The vertical structure of annual wave energy flux in the tropical Indian Ocean

A recently developed energy flux diagnosis scheme, which incorporates a smooth connection between the tropical and subtropical zones, is used in the present study to investigate vertically propagating waves in the tropical Indian Ocean (IO) based on the result of a linear, continuously stratified ocean model driven by climatological wind forcing. This extended diagnosis reveals deep-reaching eastward energy fluxes at the equator which develop four times per year and are associated with equatorial Kelvin waves (KWs) generated by semiannual winds. The authors find that the downward transfer of wave energy is particularly deep in the southern Bay of Bengal (SBoB). This downward flux is attributed to off-equatorial Rossby waves and appears four times per year, maximizing its amplitude during November–December. Southwesterly winds in the Arabian Sea intensify eastward energy flux of KWs at mid-depth, which maximizes in amplitude in August. This is contrastive to KW energy flux at the surface which peaks in May. These mid-depth equatorial KW packets subsequently arrive at the eastern boundary of the IO and are diffracted poleward to produce downward energy flux in November and December detected in the SBoB.

Mixing by Oceanic Lee Waves

Oceanic lee waves are generated in the deep stratified ocean by the flow of ocean currents over sea floor topography, and when they break, they can lead to mixing in the stably stratified ocean interior. While the theory of linear lee waves is well established, the nonlinear mechanisms leading to mixing are still under investigation. Tidally driven lee waves have long been observed in the ocean, along with associated mixing, but observations of lee waves forced by geostrophic eddies are relatively sparse and largely indirect. Parameterizations of the mixing due to ocean lee waves are now being developed and implemented in ocean climate models. This review summarizes current theory and observations of lee wave generation and mixing driven by lee wave breaking, distinguishing between steady and tidally oscillating forcing. The existing parameterizations of lee wave–driven mixing informed by theory and observations are outlined, and the impacts of the parameterized lee wave–driven mixing on simulations of large-scale ocean circulation are summarized.