EAPS

Title: Ocean mixing driven by tides: breaking lee waves, hydraulic jumps and the influence of subinertial internal tides

Abstract: The dissipation of tidal energy in the abyssal ocean leads to diapycnal mixing, raising the deep cold waters that form at high latitudes, and providing an important link in the energetics of the overturning circulation. Observations of deep ocean mixing show that it is both intermittent and highly inhomogeneous, with hotspots over regions of rough topography. Determining when, where and how much the ocean mixes is of critical importance for understanding the large scale circulation, with implications for biogeochemistry, heat transport and ocean modeling in a changing climate.

In this talk, I will focus on specific processes associated with tidally driven turbulence, combining field observations and numerical modeling of flows through a small channel that transects the crest of the Mendocino Ridge in the eastern Pacific, a site of mixed (diurnal and semidiurnal) tides. At this latitude the diurnal tide is subinertial and evanescent away from the topography, in contrast to the semidiurnal tide which is superinertial and radiating. We construct two numerical simulations to interpret our observations. First, we use a two-dimensional, nonhydrostatic, high resolution simulation to examine flow through the channel, and show that the observed turbulence arises from both hydraulic jumps and breaking internal lee waves. To place the processes in a regional context, we use a second simulation of tidally driven flow in a three-dimensional domain using realistic bathymetry, demonstrating the presence of diurnal bottom-trapped waves. These energetic internal waves generate strong currents close to the topography, and their interplay with the superinertial tidal constituent is of leading order importance in determining the timing and magnitude of the observed turbulence at the ridge.

Our observations and models illustrate some of the mechanisms by which energy is transferred from the tides to the large scale circulation, and highlight the role of subinertial trapped waves at this location. We posit that these waves may be important contributors to ocean mixing, especially at climatically sensitive high latitudes.

Event link: https://eapsweb.mit.edu/paoc-colloquium-ruth-musgrave-mit