Topographic Stabilization of Ocean Rings

Abstract

Coherent large‐scale vortices in the open ocean can retain their structure and properties for periods as long as several years. However, the patterns of potential vorticity in such vortices suggest that they are baroclinically unstable and therefore should rapidly disintegrate. This study proposes a plausible explanation of the longevity of large‐scale ocean rings based on bottom roughness, which restricts flow in the lower layer and thereby stabilizes the eddy. We perform a series of simulations in which topography is represented by the observationally derived Goff‐Jordan spectrum. We demonstrate that topography stabilizes coherent vortices and dramatically prolongs their lifespan. In contrast, the same vortices in the flat‐bottom model exhibit strong instability and fragmentation on the timescale of weeks. A critical depth variance exists that allows vortices to remain stable and circularly symmetric indefinitely. Our investigation underscores the essential role played by topography in the dynamics of large‐ and meso‐scale flows.

Spin-down of a baroclinic vortex by irregular small-scale topography

“…In contrast, the vortex above irregular topography remains coherent and nearly steady throughout the entire simulation. This topographic stabilization is consistent with our earlier findings (Gulliver & Radko 2022) and calls for a more systematic analysis.

Figure 6.Snapshots of at ( a , b ) t = 70, ( c , d ) t = 75 and ( e , f ) t = 80 in the topography-resolving ( a , c , e ) and flat-bottom ( f - b ) ( b , d , f ) simulations.

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“…This form of instability is caused by the interaction of perturbations at different levels (Phillips 1951) and, therefore, the topographic arrest of abyssal motions dramatically reduces its intensity. We hypothesize that topographic stabilization, illustrated by the sandpaper model, contributes to the surprising sensitivity of oceanic flows to seafloor roughness (LaCasce et al 2019; Radko 2020; Gulliver & Radko 2022).

Figure 1.Schematic diagram illustrating the set-up of the stratified sandpaper model.

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“…Our study is focused on less explored, and more difficult to quantify, indirect effects of topography. We are interested in the tendency of rough topography to interact with large-scale currents by generating small-scale eddies associated with considerable Reynolds stresses that, in turn, influence the evolution of primary flows (Radko 2020; Gulliver & Radko 2022). The present investigation builds on the previously reported barotropic model of topographic control (Radko 2022), which contains further background information.…”

Spin-down of a barotropic vortex by irregular small-scale topography

“…Particularly relevant to the present investigation are the findings of Gulliver & Radko (2022), who analysed the effects of irregular topography on the stability and longevity of ocean rings. This study explored the parameter regime in which the lateral extent of primary flows greatly exceeded that of individual topographic features – the configuration aptly dubbed the ‘sandpaper model’.…”

“…The name was chosen to invoke the associations with fine abrasive particles of sandpaper that may be individually insignificant but have a tangible cumulative effect in grinding down much larger objects. A series of simulations in Gulliver & Radko (2022) revealed dramatic dissimilarities in the evolution of coherent vortices in flat-bottom basins and in the presence of realistic topographic patterns. The set-up of these experiments is illustrated by the schematic diagram in figure 1.…”