The Impact of Small-Scale Topography on the Dynamical Balance of the Ocean

Abstract

The impact of small-scale topography on the ocean's dynamical balance is investigated by quantifying the rates at which internal wave drag extracts (angular) momentum and vorticity from the general circulation. The calculation exploits the recent advent of two near-global descriptions of topographic roughness on horizontal scales on the order of 1-10 km, which play a central role in the generation of internal lee waves by geostrophic flows impinging on topography and have been hitherto unresolved by bathymetric datasets and ocean general circulation models alike. It is found that, while internal wave drag is a minor contributor to the ocean's dynamical balance over much of the globe, it is a significant player in the dynamics of extensive areas of the ocean, most notably the Antarctic Circumpolar Current and several regions of enhanced small-scale topographic variance in the equatorial and Southern Hemisphere oceans. There, the contribution of internal wave drag to the ocean's (angular) momentum and vorticity balances is generally on the order of ten to a few tens of percent of the dominant source and sink terms in each dynamical budget, which are respectively associated with wind forcing and form drag by topography with horizontal scales from 500 to 1000 km. It is thus suggested that the representation of internal wave drag in general circulation models may lead to significant changes in the deep ocean circulation of those regions. A theoretical scaling is derived that captures the basic dependence of internal wave drag on topographic roughness and near-bottom flow speed for most oceanographically relevant regimes.

Suppression of Internal Wave Breaking in the Antarctic Circumpolar Current near Topography

“…These observations allow the first direct studies of the relationship between turbulence and the internal wave field in the deep ACC and have provided evidence of enhanced near-bottom turbulent dissipation in association with strong near-bottom flows, rough topography, and regions where the internal wave field is found to have enhanced energy, less inertial frequency content, and a dominance of upward-propagating energy. As such, the data provide strong support for the view that deep turbulent dissipation and mixing in the Southern Ocean are primarily underpinned by the breaking of internal waves generated as deep-reaching geostrophic flows impinge on rough seafloor topography, a view that has been suggested by a number of previous indirect studies (e.g., Naveira Garabato et al 2004;Sloyan 2005;Kunze et al 2006;Nikurashin and Ferrari 2010a;Wu et al 2011).…”

“…Testing this hypothesis is also the subject of ongoing work. Nevertheless, this interpretation is useful as the overprediction can be considered as an effective suppression of internal wave breaking in widespread instances where finescale parameterization results are interpreted as a measure of the internal wave-driven turbulent mixing rates (e.g., Naveira Garabato et al 2004;Kunze et al 2006;Wu et al 2011). The lack of account of this ''effective suppression'' may contribute in part to relatively large turbulent dissipation and mixing rates predicted in the Southern Ocean in regions of rough topography by these finestructure-based studies.…”

Energy Flux into Internal Lee Waves: Sensitivity to Future Climate Changes Using Linear Theory and a Climate Model

“…The wind power input into geostrophic flows, estimated to be of O(1) TW (Wunsch 1998;Scott and Xu 2009), is then mostly transferred to geostrophic eddies through baroclinic instability (Wunsch and Ferrari 2004). Among them are bottom drag (Arbic and Flierl 2004), with an estimated dissipation of 0.2-0.8 TW (Sen et al 2008), dissipation in the western boundary of the oceanic basins, accounting for an estimated dissipation of 0.1-0.3 TW poleward of 108N/S (Zhai et al 2010), and the generation and subsequent dissipation of internal lee waves (e.g., Naveira Garabato et al 2004;Marshall and Naveira Garabato 2008;Nikurashin and Ferrari 2010b;Nikurashin et al 2013), with an estimated dissipation of 0.2-0.4 TW (Nikurashin and Ferrari 2011;Scott et al 2011). Among them are bottom drag (Arbic and Flierl 2004), with an estimated dissipation of 0.2-0.8 TW (Sen et al 2008), dissipation in the western boundary of the oceanic basins, accounting for an estimated dissipation of 0.1-0.3 TW poleward of 108N/S (Zhai et al 2010), and the generation and subsequent dissipation of internal lee waves (e.g., Naveira Garabato et al 2004;Marshall and Naveira Garabato 2008;Nikurashin and Ferrari 2010b;Nikurashin et al 2013), with an estimated dissipation of 0.2-0.4 TW (Nikurashin and Ferrari 2011;Scott et al 2011).…”

“…When lee waves break due to shear or convective instabilities, the energy converted from geostrophic motions into lee waves is dissipated and a fraction of it goes into turbulent mixing in the deep ocean (e.g., Naveira Garabato et al 2004;Nikurashin et al 2013). How far from their generation site and how high in the water column lee waves dissipate their energy is largely unknown (e.g., Melet et al 2014).…”

The Impact of a Variable Mixing Efficiency on the Abyssal Overturning

“…Nonetheless, the energy required to sustain passage flows and the 470 associated mixing likely originates to a large extent in the surface, large-scale wind and buoyancy forcing of the general circulation (Hughes and Griffiths 2006, Hughes et al 2009, Saenz et al 472 2012. Except for the relatively small fraction of the large-scale wind work dissipated by lee wave generation (Naveira Garabato et al 2013), these forcings are absent from our calculations, 474 which only include the breaking of lee and tidally-forced internal waves as a direct mixing source.…”