Abstract
AbstractThe 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.
Highlights
The dynamical balance of the ocean’s general circulation, commonly expressed in terms of the steady, depth-integrated momentum and vorticity budgets, is presently understood as a leadingorder balance between an acceleration of the ocean by wind stress and a deceleration by pressure forces on the bottom topography (e.g., Hughes and de Cuevas 2001; Vallis 2006)
To assess the role of small-scale topography on the ocean’s dynamical balance, we examine the significance of internal wave drag in the context of other source and sink terms in the oceanic budgets of momentum, angular momentum, and vorticity
The other sink is not shown because displaying it in this manner would reveal very large numbers of alternating sign associated with reversing topographic slopes and would be uninformative
Summary
The dynamical balance of the ocean’s general circulation, commonly expressed in terms of the steady, depth-integrated (angular) momentum and (potential) vorticity budgets, is presently understood as a leadingorder balance between an acceleration of the ocean by wind stress and a deceleration by pressure forces on the bottom topography (e.g., Hughes and de Cuevas 2001; Vallis 2006).
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