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Ge Chen, Xiaoyan Chen, and Chuanchuan Cao

Abstract

It is well understood that isolated eddies are presumed to propagate westward intrinsically at the speed of the annual baroclinic Rossby wave. This classic description, however, is known to be frequently violated in both propagation speed and its direction in the real ocean. Here, we present a systematic analysis on the divergence of eddy propagation direction (i.e., global pattern of departure from due west) and dispersion of eddy propagation speed (i.e., zonal pattern of departure from Rossby wave phase speed). Our main findings include the following: 1) A global climatological phase map (the first of its kind to our knowledge) indicating localized direction of most likely eddy propagation has been derived from 28 years (1993–2020) of satellite altimetry, leading to a leaf-like full-angle pattern in its overall divergence. 2) A meridional deflection map of eddy motion is created with prominent equatorward/poleward deflecting zones identified, revealing that it is more geographically correlated rather than polarity determined as previously thought (i.e., poleward for cyclonic eddies and equatorward for anticyclonic ones). 3) The eddy–Rossby wave relationship has a duality nature (waves riding by eddies) in five subtropical bands centered around 27°N and 26°S in the two hemispheres, outside which their relationship has a dispersive nature with dominant waves (eddies) propagating faster in the tropical (extratropical) oceans. Current, wind, and topographic effects are major external forcings responsible for the observed divergence and dispersion of eddy propagations. These results are expected to make a significant contribution to eddy trajectory prediction using physically based and/or data-driven models.

Open access
Xiaomei Yan, Dujuan Kang, Chongguang Pang, Linlin Zhang, and Hongwei Liu

Abstract

The three-dimensional energetics evolution during eddy–Kuroshio interactions east of Taiwan is systematically investigated in a time-dependent theoretical framework using outputs from an eddy-resolving ocean general circulation model. Composite analyses are conducted based on 17 anticyclonic eddies (AEs) and 19 cyclonic eddies (CEs). These westward propagating mesoscale eddies impinge on the Kuroshio at ∼22°N, ∼124.5°E and interact with the Kuroshio with a mean duration of ∼70 days. During the interaction, all the eddy energy reservoirs and eddy–mean flow energy conversions exhibit complex spatial–temporal variations. In particular, during the strong interaction period (days 18–54), both AEs and CEs are deformed into an elliptic shape, with the major axis in the northeast–southwest direction due to the squeeze of surrounding eddies, and obtain kinetic energy from the mean flow. Overall, the eddies are weakened gradually after encountering the Kuroshio, with the energy of CEs decreased more rapidly than that of AEs. The eddies decay through two pathways: transferring ∼8% of eddy available potential energy (EPE) to the mean flow, and converting ∼64% of EPE to eddy kinetic energy (EKE) via the baroclinic instability with the majority of the EKE finally dissipated. The results suggest that although the time-dependent energy conversion terms vanish upon time averaging, they play important but opposite roles in the evolution of AEs and CEs. The analysis in this work is on the synoptic and intraseasonal time scales; hence, it provides a basis for understanding the long-term variations of the eddy–Kuroshio interaction and associated climate change.

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Hiromichi Ueno, Masato Oda, Katsura Yasui, Ryo Dobashi, and Humio Mitsudera

Abstract

The distribution and interannual variation in the winter halocline in the upper layers of the World Ocean were investigated via analyses of hydrographic data from the World Ocean Database 2013 using a simple definition of the halocline. A halocline was generally observed in the tropics, equatorward portions of subtropical regions, subarctic North Pacific, and Southern Ocean. A strong halocline tended to occur in areas where the sea surface salinity (SSS) was low. The interannual variation in halocline strength was correlated with variation in SSS. The correlation coefficients were usually negative: the halocline was strong when the SSS was low. However, in the Gulf of Alaska in the northeastern North Pacific, the correlation coefficient was positive. There, halocline strength was influenced by interannual variation in Ekman pumping.

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P. Tedesco, J. Gula, P. Penven, and C. Ménesguen

Abstract

Western boundary currents are hotspots of mesoscale variability and eddy–topography interactions, which channel energy toward smaller scales and eventually down to dissipation. Here, we assess the main mesoscale eddies energy sinks in the Agulhas Current region from a regional numerical simulation. We derive an eddy kinetic energy (EKE¯) budget in the framework of the vertical modes. It accounts for energy transfers between energy reservoirs and vertical modes, including transfers channeled by topography. The variability is dominated by mesoscale eddies (barotropic and first baroclinic modes) in the path of intense mean currents. Eddy–topography interactions result in a major mesoscale eddy energy sink, along three different energy routes, with comparable importance: transfers toward bottom-intensified time-mean currents, generation of higher baroclinic modes, and bottom friction. The generation of higher baroclinic modes takes different forms in the Northern Agulhas Current, where it corresponds to nonlinear transfers to smaller vertical eddies on the slope, and in the Southern Agulhas Current, where it is dominated by a (linear) generation of internal gravity waves over topography. Away from the shelf, mesoscale eddies gain energy by an inverse vertical turbulent cascade. However, the Agulhas Current region remains a net source of mesoscale eddy energy due to the strong generation of eddies, modulated by the topography, especially in the Southern Agulhas Current. It shows that the local generation of mesoscale eddies dominates the net EKE¯ budget, contrary to the paradigm of mesoscale eddies decay upon western boundaries.

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Lin Jiang, Wansuo Duan, and Hailong Liu

Abstract

We used the conditional nonlinear optimal perturbation (CNOP) approach to investigate the most sensitive initial error of sea surface height anomaly (SSHA) forecasts by using a two-layer quasigeostrophic model and revealed the importance of mesoscale eddies in initialization of the SSHA forecasts. Then, the CNOP-type initial errors for individual mesoscale eddies were calculated, revealing that the errors tend to occur in locations where the eddies present a clear high- to low-velocity gradient along the eddy rotation and the errors often have a shear SSHA structure present. Physically, we interpreted the rationality of the particular location and shear structure of the CNOP-type errors by barotropic instability from the perspective of the Lagrange expression of fluid motions. Numerically, we examined the sensitivity of the CNOP-type errors by using observing system simulation experiments (OSSEs). We concluded that if additional observations are preferentially implemented in the location where CNOP-type errors occur, especially with a particular array indicated by their shear structure, the forecast ability of the SSHA can be significantly improved. These results provide scientific guidance for the target observation of mesoscale eddies and therefore are very instructive for improving ocean state SSHA forecasts.

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Yu Gao, Igor Kamenkovich, Natalie Perlin, and Benjamin Kirtman

Abstract

We analyze the role of mesoscale heat advection in a mixed layer (ML) heat budget, using a regional high-resolution coupled model with realistic atmospheric forcing and an idealized ocean component. The model represents two regions in the Southern Ocean, one with strong ocean currents and the other with weak ocean currents. We conclude that heat advection by oceanic currents creates mesoscale anomalies in sea surface temperature (SST), while the atmospheric turbulent heat fluxes dampen these SST anomalies. This relationship depends on the spatial scale, the strength of the currents, and the mixed layer depth (MLD). At the oceanic mesoscale, there is a positive correlation between the advection and SST anomalies, especially when the currents are strong overall. For large-scale zonal anomalies, the ML-integrated advection determines the heating/cooling of the ML, while the SST anomalies tend to be larger in size than the advection and the spatial correlation between these two fields is weak. The effects of atmospheric forcing on the ocean are modulated by the MLD variability. The significance of Ekman advection and diabatic heating is secondary to geostrophic advection except in summer when the MLD is shallow. This study links heat advection, SST anomalies, and air–sea heat fluxes at ocean mesoscales, and emphasizes the overall dominance of intrinsic oceanic variability in mesoscale air–sea heat exchange in the Southern Ocean.

Open access
Giovanni Dematteis, Kurt Polzin, and Yuri V. Lvov

Abstract

We provide a first-principles analysis of the energy fluxes in the oceanic internal wave field. The resulting formula is remarkably similar to the renowned phenomenological formula for the turbulent dissipation rate in the ocean, which is known as the finescale parameterization. The prediction is based on the wave turbulence theory of internal gravity waves and on a new methodology devised for the computation of the associated energy fluxes. In the standard spectral representation of the wave energy density, in the two-dimensional vertical wavenumber–frequency (mω) domain, the energy fluxes associated with the steady state are found to be directed downscale in both coordinates, closely matching the finescale parameterization formula in functional form and in magnitude. These energy transfers are composed of a “local” and a “scale-separated” contributions; while the former is quantified numerically, the latter is dominated by the induced diffusion process and is amenable to analytical treatment. Contrary to previous results indicating an inverse energy cascade from high frequency to low, at odds with observations, our analysis of all nonzero coefficients of the diffusion tensor predicts a direct energy cascade. Moreover, by the same analysis fundamental spectra that had been deemed “no-flux” solutions are reinstated to the status of “constant-downscale-flux” solutions. This is consequential for an understanding of energy fluxes, sources, and sinks that fits in the observational paradigm of the finescale parameterization, solving at once two long-standing paradoxes that had earned the name of “oceanic ultraviolet catastrophe.”

Significance Statement

The global circulation models cannot resolve the scales of the oceanic internal waves. The finescale parameterization of turbulent dissipation, a formula grounded in observations, is the standard tool by which the energy transfers due to internal waves are incorporated in the global models. Here, we provide an interpretation of this parameterization formula building on the first-principles statistical theory describing energy transfers between waves at different scales. Our result is in agreement with the finescale parameterization and points out a large contribution to the energy fluxes due to a type of wave interactions (local) usually disregarded. Moreover, the theory on which the traditional understanding of the parameterization is mainly built, a “diffusion approximation,” is known to be partly in contradiction with observations. We put forward a solution to this problem, visualized by means of “streamlines” that improve the intuition of the direction of the energy cascade.

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Chao Yan, James C. McWilliams, and Marcelo Chamecki

Abstract

Boundary layer turbulence in coastal regions differs from that in deep ocean because of bottom interactions. In this paper, we focus on the merging of surface and bottom boundary layers in a finite-depth coastal ocean by numerically solving the wave-averaged equations using a large-eddy simulation method. The ocean fluid is driven by combined effects of wind stress, surface wave, and a steady current in the presence of stable vertical stratification. The resulting flow consists of two overlapping boundary layers, i.e., surface and bottom boundary layers, separated by an interior stratification. The overlapping boundary layers evolve through three phases, i.e., a rapid deepening, an oscillatory equilibrium and a prompt merger, separated by two transitions. Before the merger, internal waves are observed in the stratified layer, and they are excited mainly by Langmuir turbulence in the surface boundary layer. These waves induce a clear modulation on the bottom-generated turbulence, facilitating the interaction between the surface and bottom boundary layers. After the merger, the Langmuir circulations originally confined to the surface layer are found to grow in size and extend down to the sea bottom (even though the surface waves do not feel the bottom), reminiscent of the well-organized Langmuir supercells. These full-depth Langmuir circulations promote the vertical mixing and enhance the bottom shear, leading to a significant enhancement of turbulence levels in the vertical column.

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Yu-Kun Qian, Shiqiu Peng, Xixi Wen, and Tongya Liu

Abstract

Based on the dispersion of Lagrangian particles relative to the contours of a quasi-conservative tracer field, the present study proposes two new diffusivity diagnostics: the local Lagrangian diffusivity K˜L and local effective diffusivity K˜eff, to quantify localized, instantaneous, irreversible mixing. The attractiveness of these two diagnostics is that 1) they both recover exactly the effective diffusivity K eff proposed by when averaged along a contour and 2) they share very similar spatial patterns at each time step and hence a local equivalence between particle-based and tracer-based diffusivities can be obtained instantaneously. From a particle perspective, K˜L represents the local magnifying of the mixing length; from a contour perspective, K˜eff represents the local strengthening of tracer gradient and elongation of the contour interface. Both of these enhancements are relative to an unstirred (meridionally sorted) state. While K eff cannot quantify the along-contour variation of irreversible mixing, K˜L is able to identify the portion of a (quasi-conservative) contour where it is leaky and thus easily penetrated through by Lagrangian particles. Also, unlike traditional Lagrangian diffusivity, K˜L is able to capture the fine-scale spatial structure of mixing. These two new diagnostics allows one to explore the interrelations among three types (Eulerian, Lagrangian, and tracer-based) of mixing diagnostics. Through a time mean, K˜eff has a very similar expression with the Eulerian Osborn–Cox diffusivity. The main difference lies in the definition of their denominators. That is, the non-eddying tracer background state, representing the lowest mixing efficiency, differs in each definition. Discrepancies between these three types of diffusivities are then reconciled both theoretically and practically.

Significance Statement

Large discrepancies are reported in the estimates of mixing using different types of diffusivity diagnostics, specifically the particle-based, tracer-based, and Eulerian-based diffusivities, as their definitions are quite different from each other. Here we propose two local mixing diagnostics according to the particle- and tracer-based diffusivities. It is then shown that the theoretical discrepancies between the three types of mixing diagnostics can be clearly reconciled based on these two local diagnostics. Therefore, an updated, consistent, and unified view of different mixing models becomes clear and discrepancies between different estimates can be minimized.

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Henry G. Peterson and Jörn Callies

Abstract

The near-bottom mixing that allows abyssal waters to upwell tilts isopycnals and spins up flow over the flanks of midocean ridges. Meso- and large-scale currents along sloping topography are subjected to a delicate balance of Ekman arrest and spindown. These two seemingly disparate oceanographic phenomena share a common theory, which is based on a one-dimensional model of rotating, stratified flow over a sloping, insulated boundary. This commonly used model, however, lacks rapid adjustment of interior flows, limiting its ability to capture the full physics of spinup and spindown of along-slope flow. Motivated by two-dimensional dynamics, the present work extends the one-dimensional model by constraining the vertically integrated cross-slope transport and allowing for a barotropic cross-slope pressure gradient. This produces a closed secondary circulation by forcing Ekman transport in the bottom boundary layer to return in the interior. The extended model can thus capture Ekman spinup and spindown physics: the interior return flow is turned by the Coriolis acceleration, leading to rapid rather than slow diffusive adjustment of the along-slope flow. This transport-constrained one-dimensional model accurately describes two-dimensional mixing-generated spinup over an idealized ridge and provides a unified framework for understanding the relative importance of Ekman arrest and spindown of flow along a slope.

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