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

Abstract

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 feff, while both relative vorticity and baroclinicity modify the minimum wave frequency of freely propagating waves ωmin. On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where feff was less than the Coriolis frequency f. If everywhere feff > f then NIWs were generated where ωmin < f and feff 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 ωmin > 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 ω within the anomalously low-frequency band (i.e., ωmin < ω < feff), which only exists if the baroclinicity is nonzero. We observed two periods when ωmin < f on the shelf, but the relative vorticity was positive (i.e., feff > f) for one of these periods. The process of NIW propagation remained consistent with the local ωmin, and not feff, 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.

Slope‐Intensified Storm‐Induced Near‐Inertial Oscillations in the South China Sea

“…A similar slope‐intensified feature was observed in near‐inertial waves over the Tasmanian shelf Australia by Schlosser et al. (2019). They found that the inertial energy can be trapped over the slope by the subtidal flow with negative vorticity.…”

“…The resulting slope-intensified NIOs are under a balance of the cross-shelf gradient of surface elevation and vertical gradient of Reynolds stress. A similar slope-intensified feature was observed in near-inertial waves over the Tasmanian shelf Australia by Schlosser et al (2019). They found that the inertial energy can be trapped over the slope by the subtidal flow with negative vorticity.…”

“…The slope-intensified structure of storm-induced NIOs over the northern shelf of the SCS was very similar to that caused by passages of low air pressure fronts over the LATEX shelf (Chen et al, 1996) and the Tasmanian shelf (Schlosser et al, 2019). It is likely to be a ubiquitous feature over the slope wherever wind-induced NIOs exist.…”

Impact of Background Geostrophic Currents With Vorticity on Resonant Triad Interaction Over Mid‐Ocean Ridges

“…In other words, anti-cyclonic eddies may have greater influences on RTI than cyclonic eddies in the real ocean, especially for regions where strong IT beams are generated. In addition, the vertical structure of mesoscale eddies may modulate RTI in another way: the baroclinic mesoscale eddies provide background vertical shear, that is, horizontal vorticity, which could change the vertical group velocity of the near-inertial waves (Buyn et al, 2010;Kunze, 1985;Schlosser et al, 2019). Nevertheless, compared with the impact of relative vorticity, the impact of baroclinicity of background currents is indeed poorly understood.…”

“…Meanwhile, the Northern Pacific is filled with large-scale circulation and mesoscale eddies, which provide vertical relative vorticity, especially during summer (Figures 1b and 1c). The presence of such background geostrophic currents (BGCs) with vertical vorticity can shift the local Coriolis frequency, and the effective Coriolis frequency should be introduced as (Chavanne et al, 2012;Kunze, 1985;Schlosser et al, 2019;Whitt & Thomas, 2015):…”