Eddy Energy Sources Research Articles

On the Genesis of the South China Sea Mesoscale Eddies

The climatology of the mesoscale eddies in the upper layer of the South China Sea (SCS) is investigated for an understanding of its genesis using the outputs from a 1/12.5° ocean reanalysis. Employed is a recently developed multiscale energetics formalism on the basis of a multiscale window transform (MWT) and the theory of canonical transfer. Three scale windows, namely, background flow, mesoscale eddy and synoptic eddy, are differentiated, and fields on different scales are reconstructed henceforth. Diagnosis of the mesoscale eddy energy budget reveals that barotropic and baroclinic instabilities, wind work, advection and pressure work are essential ingredients of the eddy energy sources and sinks in the SCS, but their contributions vary from region to region. In the southwestern part of the SCS, the regional mesoscale eddy energy is mainly generated by barotropic instability, while in the northeastern SCS, baroclinic instability and the wind working directly on the eddies are the two dominant eddy generation processes. The eddies southwest of Taiwan are damped by outward energy transport via advection, while the decay of those southeast of Vietnam is due to pressure work. The three-scale framework also reveals that the interaction between the mesoscale eddies and higher-frequency synoptic eddies mainly serves as a sink for the mesoscale eddy energy in the SCS, except for the northeastern SCS, where significant inverse cascade of kinetic energy is found.

Eddy energy sources and mesoscale eddies in the Sea of Okhotsk

Based on an eddy-permitting ocean circulation model, the eddy kinetic energy (EKE) sources are studied in the Sea of Okhotsk. An analysis of the spatial distribution of the EKE showed that intense mesoscale variability occurs along the western boundary of the Sea of Okhotsk, where the East-Sakhalin Current extends. It was found a pronounced seasonally varying EKE with its maximal magnitudes in winter, and its minimal magnitudes in summer. An analysis of the EKE sources and the energy conversions showed that time-varying (turbulent) wind stress is a main contribution to mesoscale variability along the western boundary of the Sea of Okhotsk. The contribution of baroclinic instability to the generation of mesoscale variability predominates over that of barotropic instability along the western boundary of the Sea of Okhotsk. To demonstrate the mechanism of baroclinic instability, the circulation was considered along the western boundary of the Sea of Okhotsk from January to April 2005. An analysis of hydrological conditions showed outcropping isopycnals and being strong vertical shear of the along-shore velocity from January to May 2005. In April, mesoscale eddies are observed along the western boundary of the Sea of Okhotsk. It was established that seasonal variability of turbulent wind stress and the baroclinic instability of the East-Sakhalin Current are major reasons of mesoscale variability along the western boundary of the Sea of Okhotsk.

Three-dimensional structure of mesoscale eddies in the western tropical Pacific as revealed by a high-resolution ocean simulation

The three-dimensional structure of mesoscale eddies in the western tropical Pacific (6°S–20°N, 120°E–150°E) is investigated using a high-resolution ocean model simulation. Eddy detection and eddy tracking algorithms are applied to simulated horizontal velocity vectors, and the anticyclonic and cyclonic eddies identified are composited to obtain their three-dimensional structures. The mean lifetime of all long-lived eddies is about 52 days, and their mean diameter is 147 km. Two typical characteristics of mesoscale eddies are revealed and possible dynamic explanations are analyzed. One typical characteristic is that surface eddies are generally separated from subthermocline eddies along the bifurcation latitude (~13°N) of the North Equatorial Current in the western tropical Pacific, which may be associated with different eddy energy sources and vertical eddy energy fluxes in subtropical and tropical gyres. Surface eddies have maximum swirl velocities of 8–9 cm s−1 and can extend to about 1500 m depth. Subthermocline eddies occur below 200 m, with their cores at about 400–600 m depth, and their maximum swirl velocities can reach 10 cm s−1. The other typical characteristic is that the meridional velocity component of the eddy is much larger than the zonal component. This characteristic might be due to more zonal eddy pairs (two eddies at the same latitude), which is also supported by the zonal wavelength (about 200 km) in the high-frequency meridional velocity component of the horizontal velocity.

Eddy energy sources and sinks in the South China Sea

The eddy energy sources and sinks in the South China Sea (SCS) are studied based on a high‐resolution ocean circulation model. Eddies are found to acquire their energy from barotropic instability (BT), the release of available potential energy (APE) associated with baroclinic instability (BC) and horizontal convergence from surrounding areas, while they lose energy due to turbulent processes. Both the eddy energy sources and sinks show western intensification with their maximum around the southwestern SCS where most of the wind‐input energy occurs, and the Luzon Strait where the Kuroshio intrudes the SCS in winter. Unlike that in the open ocean, the eddy energy in the inner SCS is confined to the upper layer. Besides, the eddy energy indicates clear seasonal variability, especially at the Luzon Strait where the eddy activity is much stronger in winter than in summer. Energy budget analysis suggests that APE releasing is found to be the most important eddy energy source, accounting for over 60% of the total, followed by energy convergence from surrounding areas in horizontal direction (around 20%) and energy released from BT (around 15%). Most of the generated energy is mainly balanced by the turbulent processes, while both of the downward energy flux and tendency term are negligible.

Eddy Amplitudes in Baroclinic Turbulence Driven by Nonzonal Mean Flow: Shear Dispersion of Potential Vorticity

Abstract As in the midlatitude atmosphere, midocean eddies are primarily generated by baroclinically unstable mean currents. In contrast to the atmosphere, however, oceanic currents are significantly nonzonal. Even weak nonzonal currents are linearly unstable since β does not suppress growing meridional waves. Theories for the nonlinear equilibration of baroclinic instability, and hence theories for the amplitudes of midocean eddies, must therefore take into account the different dynamics of nonzonal flow. It is shown here that the amplitude of fully developed baroclinic turbulence due to nonzonal shears differs from that due to zonal shears primarily in the nature of the eddy generation. Since β will act to create large-scale zonal jet structures regardless of the generation source, the nature of eddy fluxes of potential vorticity (the source of eddy energy) in the zonal and meridional directions are fundamentally different. The cross-jet mixing has been shown previously to obey a mixing-length scaling, and this corresponds to the generation due to unstable zonal flow. The along-jet mixing, which corresponds to the generation due to the meridional shear, is shown here to be best described by a shear dispersion model. The resulting flux is orders of magnitude higher than in the cross-jet direction, and thus eddy energies driven by baroclinically unstable mean flows with a nonzero meridional component are much larger. This provides an explanation for recently reported results. Moreover, given recent observational and modeling studies showing the ubiquitous presence of zonal jets in the oceans, the results presented here indicate a powerful source of eddy energy.

Assessment of mesoscale eddy parameterizations for a single‐basin coarse‐resolution ocean model

In this study we explore the ability of three parameterization schemes to represent mesoscale eddies' effects in a single, idealized ocean basin. We first carry out an eddy‐resolving simulation with an ocean general circulation model (GCM). The three schemes tested are the traditional Fickian diffusion and the Green‐Stone (GS) and Gent‐McWilliams (GM) parameterizations. Evaluation of the divergence of the eddy heat flux from the reference simulation in the time‐averaged thermal balance shows that eddies contribute significantly to the balance only in a geographically limited area adjacent to the western boundary around the separation point of the midlatitude jet. For this region, diagnostic tests are carried out comparing the local properties of the eddy heat flux evaluated from the fine‐resolution eddy‐resolving reference simulation with the parameterized fluxes diagnosed according to the three schemes. Evaluation of the Fickian diffusion shows that the components of the eddy heat flux are not down the temperature gradient in about half of the domain. The GS parameterization is assessed by examining the relative orientation of the isopycnal surface and the eddy heat flux vector. The relative orientation falls outside the range implicitly assumed by the GS parameterization in about half of the domain. Similarly, the GM scheme simulates the wrong sign for the divergence of the eddy heat flux in about half of the domain. Comparison of the divergences predicted with the schemes in the most energetic subdomain of the western region shows that all of them have some skill but does not identify any one scheme as being superior to the others, and two of them, GS and GM, fail in the western boundary current by predicting the wrong sign of the divergence. The incorrect orientation of the eddy heat flux vector in the test of the GS scheme implies that mean flow kinetic energy is an important source of eddy energy, and this is a likely reason why the GS and GM schemes do not do better. We carry out a further assessment of the parameterization schemes by implementing them in an ocean GCM in coarse‐resolution experiments, thus mimicking the actual procedure used in climate simulations. All the parameterization schemes can be tuned to reproduce with some accuracy some of the climatological diagnostic quantities, hence their relative success in climatological coarse‐resolution simulations. However, once again, no one scheme is noticeably better than the others. We note that our results only apply to parameterizations of eddies generated in western boundary currents and their extensions.

The Life Cycle of a Cyclone Wave in the Southern Hemisphere. Part I: Eddy Energy Budget

Abstract The energetics of a Southern Hemisphere cyclone wave have been analyzed using ECMWF data and the results of a limited-area model simulation. An analysis of the energy budget for a storm that developed in the eastern Pacific on 4–6 September 1987 showed the advection of the geopotential height field by the ageostrophic wind to be both a significant source and the primary sink of eddy kinetic energy. Air flowing through the wave gained kinetic energy via this term as it approached the energy maximum and then lost it upon exiting. Energy removal by diffusion, friction, and Reynolds stresses was found to be small. The most important conclusion was that, while the wave grew initially by poleward advection of heat as expected from baroclinic theory, the system evolved only up to the point where this source of eddy energy and the conversion of eddy potential to eddy kinetic energy (typically denoted “ωα”) was compensated for by energy flux divergence (dispersion of energy), mainly of the ageostrophic ge...

Mean circulation and eddy kinetic energy in the eastern North Atlantic

Sixty‐two satellite‐tracked buoys have been released in the northern part of the western and central North Atlantic during 1981 and 1982 by the Institut für Meereskunde, Universität Kiel, Federal Republic of Germany. Additionally, six buoys deployed in spring 1980 by the Bedford Institute have been taken over. Mean values and variances about the mean have been calculated for 3° by 3° areas. The mean velocities confirm the large‐scale gyre circulation as deduced from hydrographie data and numerical models. The center of the gyre is located at about 33°N. Eddy kinetic energy decreases east‐ and southward and is concentrated along the North Atlantic Current. It appears that the North Atlantic Current (Polar Front) is the main source of eddy kinetic energy for the northern North Atlantic, whereas the Mid‐Atlantic Ridge is no source of eddy energy. Eddy kinetic energy along the North Atlantic Current decreases from about 1000 cm2 s−2 near Newfoundland to 300 cm2 s−2 west of Scotland. A homogeneous pool of low eddy kinetic energy (less than 100 cm2 s−2) appears in the central North Atlantic east of the Mid‐Atlantic Ridge. The level increases towards the European and African shelf where the mean southward flow also becomes stronger. The results concern the upper ocean.

A numerical study of rotating annulus flows using a modified Galerkin method

Physical phenomena fundamental to rotating, baroclinically driven flows are studied with reference to results of numerical simulation of rotating annulus flows, using a modified Galerkin Model. Both local and global effects of sources, sinks, and transports of heat and momentum are discussed. A convenient ‘energy exchange” diagram reveals detailed information that is used to analyze nonlinear equilibration and amplitude vacillation of quasi-geostrophic baroclinic eddies. Transient inertial oscillations, sidewall boundary layers, and internal boundary layers are also discussed. A detailed study of symmetric flows is made, eleven of which are tested numerically for stability with respect to three-dimensional disturbances of a given zonal wave number. Two of the four unstable cases are integrated to a numerical steady state with finite-amplitude, quasi-geostrophic baroclinic waves. With the ‘rigid-lid’ geometry assumed, the average zonal velocity is zero, resulting in zero phase velocity of the waves. The structure of the thermal wave is nearly coherent in the vertical. These numerical results are consistent with laboratory observations. The eddy flow is quasi-geostrophic except in horizontal boundary layers, where the flow is driven toward low pressure. A small cross-isotherm advection is sufficient to maintain the temperature wave against diffusion and vertical advection. The eddy flow adjusts spontaneously toward the form of the fastest growing or slowest decaying disturbance representable by the truncated space resolution. The eddy flow feeds energy into the mean zonal flow in ‘barotropic-type’ interactions reflected mainly by the familiar ‘tilted trough’. During equilibration, the eddy flow alters the mean zonal flow in such a way that eddy energy sources are reduced relative to energy sinks. However, this adjustment is small compared to the change of total flow, which reflects a relatively large change of eddy amplitude. This suggests that small errors in the mean zonal flow representation can lead to relatively large errors in total flow representation. In most flows studied the kinetic energy dissipation is concentrated in thin boundary layers. In spite of this thinness, the basically laminar character of these dissipative boundary layers allows accurate and economical numerical simulation through the use of characteristic functions, which is a natural refinernent of the basic Galerkin method used. (In this prototype study, only “moderately characteristic’ functions are used, thus sacrificing numerical economy while simplifying the programming.) Similarly, the generation of potential energy, which is transformed into the kinetic energy of the flow, is accurately simulated. In most cases studied, this generation is also concentrated in thin boundary layers where thermal energy is extracted from cold fluid and added to warm fluid.