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Depth‐averaged eddy buoyancy diffusivities across continental shelves and slopes are investigated using a suite of eddy‐resolving, process‐oriented simulations of prograde frontal currents characterized by isopycnals tilted in the opposite direction to the seafloor, a flow regime commonly found along continental margins under downwelling‐favorable winds or occupied by buoyant boundary currents. The diagnosed cross‐slope eddy diffusivity varies by up to three orders of magnitude, decaying fromin the relatively flat‐bottomed region toover the steep continental slope, consistent with previously reported suppression effects of steep topography on baroclinic eddy fluxes. To theoretically constrain the simulated cross‐slope eddy fluxes, we examine extant scalings for eddy buoyancy diffusivities across prograde shelf/slope fronts and in flat‐bottomed oceans. Among all tested scalings, the GEOMETRIC framework developed by D. P. Marshall et al. (2012,https://doi.org/10.1175/JPO-D-11-048.1) and a parametrically similar Eady scale‐based scaling proposed by Jansen et al. (2015,https://doi.org/10.1016/j.ocemod.2015.05.007) most accurately reproduce the diagnosed eddy diffusivities across the entire shelf‐to‐open‐ocean analysis regions in our simulations. This result relies upon the incorporation of the topographic suppression effects on eddy fluxes, quantified via analytical functions of the slope Burger number, into the scaling prefactor coefficients. The predictive skills of the GEOMETRIC and Eady scale‐based scalings are shown to be insensitive to the presence of along‐slope topographic corrugations. This work lays a foundation for parameterizing eddy buoyancy fluxes across large‐scale prograde shelf/slope fronts in coarse‐resolution ocean models.

 

A current along a sloping bottom gives rise to upwelling, or downwelling Ekman transport within the stratified bottom boundary layer (BBL), also known as the bottom Ekman layer. In 1D models of slope currents, geostrophic vertical shear resulting from horizontal buoyancy gradients within the BBL is predicted to eventually bring the bottom stress to zero, leading to a shutdown, or “arrest,” of the BBL. Using 3D ROMS simulations, we explore how the dynamics of buoyancy adjustment in a current‐ridge encounter problem differs from 1D and 2D temporal spin up problems. We show that in a downwelling BBL, the destruction of the ageostrophic BBL shear, and hence the bottom stress, is accomplished primarily by nonlinear straining effects during the initial topographic encounter. As the current advects along the ridge slopes, the BBL deepens and evolves toward thermal wind balance. The onset of negative potential vorticitymodes of instability and their subsequent dissipation partially offsets the reduction of the BBL dissipation during the ridge‐current interaction. On the upwelling side, although the bottom stress weakens substantially during the encounter, the BBL experiences a horizontal inflectional point instability and separates from the slopes before sustained along‐slope stress reduction can occur. In all our solutions, both the upwelling and downwelling BBLs are in a partially arrested state when the current separates from the ridge slope, characterized by a reduced, but non‐zero bottom stress on the slopes.

 

Fjord circulation modulates the connection between marine‐terminating glaciers and the ocean currents offshore. These fjords exhibit both overturning and horizontal recirculations, which are driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about how various fjord characteristics influence the interaction between 3D fjord circulation and glacial melt. In this study, high‐resolution numerical simulations of idealized glacial fjords demonstrate that recirculation strength controls melt, which feeds back on overturning and recirculation. The relationships between overturning, recirculation, and melt rate are well predicted by vorticity balance, reduced‐order melt parameterizations, and empirical scaling arguments. These theories allow us to take into account the near‐glacier horizontal velocities, which yield improved predictions of fjord overturning, recirculation, and glacial melt.

 

The ongoing Arctic warming has been pronounced in winter and has been associated with an increase in downward longwave radiation. While previous studies have demonstrated that poleward moisture flux into the Arctic strengthens downward longwave radiation, less attention has been given to the impact of the accompanying increase in snowfall. Here, utilizing state-of-the-art sea ice models, we show that typical winter snowfall (snow water equivalent) anomalies of around 1.0 cm, accompanied by positive downward longwave radiation anomalies of ∼5 W m⁻², can cause basinwide sea ice thinning by around 5 cm in the following spring over the Arctic seas in the Eurasian–Pacific seas. In extreme cases, this is followed by a shrinking of summer ice extent. In the winter of 2016/17, anomalously strong warm, moist air transport combined with ∼2.5-cm increase in snowfall (snow water equivalent) decreased spring ice thickness by ∼10 cm and decreased the following summer sea ice extent by 5%–30%. This study suggests that small changes in the pattern and volume of winter snowfall can strongly impact the sea ice thickness and extent in the following seasons.

 

The Antarctic Slope Current (ASC) is a coherent circulation feature that rings the Antarctic continental shelf and regulates the flow of water toward the Antarctic coastline. The structure and variability of the ASC influences key processes near the Antarctic coastline that have global implications, such as the melting of Antarctic ice shelves and water mass formation that determines the strength of the global overturning circulation. Recent theoretical, modeling, and observational advances have revealed new dynamical properties of the ASC, making it timely to review. Earlier reviews of the ASC focused largely on local classifications of water properties of the ASC's primary front. Here we instead provide a classification of the current's frontal structure based on the dynamical mechanisms that govern both the along‐slope and cross‐slope circulation; these two modes of circulation are strongly coupled, similar to the Antarctic Circumpolar Current. Highly variable motions, such as dense overflows, tides, and eddies are shown to be critical components of cross‐slope and cross‐shelf exchange, but understanding of how the distribution and intensity of these processes will evolve in a changing climate remains poor due to observational and modeling limitations. Results linking the ASC to larger modes of climate variability, such as El Niño, show that the ASC is an integral part of global climate. An improved dynamical understanding of the ASC is still needed to accurately model and predict future Antarctic sea ice extent, the stability of the Antarctic ice sheets, and the Southern Ocean's contribution to the global carbon cycle.

 

High-resolution process modeling reveals a positive feedback of poleward ocean heat transport due to Antarctic ice shelf melt. 

Abstract It is now well established that changes in the zonal wind stress over the Antarctic Circumpolar Current (ACC) do not lead to changes in its baroclinicity nor baroclinic transport, a phenomenon referred to as “eddy saturation.” Previous studies provide contrasting dynamical mechanisms for this phenomenon: on one extreme, changes in the winds lead to changes in the efficiency with which transient eddies transfer momentum to the sea floor; on the other extreme, structural adjustments of the ACC’s standing meanders increase the efficiency of momentum transfer. In this study the authors investigate the relative importance of these mechanisms using an idealized, isopycnal channel model of the ACC. Via separate diagnoses of the model’s time-mean flow and eddy diffusivity, the authors decompose the model’s response to changes in wind stress into contributions from transient eddies and the mean flow. A key result is that holding the transient eddy diffusivity constant while varying the mean flow very closely compensates for changes in the wind stress, whereas holding the mean flow constant and varying the eddy diffusivity does not. This implies that eddy saturation primarily occurs due to adjustments in the ACC’s standing waves/meanders, rather than due to adjustments of transient eddy behavior. The authors derive a quasigeostrophic theory for ACC transport saturation by standing waves, in which the transient eddy diffusivity is held fixed, and thus provides dynamical insights into standing wave adjustment to wind changes. These findings imply that representing eddy saturation in global models requires adequate resolution of the ACC’s standing meanders, with wind-responsive parameterizations of the transient eddies being of secondary importance. 

Abstract Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up, and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: 1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation and 2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies is an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations. 

Abstract The Beaufort Gyre (BG) is hypothesized to be partially equilibrated by those mesoscale eddies that form via baroclinic instabilities of its currents. However, our understanding of the eddy field’s dependence on the mean BG currents and the role of sea ice remains incomplete. This theoretical study explores the scales and vertical structures of eddies forming specifically due to baroclinic instabilities of interior BG flows. An idealized quasi-geostrophic model is used to show that flows driven only by the Ekman pumping contain no interior potential vorticity (PV) gradients and generate weak and large eddies, ℴ(200km) in size, with predominantly barotropic and first baroclinic mode energy. However, flows containing realistic interior PV gradients in the Pacific halocline layer generate significantly smaller eddies of about 50 km in size, with a distinct second baroclinic mode structure and a subsurface kinetic energy maximum. The dramatic change in eddy characteristics is shown to be caused by the stirring of interior PV gradients by large-scale barotropic eddies. The sea ice-ocean drag is identified as the dominant eddy dissipation mechanism, leading to realistic sub-surface maxima of eddy kinetic energy for drag coefficients higher than about 2×10 −3 . A scaling law is developed for the eddy potential enstrophy, demonstrating that it is directly proportional to the interior PV gradient and the square root of the barotropic eddy kinetic energy. This study proposes a possible formation mechanism of large BG eddies and points to the importance of accurate representation of the interior PV gradients and eddy dissipation by ice-ocean drag in BG simulations and theory.