Explore the vast range of titles available.

The Southern Ocean's eddy response to changing climate remains unclear, with observations suggesting non‐monotonic changes in eddy kinetic energy (EKE) across scales. Here simulations reappear that smaller‐mesoscale EKE is suppressed while larger‐mesoscale EKE increases with strengthened winds. This change was linked to scale‐wise changes in the kinetic energy cycle, where a sensitive balance between the dominant mesoscale energy sinks—inverse KE cascade, and source—baroclinic energization. Such balance induced a strong (weak) mesoscale suppression in the flat (ridge) channel. Mechanistically, this mesoscale suppression is attributed to stronger zonal jets weakening smaller mesoscale eddies and promoting larger‐scale waves. These EKE multiscale changes lead to multiscale changes in meridional and vertical eddy transport, which can be parameterized using a scale‐dependent diffusivity linked to the EKE spectrum. This multiscale eddy response may have significant implications for understanding and modeling the Southern Ocean eddy activity and transport under a changing climate. Plain Language Summary The response of eddies in the Southern Ocean to climate change is not well understood. In this study, we used a channel model that simulates the effects of wind on eddies. We found that smaller eddies have less kinetic energy (KE) when the winds are stronger. On the other hand, larger‐scale eddies have more KE with stronger winds. Similar phenomena are also observed in the observations. By analyzing the eddy's KE budget, the interaction between different scales of eddies and the interaction between the eddies and mean flow are strengthened when the winds get stronger. This leads to a reduction of eddy KE at smaller mesoscale scales and an increase at larger scales. From the observational view, stronger winds weaken smaller eddies and promote larger waves. This change in eddy KE also affects how eddies meridionally transport materials and how eddy diffusivity varies at different scales. Smaller eddies transport materials less when their KE is weakened, while larger eddies become stronger in transporting materials. These findings determine how eddy diffusivity responds to the changed eddy KE at different scales. The multi‐scale response of eddies to wind has important implications for understanding the behavior of Southern Ocean eddies in a changing climate. Key Points Larger eddies got stronger and smaller eddies got weaker as Southern Ocean westerlies strengthened Both flat and ridge channel simulations suggest that these changes may be linked to changes in the inverse energy cascade The corresponding changes in scale‐wise meridional and vertical transport are also non‐monotonic

The mechanisms of the propagation and maintenance of the South Asian jet wave train are investigated in this study by examining the eddy kinetic energy (EKE) balance along the wave train. The intensity and evolution of the disturbances along the wave train can be well represented by the variation of EKE centers. The east-to-west discrepancy in the intensity of EKE centers indicates the wave train is not only maintained by the waveguide effect of the jet stream, but also fueled and damped by other physical processes along the wave train. It is found that both baroclinic and barotropic conversions are key energy sources during the lifespan of the wave train, characterized by regional differences. They evolve with the eddies and are offset mainly by energy exported by the ageostrophic geopotential fluxes after the peak of the eddies. They are strong upstream of the wave train, as extremely strong cooling/warming advection appears behind/ahead of the trough over the eastern Mediterranean Sea-Middle East where abnormal descent/ascent occurs, and potential energy is transformed into kinetic energy. As this trough tilts southwestward, kinetic energy can be converted from the mean flow to eddies on the north flank of the South Asian jet. Strong dissipation occurs over the Middle East, which might be responsible for the relatively weaker EKE centers downstream. Over China, the eddy is fueled by strong barotropic conversion again, as a result of the northward shift of the jet axis.

Informed by large-eddy simulation (LES) data and resolvent analysis of the mean flow, we examine the structure of turbulence in jets in the subsonic, transonic and supersonic regimes. Spectral (frequency-space) proper orthogonal decomposition is used to extract energy spectra and decompose the flow into energy-ranked coherent structures. The educed structures are generally well predicted by the resolvent analysis. Over a range of low frequencies and the first few azimuthal mode numbers, these jets exhibit a low-rank response characterized by Kelvin–Helmholtz (KH) type wavepackets associated with the annular shear layer up to the end of the potential core and that are excited by forcing in the very-near-nozzle shear layer. These modes too have been experimentally observed before and predicted by quasi-parallel stability theory and other approximations – they comprise a considerable portion of the total turbulent energy. At still lower frequencies, particularly for the axisymmetric mode, and again at high frequencies for all azimuthal wavenumbers, the response is not low-rank, but consists of a family of similarly amplified modes. These modes, which are primarily active downstream of the potential core, are associated with the Orr mechanism. They occur also as subdominant modes in the range of frequencies dominated by the KH response. Our global analysis helps tie together previous observations based on local spatial stability theory, and explains why quasi-parallel predictions were successful at some frequencies and azimuthal wavenumbers, but failed at others.

In three-dimensional turbulence there is on average a cascade of kinetic energy from the largest to the smallest scales of the flow. While the dominant idea is that the cascade occurs through the process of vortex stretching, evidence for this is debated. Here we show theoretically and numerically that vortex stretching is in fact not the main contributor to the average cascade. The main contributor is the self-amplification of the strain-rate field, and we provide several arguments for why its role must not be conflated with that of vortex stretching. Numerical results, however, indicate that vortex stretching plays a more important role during fluctuations of the cascade about its average behaviour. We also resolve a paradox regarding the differing role of vortex stretching on the energy cascade and energy dissipation rate dynamics.

In 1945–1949, Lars Onsager made an exact analysis of the high-Reynolds-number limit for individual turbulent flow realisations modelled by incompressible Navier–Stokes equations, motivated by experimental observations that dissipation of kinetic energy does not vanish. I review here developments spurred by his key idea that such flows are well described by distributional or ‘weak’ solutions of ideal Euler equations. 1/3 Hölder singularities of the velocity field were predicted by Onsager and since observed. His theory describes turbulent energy cascade without probabilistic assumptions and yields a local, deterministic version of the Kolmogorov $4/5$th law. The approach is closely related to renormalisation group methods in physics and envisages ‘conservation-law anomalies’, as discovered later in quantum field theory. There are also deep connections with large-eddy simulation modelling. More recently, dissipative Euler solutions of the type conjectured by Onsager have been constructed and his $1/3$ Hölder singularity proved to be the sharp threshold for anomalous dissipation. This progress has been achieved by an unexpected connection with work of John Nash on isometric embeddings of low regularity or ‘convex integration’ techniques. The dissipative Euler solutions yielded by this method are wildly non-unique for fixed initial data, suggesting ‘spontaneously stochastic’ behaviour of high-Reynolds-number solutions. I focus in particular on applications to wall-bounded turbulence, leading to novel concepts of spatial cascades of momentum, energy and vorticity to or from the wall as deterministic, space–time local phenomena. This theory thus makes testable predictions and offers new perspectives on large-eddy simulation in the presence of solid walls.

Rossby wave packets (RWPs) are Rossby waves for which the amplitude has a local maximum and decays to smaller values at larger distances. This review focuses on upper-tropospheric transient RWPs along the midlatitude jet stream. Their central characteristic is the propagation in the zonal direction as well as the transfer of wave energy from one individual trough or ridge to its downstream neighbor, a process called “downstream development.” These RWPs sometimes act as long-range precursors to extreme weather and presumably have an influence on the predictability of midlatitude weather systems. The paper reviews research progress in this area with an emphasis on developments during the last 15 years. The current state of knowledge is summarized including a discussion of the RWP life cycle as well as Rossby waveguides. Recent progress in the dynamical understanding of RWPs has been based, in part, on the development of diagnostic methods. These methods include algorithms to identify and track RWPs in an automated manner, which can be used to extract the climatological properties of RWPs. RWP dynamics have traditionally been investigated using the eddy kinetic energy framework; alternative approaches based on potential vorticity and wave activity fluxes are discussed and put into perspective with the more traditional approach. The different diagnostics are compared to each other and the strengths and weaknesses of individual methods are highlighted. A recurrent theme is the role of diabatic processes, which can be a source for forecast errors. Finally, the paper points to important open research questions and suggests avenues for future research.

In this study, uncoupled and coupled ocean–atmosphere simulations are carried out for the California Upwelling System to assess the dynamic ocean–atmosphere interactions, namely, the ocean surface current feedback to the atmosphere. The authors show the current feedback, by modulating the energy transfer from the atmosphere to the ocean, controls the oceanic eddy kinetic energy (EKE). For the first time, it is demonstrated that the current feedback has an effect on the surface stress and a counteracting effect on the wind itself. The current feedback acts as an oceanic eddy killer, reducing by half the surface EKE, and by 27% the depth-integrated EKE. On one hand, it reduces the coastal generation of eddies by weakening the surface stress and hence the nearshore supply of positive wind work (i.e., the work done by the wind on the ocean). On the other hand, by inducing a surface stress curl opposite to the current vorticity, it deflects energy from the geostrophic current into the atmosphere and dampens eddies. The wind response counteracts the surface stress response. It partly reenergizes the ocean in the coastal region and decreases the offshore return of energy to the atmosphere. Eddy statistics confirm the current feedback dampens the eddies and reduces their lifetime, improving the realism of the simulation. Finally, the authors propose an additional energy element in the Lorenz diagram of energy conversion: namely, the current-induced transfer of energy from the ocean to the atmosphere at the eddy scale.

Mesoscale ocean eddies, an important element of the climate system, impact ocean circulation, heat uptake, gas exchange, carbon sequestration and nutrient transport. Much of what is known about ongoing changes in ocean eddy activity is based on satellite altimetry; however, the length of the altimetry record is limited, making it difficult to distinguish anthropogenic change from natural variability. Using a climate model that exploits a variable-resolution unstructured mesh in the ocean component to enhance grid resolution in eddy-rich regions, we investigate the long-term response of ocean eddy activity to anthropogenic climate change. Eddy kinetic energy is projected to shift poleward in most eddy-rich regions, to intensify in the Kuroshio Current, Brazil and Malvinas currents and Antarctic Circumpolar Current and to decrease in the Gulf Stream. Modelled changes are linked to elements of the broader climate including Atlantic meridional overturning circulation decline, intensifying Agulhas leakage and shifting Southern Hemisphere westerlies.Anthropogenic changes in ocean eddies are difficult to distinguish from natural variability due to short satellite records. Here model projections show a poleward shift and intensification of eddy kinetic energy in most eddy-rich regions; however, Gulf Stream eddy activity decreases.

Although the surface eddy kinetic energy (EKE) has been well studied using satellite altimeter and surface drifter observations, our knowledge of EKE in the ocean interior is much more limited due to the sparsity of subsurface current measurements. Here we develop a new approach for estimating EKE over the full depth of the global ocean by combining 20 years of satellite altimeter and Argo float data to infer the vertical profile of eddies. The inferred eddy profiles are surface‐intensified at low latitudes and deep‐reaching at mid‐ and high latitudes. They compare favorably to the first empirical orthogonal function obtained from current meter velocities. The global‐integrated EKE estimated from the inferred profiles is about 3.1 × 1018 J, which is close to that estimated from the surface mode (3.0 × 1018 J) but about 30% smaller than that estimated from the traditional flat bottom modes (4.6 × 1018 J). Plain Language Summary The ocean is full of mesoscale eddies, analogous to weather systems in the atmosphere. Eddy kinetic energy in the surface ocean is generally well studied thanks to the availability of satellite and drifter data. The subsurface eddy energy, on the other hand, is not well known due to the relative lack of subsurface current observations. Using vertical eddy structures inferred from satellite altimeter and Argo float data, we provide the first observational estimate of eddy kinetic energy over the full depth of the global ocean. Our results have important implications for understanding the ocean energy budget and for representing the effects of mesoscale eddies in ocean and climate models. Key Points A new method is developed for estimating full‐depth eddy kinetic energy (EKE) from satellite altimeter and Argo float data Mesoscale eddy structures are surface‐intensified at low latitudes and deep‐reaching at high latitudes The total EKE in the global ocean is estimated to be about 3.1 × 1018 J

Mesoscale eddies help regulate ocean energy cascades. Eddies deformation influences barotropic instability, which represents kinetic energy transfer between scales; however, the barotropic instability structure has not been well studied. We investigated an intra‐thermocline eddy (ITE) and developed a novel anisotropic method to examine the horizontal barotropic instability. The development of the ITE was monitored using a state‐of‐the‐art autonomous underwater vehicle from May to July. The ITE became trapped in June and moved eastward in July. Based on anisotropic theory, the barotropic instability was separated into isotropic and anisotropic productions. The anisotropy contained information regarding shape and mean flow feedback of the eddy. Barotropic instability was the main source for ITE eastward propagation and was dominated by anisotropic production. Following a shape and anisotropy change, the ITE gained the mean‐flow kinetic energy by the anisotropy shear production in June and by the anisotropy stretch production when moving eastward in July. Plain Language Summary Mesoscale eddies play vital roles in ocean circulation and are important in energy cascades between large‐scale ocean circulation and dissipate scales. The barotropic instability could induce kinetic energy transition between scales, however, the underlying mechanism has not been well studied. We developed a novel method to decompose the horizontal barotropic instability into isotropic production and anisotropic production. The development of a mesoscale eddy was monitored continuously using a state‐of‐the‐art autonomous underwater vehicle. This mesoscale eddy became stationary in June and began to move eastward in July. In the observation case, the barotropic instability mainly controlled the eddy kinetic energy budget and is dominated by anisotropic production, indicating the contribution of the mean‐flow strain. We found that, the mesoscale eddy gained the mean‐flow kinetic energy via the anisotropy shear production when it was stationary, while via anisotropy stretch production when it moved eastward. Key Points For 3 months, a state‐of‐the‐art AUV monitored the development of an intra‐thermocline eddy (ITE) in terms of vertical transects Novel anisotropy analysis was developed and revealed interactions between eddy anisotropy and mean‐flow strain in barotropic instability The anisotropic partitioning of the ITE at different stages helped gain kinetic energy from the mean flow