Internal Waves Generated by Mixed Region Collapse in the Ocean

Tropical cyclones are known to mix the relatively warm near-surface fluid with the cooler underlying fluid creating a well-mixed region of uniform density. The well-mixed region collapses into the stably stratified ambient and forms an intrusive gravity current. This motion is a mechanism for the generation of downward propagating internal waves. We will present a series of laboratory experiments used to examine the axisymmetric collapse of a well-mixed region in a uniformly stratified and rotating fluid. A square tank was filled with uniformly stratified fluid and a hollow cylinder of radius $R_c\sim 5$\,cm and depth $H_m$ between $5$ and $15$\,cm was\, suspended at the center of the tank. Synthetic Schlieren was used to determine wave characteristics such as the frequency $\omega$ and radial wavenumber $k_r$. We found that internal wave frequencies were set by the buoyancy frequency $\omega \approx 0.75N$, the radial wavenumber scaled with $R_c$ so that $k_rR_c \approx 3.5$ and the vertical displacement amplitude increases with $H_m$. To explore a wider range of parameters the experimental data was used to calibrate a numerical model of the axisymmetric collapse. We examined the effects of changing the aspect ratio of the lock $R_c$/$H_m$ and the Rossby number $Ro$.