Topographically generated internal waves in the open ocean

Abstract

The generation of internal waves by the interaction of deep ocean flows with bottom topography is considered. The interaction of quasi‐steady currents with the bottom topography is found to produce an internal wave drag and associated momentum flux of roughly ½ dyn/cm2, which is comparable to the average wind stress on the ocean surface. The waves are probably dissipated by critical layer phenomena in the lowest kilometer or so of the ocean, the result being a wave‐induced vertical mixing coefficient of order 10 cm2/s in that portion of the water column. The interaction of the barotropic tide with the bottom topography is found to result in a flux of energy of order 1 erg/cm2 s into the internal tide, which may represent a nonnegligible sink of the tidal energy. This mechanism appears to be capable of supplying a significant portion of the observed internal wave energy in the ocean. The internal tide is probably dissipated by inherently nonlinear phenomena: spectral transport by weak interactions and, in the upper oceans, strong interactions manifested by localized instabilities resulting in isolated patches of turbulence.

Preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography

“…Earlier studies (Bell, 1975) focused on the conversion of barotropic to baroclinic energy by flow over subcritical topography; recent studies have extended understanding into the supercritical regime, analytically for both γ → 1 (Balmforth et al, 2002) and for γ = ∞ , and numerically for γ > 1 (Khatiwala, 2003). For steep slopes and deep fluid, energy conversion is enhanced by a factor 2, relative to predictions made assuming subcritical slopes Llewellyn Smith and Young, 2003).…”

“…The studies of Bell (1975) assumed γ << 1, but examined the role of finite amplitude R L = U 0 /(ω 0 L), the tidal excursion parameter. For small R L , the response is entirely at the forcing frequency ω 0 , but for R L > 1, waves at higher harmonic frequencies nω 0 are generated.…”

High mixing rates in the abyssal Southern Ocean

“…This is one of the least well de®ned quantities in the heat budget calculation, because of poor vertical and horizontal data density. If we grid the same 83 stations and produce an area-weighted mean, the NATURE | VOL 415 | 28 …”

“…Global maps of the energy estimated to be dissipated by tides 26,27 imply that the Scotia Sea may be a region where large amounts of energy are supplied to internal tides by the tidal¯ow of a strati®ed ocean over rough topography. Other possible sources for the energy are internal lee waves generated by interaction of the currents with the rough topography 28 , and basin-trapped barotropic planetary wave modes 29 .…”

Internal tides resulting from the passage of surface tides through an eddy field

“…Barbee et al [1975] claim that their array of four moored temperature sensors shows a peak in energy at a depth only slightly different from that predicted by theory. Schott [1977] Rough bottom topography in the deep ocean has also been considered as a possible candidate [Cox and Sandriim, 1962;Bell, 1975] …”