Topographic Stabilization of Ocean Rings.

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. Plain Language Summary: Swirls of circular currents tens to hundreds of kilometers in diameter known as ocean rings are critically important for transporting heat and nutrients throughout the ocean. Such structures usually reside in the upper ocean (top 1,000 m) and can last from months to several years. Ocean rings are often emitted by strong currents, such as the Gulf Stream, Agulhas, and Kuroshio. However, scientists are not entirely sure what allows rings to maintain their strength and persist for long periods. In particular, early theoretical models suggest that such large‐scale vortices are unstable and therefore should quickly disintegrate. In this study, we suggest that the resolution of the vortex longevity conundrum could lie in an unexpected direction—topography. The seafloor contains numerous underwater mountains, ridges, and valleys which, surprisingly, can dramatically affect rings that spin several kilometers above the bottom. Using numerical simulations with flat and realistically varying bottom, we demonstrate that eddies above rough topography persist much longer than their counterparts with identical parameters above a flat seafloor. We also show that there is a critical height of the bottom roughness which allows the surface‐intensified rings to remain stable and maintain their structure for years. Key Points: Coherent surface‐intensified vortices with scales greater than the radius of deformation are stabilized by rough bottom topographyThe lifespan of large vortex rings increases with the increasing amplitude of bottom topographyA critical threshold of the depth variance exists, above which the vortices are linearly stable [ABSTRACT FROM AUTHOR]