Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography

• Published in: Journal of advances in modeling earth systems Vol. 16; no. 3
• Main Authors: Nummelin, Aleksi, Isachsen, Pål Erik
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.03.2024; American Geophysical Union (AGU)
• Subjects: Anisotropy; Buoyancy; Eddies; Energy; Friction; Kinetic energy; mesoscale eddies; Mixing length; Ocean circulation; Ocean floor; Oceans; Parameterization; Potential energy; Potential vorticity; Salinity; Simulation; Slopes; Thermal winds; Topographic waves; Topography; Tracers; Velocity; Wave propagation
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1029/2023MS003806

Most of the ocean's kinetic energy is contained within the mesoscale eddy field. Models that do not resolve these eddies tend to parameterize their impacts such that the parameterized transport of buoyancy and tracers reduces the large‐scale available potential energy and spreads tracers. However, the parameterizations used in the ocean components of current generation Earth System Models rely on an assumption of a flat ocean floor even though observations and high‐resolution modeling show that eddy transport is sensitive to the potential vorticity gradients associated with a sloping seafloor. We show that buoyancy transport coefficient diagnosed from idealized eddy‐resolving simulations is indeed reduced over both prograde and retrograde bottom slopes (topographic wave propagation along or against the mean flow, respectively) and that the reduction can be skilfully captured by a mixing length parameterization by introducing the topographic Rhines scale as a length scale. This modified “GM” parameterization enhances the strength of thermal wind currents over the slopes in coarse‐resolution, non‐eddying, simulations. We find that in realistic global coarse‐resolution simulations the impact of topography is most pronounced at high latitudes, enhancing the mean flow strength and reducing temperature and salinity biases. Reducing the buoyancy transport coefficient further with a mean‐flow dependent eddy efficiency factor, has notable effects also at lower latitudes and leads to reduction of global mean biases. Plain Language Summary Due to their high computational costs, global climate models are usually run at coarse spatial resolution, which does not allow them to resolve the ocean weather—mesoscale eddies—which are an important part of the ocean energy cycle and contribute to mixing of tracers such as heat and carbon. Eddies are instead parameterized in an idealized manner which relates the eddy‐driven transport to the strength of the vertical and horizontal density gradients in the ocean. Such parameterizations do not take into account impacts of large‐scale bottom bathymetry which have been shown to weaken the eddy driven transport. Here we use high‐resolution eddy‐resolving simulations to improve existing parameterizations so that they become sensitive to the bottom slope. We show that such a parameterization qualitatively captures the transport reduction seen in idealized high‐resolution simulations and can also reduce errors in realistic global simulations. Key Points Eddy buoyancy diffusivity reduction over bottom slopes can be parameterized using the Eady growth rate and topographic Rhines scale Realistic reduction in buoyancy diffusivity in a coarse‐resolution model strengthens baroclinic boundary currents A topographically‐aware eddy efficiency factor improves the parameterization and further reduces biases in global simulations