Generation and Propagation of Near-Inertial Waves in a Baroclinic Current on the Tasmanian Shelf

Near-inertial waves (NIWs) are often an energetic component of the internal wave field on windy continental shelves. The effect of baroclinic geostrophic currents, which introduce both relative vorticity and baroclinicity, on NIWs is not well understood. Relative vorticity affects the resonant frequency f(eff), while both relative vorticity and baroclinicity modify the minimum wave frequency of freely propagating waves omega(min). On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where f(eff) was less than the Coriolis frequency f. If everywhere f(eff) > f then NIWs were generated where omega(min) <f and f(eff) was smallest. The background current not only affected the location of generation, but also the NIWs' propagation direction. The estimated NIW energy fluxes show that NIWs propagated predominantly toward the equator because omega(min) > f on the continental slope for the entire sample period. In addition to being laterally trapped on the shelf, we observed vertically trapped and intensified NIWs that had a frequency omega within the anomalously low-frequency band (i.e., omega(min) <omega <f(eff)), which only exists if the baroclinicity is nonzero. We observed two periods when omega(min) <f on the shelf, but the relative vorticity was positive (i.e., f(eff) > f) for one of these periods. The process of NIW propagation remained consistent with the local omega(min), and not f(eff), emphasizing the importance of baroclinicity on the NIW dynamics. We conclude that windy shelves with baroclinic background currents are likely to have energetic NIWs, but the current and seabed will adjust the spatial distribution and energetics of these NIWs.