Coriolis and buoyancy respectively, give rise to inertia-gravity waves. Near-inertial waves can be found at the low frequency end of this spectrum where, unsurprisingly, the frequencies are close to the inertial, or Coriolis, frequency.  These waves form a major part of the transient response to wind forcing and often dominate the kinetic energy budget of the upper ocean. They play a crucial role in deepening the mixed layer but, more pertinently for this talk, they are also associated with large vertical shears and the transport of energy from the ocean surface into the interior. Being both energetic and highly susceptible to shear instabilities, near-inertial waves are thought to be an important source of mixing in the pycnocline and below. On the Texas-Louisiana shelf, the situation is more extreme as the shallow depths mean that near-inertial waves can drive mixing throughout the entire water column.

Submesoscale flows are characterised by short horizontal length scales and strong lateral gradients.  Challenging to model and even harder to observe, they have been the subject of an increasing amount of attention over the past 20 years. Associated with strong vertical velocities, they make a leading order contribution to the Earth system through the vertical transport of heat, nutrients and dissolved gases. However, in this talk we look at the less well-known but crucial role that the submesoscales play in controlling internal wave dynamics.