Eddy Potential Energy Research Articles

Sensitivity of Idealized Moist Baroclinic Waves to Environmental Temperature and Moisture Content

Idealized simulations are used to examine the sensitivity of moist baroclinic wave growth to environmental temperature and moisture content. With relative humidity held fixed, the surface temperature at 45°N, denoted T0, is varied from 275 to 290 K. As T0 increases, the atmospheric moisture content, moist instability, and moist available potential energy also increase. For the chosen initial configuration, moist waves develop larger eddy kinetic energy K e than corresponding dry waves, but enhanced diabatic heating at larger T0 does not further increase K e. This finding is linked to a warm-frontal cyclonic potential vorticity (PV) anomaly that strengthens and shifts downstream at larger T0 owing to increased diabatic heating along the frontal cloud band. This eastward shift feeds back negatively on the parent cyclone by increasing the downstream export of mechanical energy aloft and degrading the phasing between dry baroclinic vertical motion and buoyancy within the warm sector. The latter suppresses the conversion from eddy potential energy to K e [ C( P e, K e)], offsetting a direct enhancement of C( P e, K e) by diabatic heating. Compared to their dry counterparts, isolated moist waves (initiated by a single finite-amplitude PV anomaly) display a similar sensitivity to T0, while periodic wave trains (initiated by multiple such anomalies) exhibit a stronger negative relationship. The latter stems from anticyclonic diabatic PV anomalies aloft that originate along the warm front and recirculate through the system to interact with the upper-level trough. This interaction leads to a horizontal forward wave tilt that enhances the conversion of wave K e into zonal-mean kinetic energy.

Energetics of the Brazil Current in the Rio Grande Cone region

The energetics of the Brazil Current (BC) in the region of the Rio Grande Cone (RGC, 30–35.5°S), a topographic rise in the southwest portion of the Brazilian continental margin, are analyzed using 16 years of numerical data from the Ocean General Circulation Model (OGCM) for the Earth Simulator (OFES). The main focus of this study is the eddy-mean flow interactions of the BC and the local energy budgets in the study region. The kinetic and potential energy balance equations are derived for mean and eddy flows, and the resulting terms are presented and discussed. The eddy-mean flow interactions exhibit complex spatial distributions, and the intensities of the energy budgets decrease with increasing depth. However, only the mean potential energy (MPE) budget decreases southward. Eddy kinetic energy (EKE) and eddy potential energy (EPE) exhibit similar horizontal distribution patterns. Additionally, the baroclinic and barotropic conversion rates increase downstream of the bump, where the eddy energy field exhibits along-stream variability that increases southward. Barotropic conversion is more intense between 50 and 200m, where mean kinetic energy (MKE) and EKE are concentrated, and it exhibits a horizontal cross-stream variation pattern, with mean-to-eddy energy conversion observed on the offshore side of the BC. This result indicates that the turbulence associated with the stream jet increases as the BC moves away from the coast, with the conversion term acting to stabilize the flow. Baroclinic conversion exhibits a high intensity below 300m (where MPE and EPE display peaks), and it has a greater influence on the eddy-mean flow interaction than does the barotropic conversion. The RGC directly affects the local dynamics of the BC by increasing the eddy field as soon as the BC reaches the bump. The energy diagrams illustrate a stream characterized by evolving barotropic and baroclinic instability processes throughout the water column. This result indicates an intrinsically unstable jet in the study region. Moreover, baroclinic instability is the main source of EKE in the RGC region.

Western boundary currents regulated by interaction between ocean eddies and the atmosphere.

Current climate models systematically underestimate the strength of oceanic fronts associated with strong western boundary currents, such as the Kuroshio and Gulf Stream Extensions, and have difficulty simulating their positions at the mid-latitude ocean's western boundaries. Even with an enhanced grid resolution to resolve ocean mesoscale eddies-energetic circulations with horizontal scales of about a hundred kilometres that strongly interact with the fronts and currents-the bias problem can still persist; to improve climate models we need a better understanding of the dynamics governing these oceanic frontal regimes. Yet prevailing theories about the western boundary fronts are based on ocean internal dynamics without taking into consideration the intense air-sea feedbacks in these oceanic frontal regions. Here, by focusing on the Kuroshio Extension Jet east of Japan as the direct continuation of the Kuroshio, we show that feedback between ocean mesoscale eddies and the atmosphere (OME-A) is fundamental to the dynamics and control of these energetic currents. Suppressing OME-A feedback in eddy-resolving coupled climate model simulations results in a 20-40 per cent weakening in the Kuroshio Extension Jet. This is because OME-A feedback dominates eddy potential energy destruction, which dissipates more than 70 per cent of the eddy potential energy extracted from the Kuroshio Extension Jet. The absence of OME-A feedback inevitably leads to a reduction in eddy potential energy production in order to balance the energy budget, which results in a weakened mean current. The finding has important implications for improving climate models' representation of major oceanic fronts, which are essential components in the simulation and prediction of extratropical storms and other extreme events, as well as in the projection of the effect on these events of climate change.

Numerical study on the eddy–mean flow interaction between a cyclonic eddy and Kuroshio

A three-dimensional primitive equations ocean model (POM) is employed to study the momentum and energy balance of a moving cyclonic eddy (CE) during eddy–mean flow interaction. The CE generated by an idealized typhoon forms to the east of the Philippine islands. A momentum balance analysis shows that the dynamics of the CE are generally dominated by the geostrophic current throughout the life cycle of the CE. An energy analysis suggests that the eddy kinetic energy (EKE) and the eddy potential energy (EPE) decay rapidly after generation. The maximum EPE initially appears at the surface of the eddy center and gradually appears in the subsurface layer. The largest baroclinic instability (BCI) initially occurs at the surface. For a CE moving along a trajectory, the conversion from mean potential energy (MPE) to the EPE is positive (negative) in the front (rear) part of the trajectory, and then the eddy transfers its EPE forward along its trajectory by means of the front (rear) part of the eddy obtaining (losing) EPE from (to) the mean flow. During the interaction stage, the northward flowing Kuroshio interacts with the southward flow on the western side of the eddy and the inverse velocity shear between the Kuroshio and the eddy causes the EKE to gradually develop east–west asymmetry. The largest barotropic instability (BTI) is found in the interaction zone. Advection term, pressure work, and friction term play the dominating role in eddy decay in the eddy zone, while BTI only dominates in the interaction zone.

Loop Current Eddy formation and baroclinic instability

The formation of three Loop Current Eddies, Ekman, Franklin, and Hadal, during the period April 2009 through November 2011 was observed by an array of moored current meters and bottom mounted pressure equipped inverted echo sounders. The array design, areal extent nominally 89°W to 85°W, 25°N to 27°N with 30–50km mesoscale resolution, permits quantitative mapping of the regional circulation at all depths. During Loop Current Eddy detachment and formation events, a marked increase in deep eddy kinetic energy occurs coincident with the growth of a large-scale meander along the northern and eastern parts of the Loop Current. Deep eddies develop in a pattern where the deep fields were offset and leading upper meanders consistent with developing baroclinic instability. The interaction between the upper and deep fields is quantified by evaluating the mean eddy potential energy budget. Largest down-gradient heat fluxes are found along the eastern side of the Loop Current. Where strong, the horizontal down-gradient eddy heat flux (baroclinic conversion rate) nearly balances the vertical down-gradient eddy heat flux indicating that eddies extract available potential energy from the mean field and convert eddy potential energy to eddy kinetic energy.

The role of atmospheric internal variability on the tropical instability wave dynamics

[1] The impact of atmospheric internal variability on tropical instability wave (TIW) activity in the eastern equatorial Pacific is examined. To diagnose the atmospheric internal variability, two simulations were performed with a state-of-the-art coupled general circulation model that uses an eddy permitting ocean component model. Standard coupling procedures are implemented in the control simulation. In the experimental simulation, the so-called interactive ensemble coupling is used, which systematically reduces the contribution of internal atmospheric dynamics to the air-sea fluxes of heat, momentum and fresh water. In the eastern equatorial Pacific, the reduction of the atmospheric internal variability leads to an enhancement of the available potential energy and higher exchanges from mean to eddy potential energy. The perturbations in the available potential energy and the eddy potential energy contribute to the enhancement in the TIW activity through the increased eddy kinetic energy. Due to the negative correlation between the atmospheric internal variability and TIW activity, the covariance between the momentum flux at the air-sea interface and the ocean surface currents as well as heat flux at the air-sea interface and the sea surface temperatures were nearly conserved west of 120°W between the control and the experimental simulations.

The distribution of eddy kinetic and potential energies in the global ocean

The distribution of eddy kinetic and potential energies in the global ocean

Os processos de conversão de energia nos oceanos: uma revisão do Diagrama de Lorenz

A análise energética é uma importante ferramenta diagnóstica para a identificação de processos dinâmicos associados a eventos de conversão de energia no oceano. Esta análise, de uma forma sucinta, procura mostrar as interações energéticas que ocorrem entre os campos médio e turbulento de um determinado escoamento geofísico, bem como as transferências entre os reservatórios de energia nos oceanos. Estes processos são sumarizados pelo Diagrama de Lorenz que representa, através de quatro caixas, os reservatórios das energias cinética média, cinética turbulenta, potencial média e potencial turbulenta do escoamento do fluido geofísico, identificando as interações que ocorrem entre estas quatro formas de energia, bem como as trocas com o meio externo (fontes e sumidouros). Notadamente a literatura especializada é escassa em textos que detalhem os diversos mecanismos de conversão energética entre as principais formas de energia nos oceanos, havendo algumas publicações que, dependendo da abordagem, detalham apenas alguns deles. Este trabalho apresenta uma revisão sobre o assunto, reunindo informações de diversas fontes e tendo como objetivo principal a construção, passo a passo, do Diagrama de Lorenz, com suas respectivas formulações. A discussão de cada termo do Diagrama é apresentada objetivamente, o que possibilitará um melhor entendimento desta ferramenta, a qual é de real importância para a sumarização de uma estimativa energética para uma dada região do oceano.

A Study of the Energetics of African Easterly Waves Using a Regional Climate Model

Abstract The evolution and spatial distribution of the energetics of African waves are studied. Complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation. It is found that baroclinic overturning is the dominant energy source, although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet. The generation of active waves usually results from the nearly in-phase evolution of baroclinic and barotropic conversions, which are associated with significant rainfall over Africa. Significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet. Barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet is strong. The eddy available potential energy consumed by the baroclinic overturning is compensated directly by the conversion of zonal to eddy available potential energy and the generation of eddy potential energy by diabatic heating. These direct conversions of latent heat and zonal available potential energy suggest that interactions across space scales, from convective space scales to the large scales, are important for generating African waves. The convectively induced barotropic instability may enhance baroclinic overturning through the resonance between these two instabilities. This leads to the nonlinear interaction of the waves with convection, corresponding to the formation of organized precipitation migrating with the waves. A space–time spectral analysis shows that the dispersion characteristics of African easterly waves with wavelengths between 2650 and 4000 km do not follow the dispersion relation of the shallow water waves, indicating that these waves, similar to other easterly waves in the Tropics, possess significant nonlinearity, and cannot be fully explained by linear wave theory.

Dynamical implications of the orientation of atmospheric eddies: a local energetics perspective

ABSTRACTA local quasi-geostrophic energetics analysis indicates that within the jet core, low-frequency (LF) eddies behave baroclinically essentially the same as high-frequency (HF) eddies. They both have a westward tilting vertical structure and both grow baroclinically by transporting heat poleward and by converting eddy potential energy to kinetic energy. However, the difference in the horizontal orientations of HF and LF eddies has several important implications to their amplitude and peak locations, as well as their interaction with stationary waves. The barotropic decay of meridionally elongated HF eddies tends to terminate the growth of HF eddies beyond the jet exit region. The barotropic growth of the zonally elongated LF eddies not only ensure a continuous growth of LF eddies in the jet exit region, but also results in a new baroclinic growth of LF eddies farther downstream due to the presence of the west—east temperature contrast associated with stationary waves. The continuous growth of LF eddies due to both barotropic and baroclinic processes in the jet exit region is consistent with the facts that LF eddies reach maximum variability farther downstream of the two major jet streams and that the LF variability is much stronger than HF eddies.The results of energetics analysis are confirmed by the feedback analysis, showing that HF eddies, being dominated by meridional orientations, mainly act to maintain (damp) stationary waves by locally enhancing (reducing) north—south gradient of the height (temperature) field near the jet core regions. The zonally elongated LF eddies, on the other hand, act to primarily reduce the zonal gradient associated with stationary waves both barotropically and baroclinically.

Dynamical model of mesoscales in z-coordinates

Using the equations of the dynamical mesoscale model developed previously [Ocean Modell. (2004) 8, 1–30], we derive a mesoscale model in z-coordinates to be used in coarse resolution OGCMs. We present a model for the eddy-induced velocities, mesoscale diffusivity, eddy kinetic energy, eddy potential energy, residual diapycnal flux, velocity across mean isopycnals, Reynolds stresses, E-P, PV and RV fluxes. Specifically, in the mean buoyancy equation, mesoscales give rise to two terms: an eddy-induced velocity u M = ( u +, w +) and a residual diapycnal flux Σ. While u M has received much attention, Σ has always been taken to be zero. Physical and numerical arguments are presented to show that Σ is not zero. We present the model results for both u M and Σ. The new expression for u + contains four terms, the first of which has the structure of the GM model while the remaining three terms are new. Several interpretations of the new terms are given. The boundary conditions at z = − H, 0 are satisfied by u M and Σ, thus avoiding the need for “tapering schemes” employed thus far to amend the failure of models for u M to satisfy the proper boundary conditions. We also present the model results for the mesoscale diffusivity and show that the predicted magnitude and z-dependence are in accord with recent numerical models. The residual flux Σ gives rise to a mesoscale-induced diapycnal diffusivity which in the ACC is larger than the diabatic one. The resulting “velocity across mean isopycnals” may thus significantly affect the dynamics of the thermocline (e.g., the Munk–Wunsch advective–diffusive model). The predicted magnitude of this velocity is in an accord with recent results from eddy-resolving codes. The presence of a non-zero residual flux Σ in the mean buoyancy equation implies that the effect of mesoscales in the T–S equations is not fully accounted for with only the eddy-induced velocity, as generally done. Additional Σ-like diapycnal fluxes must be added to the T–S equations. Mean momentum equations. Since the down-gradient model used in most OGCMs does not represent mesoscales, the latter have not yet been accounted for. A model for the divergence of the Reynolds stresses, the Eliassen–Palm fluxes, the PV (potential vorticity) and RV (relative vorticity) fluxes is presented. We show that the Sverdrup vorticity balance is modified by mesoscales. In particular, while the standard Sverdrup relation does not allow meridional currents to cross the equator, the presence of mesoscales allows such a possibility.

Dynamic Model of Mesoscale Eddies

Oceanic mesoscale eddies which are analogs of well known synoptic eddies (cyclones and anticyclones), are studied on the basis of the turbulence model originated by Dubovikov (Dubovikov, M.S., "Dynamical model of turbulent eddies", Int. J. Mod. Phys. B7, 4631-4645 (1993).) and further developed by Canuto and Dubovikov (Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: I. General formalism", Phys. Fluids 8, 571-586 (1996a) (CD96a); Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: II. Sheardriven flows", Phys. Fluids 8, 587-598 (1996b) (CD96b); Canuto, V.M., Dubovikov, M.S., Cheng, Y. and Dienstfrey, A., "A dynamical model for turbulence: III. Numerical results", Phys. Fluids 8, 599-613 (1996c)(CD96c); Canuto, V.M., Dubovikov, M.S. and Dienstfrey, A., "A dynamical model for turbulence: IV. Buoyancy-driven flows", Phys. Fluids 9, 2118-2131 (1997a) (CD97a); Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: V. The effect of rotation", Phys. Fluids 9, 2132-2140 (1997b) (CD97b); Canuto, V.M., Dubovikov, M.S. and Wielaard, D.J., "A dynamical model for turbulence: VI. Two dimensional turbulence", Phys. Fluids 9, 2141-2147 (1997c) (CD97c); Canuto, V.M. and Dubovikov, M.S., "Physical regimes and dimensional structure of rotating turbulence", Phys. Rev. Lett. 78, 666-669 (1997d) (CD97d); Canuto, V.M., Dubovikov, M.S. and Dienstfrey, A., "Turbulent convection in a spectral model", Phys. Rev. Lett. 78, 662-665 (1997e) (CD97e); Canuto, V.M. and Dubovikov, M.S., "A new approach to turbulence", Int. J. Mod. Phys. 12, 3121-3152 (1997f) (CD97f); Canuto, V.M. and Dubovikov, M.S., "Two scaling regimes for rotating Raleigh-Benard convection", Phys. Rev. Letters 78, 281-284, (1998) (CD98); Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: VII. The five invariants for shear driven flows", Phys. Fluids 11, 659-664 (1999a) (CD99a); Canuto, V.M., Dubovikov, M.S. and Yu, G., "A dynamical model for turbulence: VIII. IR and UV Reynolds stress spectra for shear driven flows", Phys. Fluids 11, 656-677 (1999b) (CD99b); Canuto, V.M., Dubovikov, M.S. and Yu, G., "A dynamical model for turbulence: IX. The Reynolds stress for shear driven flows", Phys. Fluids 11, 678-694 (1999c) (CD99c).). The CD model derives from general principles and does not resort to any free parameters. Yet, it successfully describes a wide variety of quite different turbulent flows. In the present work we apply CD model to the compressible ocean. The model yields mesoscale eddies generated by the baroclinic instability. The latter, in turn, arises from the nonhorizontal orientation of the surfaces of the constant potential density (isopycnals). The obtained dynamic equations for eddy fields reduce to a vertical eigen value problem, an eigen value real part yielding an eddy radius, while an imaginary part - an eddy drift velocity. The size of the eddy is about 3rd (where rd is the Rossby deformation radius). The eddy dynamics has the following distinctive features: (1) the large scale potential energy feeds the eddy potential energy (EPE) at scales ∼ rd , (2) from rd EPE cascades to the smaller scales down to ∼ l 1 determined from the condition that the spectral Rossby number Ro(q) ≡ qU'(q)f−1 becomes ∼ 1 (q is two-dimensional wave number within an isopycnal surface), (3) at scales ∼ l 1 EPE transforms into eddy kinetic energy (EKE) which cascades backwards to the larger scales up to ∼ rd , where it transforms back into EPE, thereby closing the energy flux circulation in a wavenumber space, (4) dissipation of the eddy energy (EE) occurs at scales ∼ l 1 since at those scales the fluctuating component of the vertical shear is maximal and equals to the Brunt-Vaisala frequency. The latter equality is the well known condition for generating the vertical turbulence which dissipates EE. The model enables to determine all turbulence characteristics, including the horizontal (isopycnal) diffusivity κ h in terms of the large scale mean fields. From the typical values of the latter follow estimates for the parameters of an eddy which agree well with the observational and simulational data: kh ∼ 103m2s−1, EKE K ∼ 103m2s−1, rd ∼ 3 × 104m, lI ∼ 10. In what concerns the bolus velocity, it contains additional terms (as compared to the model of Gent and McWilliams (Gent, P.R. and McWilliams, J.C., "Isopycnal mixing in ocean circulation models", J. Phys. Oceanogr. 20, 150-155 (1990)) which result from the eddy fields advection by a mean velocity ū. Since the latter varies with depth, it is inevitable to differ from the eddy drift velocity that produces a shearing force eroding the eddy coherent structures and, therefore, contributing negatively to EE production. This is in contrast with the positive contribution from the GM term (which is due to the baroclinic instability). In those regions where the disruptive action is stronger, there is no eddy generation.

Vortex Rossby Waves in a Numerically Simulated Tropical Cyclone. Part I: Overall Structure, Potential Vorticity, and Kinetic Energy Budgets*

The asymmetric structure in the inner core of a numerically simulated tropical cyclone is analyzed in this study. The simulated tropical cyclone is found to be highly asymmetric in the inner core. In the mid–lower troposphere, the asymmetry in the core is dominated by azimuthal wavenumber-1 and wavenumber-2 vortex Rossby waves. These waves propagate azimuthally upwind against the azimuthal mean cyclonic tangential flow around the eyewall, and thus have a much longer cyclonic rotation period (by a factor of 2) than the period of a parcel moving with the cyclonic mean tangential flow around the circumference. They also propagate outward against the boundary layer inflow of the azimuthal mean cyclone. The waves are only visible within a radius of about 60 km from the cyclone center. Beyond this distance, the radial gradient of potential vorticity (PV) of the azimuthal mean cyclone is too weak to support the vortex Rossby waves. Although the divergent motion remains strong, the geopotential height and wind fields of the vortex Rossby waves are quasi-balanced, with confluent cyclonic (divergent anticyclonic) flow collocated with low (high) perturbation geopotential height. The waves spiral cyclonically inward with maximum amplitudes near the radius of maximum wind (RMW) in the horizontal and tilt radially outward with height. The upward motion of the waves leads cyclonic vorticity in both azimuthal and radial directions by about one-quarter wavelength, implying that convective heating, which is coupled with low-level convergence and upward motion, is the driving force for the vortex Rossby waves. A PV budget shows that diabatic heating contributes greatly to both the azimuthal mean PV and perturbation PV budgets. The PV tendency associated with diabatic heating is largely balanced by the advective (both horizontal and vertical) flux divergence of the symmetric PV, respectively, due to the asymmetric flow (vortex beta term, similar to the planetary beta term in the large-scale vorticity equation) for the vortex Rossby waves, and due to the symmetric flow for the symmetric cyclone. The vortex Rossby waves transport cyclonic PV from the eyewall to the eye, thus mixing the PV between the eyewall and the eye and spinning up the tangential wind in the eye at the expense of weakening the tangential wind near the RMW. Moreover, the PV tendency due to nonlinear processes associated with the wavenumber-1 vortex Rossby waves is a significant PV source for the wavenumber-2 vortex Rossby waves, indicating a strong wave–wave interaction in the eyewall. An eddy kinetic energy budget indicates that within the RMW, the vortex Rossby waves receive their kinetic energy from the azimuthal mean cyclone through baroclinic conversion and flux divergence of eddy kinetic energy due to the azimuthal mean vortex. Under the eyewall and just outside the RMW in the mid–lower troposphere, the main source for eddy kinetic energy is the eddy potential energy conversion, which is related to the asymmetric diabatic heating associated with moist convection in the eyewall. An interesting finding is that, in both the barotropic and baroclinic conversions, terms related to the radial flow of the azimuthal mean vortex are dominant and contribute to the kinetic energy of the vortex Rossby waves. The horizontal shear of the azimuthal flow of the mean vortex damps eddy kinetic energy, stabilizing the vortex Rossby waves in the mid–lower troposphere. However, both barotropic and baroclinic conversions related to the tangential flow of the azimuthal mean vortex, together with the eddy potential energy conversion, are responsible for the development of asymmetry in the outflow layer.

Eddy Variability and Energetics from Direct Current Measurements in the Antarctic Circumpolar Current South of Australia

Two-year time series measurements of current velocity and temperature in the Subantarctic Front (SAF) south of Australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the Antarctic Circumpolar Current (ACC) and further insight into the variability of the ACC. The SAF was found to be an energetic, meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm s21 at 1150 dbar. A daily varying coordinate frame that follows the direction of flow allowed mesoscale variability of the SAF to be isolated from variability due to meandering of the front and proved very successful for examining eddy fluxes. Vertically averaged cross-stream eddy heat flux was 11.3 kW m 22 poleward and significantly different from zero at 95% for fluctuations in the 2‐90 day band. Zonally integrated, this eddy heat flux ( 50.9 PW) is more than large enough to balance heat lost south of the Polar Front and is as large as cross-SAF fluxes found in Drake Passage. Cross-stream eddy momentum fluxes were small, not significantly different from zero, and of indeterminate sign, but tending to decelerate the mean flow. A relationship between vertical motion and meander phase identified in the Gulf Stream was found to hold for the ACC. Eddy kinetic energy levels were similar to those in Drake Passage and southeast of New Zealand. Eddy potential energy was up to an order of magnitude larger than at other ACC sites, most likely because meandering of the front is more common here. Baroclinic conversion was found to be the dominant mechanism by which eddies grow south of Australia. Typical eddy growth time is estimated to be 30 days, approximately twice as fast as in Drake Passage, consistent with eddy energy growing rapidly downstream.

The storm tracks and the energy cycle of the Southern Hemisphere: sensitivity to sea-ice boundary conditions

Abstract. The effect of sea-ice on various aspects of the Southern Hemisphere (SH) extratropical climate is examined. Two simulations using the LMD GCM are performed: a control run with the observed sea-ice distribution and an anomaly run in which all SH sea-ice is replaced by open ocean. When sea-ice is removed, the mean sea level pressure displays anomalies predominantly negatives near the Antarctic coast. In general, the meridional temperature gradient is reduced over most of the Southern Ocean, the polar jet is weaker and the sea level pressure rises equatorward of the control ice edge. The high frequency filtered standard deviation of both the sea level pressure and the 300-hPa geopotential height decreases over the southern Pacific and southwestern Atlantic oceans, especially to the north of the ice edge (as prescribed in the control). In contrast, over the Indian Ocean the perturbed simulation exhibits less variability equatorward of about 50°S and increased variability to the south. The zonal averages of the zonal and eddy potential and kinetic energies were evaluated. The effect of removing sea-ice is to diminish the available potential energy of the mean zonal flow, the available potential energy of the perturbations, the kinetic energy of the growing disturbances and the kinetic energy of the mean zonal flow over most of the Southern Ocean. The zonally averaged intensity of the subpolar trough and the rate of the baroclinic energy conversions are also weaker.Key words. Air-sea interactions · Meteorology and atmospheric dynamics (climatology; ocean · atmosphere interactions)

The storm tracks and the energy cycle of the Southern Hemisphere: sensitivity to sea-ice boundary conditions

<strong class="journal-contentHeaderColor">Abstract.</strong> The effect of sea-ice on various aspects of the Southern Hemisphere (SH) extratropical climate is examined. Two simulations using the LMD GCM are performed: a control run with the observed sea-ice distribution and an anomaly run in which all SH sea-ice is replaced by open ocean. When sea-ice is removed, the mean sea level pressure displays anomalies predominantly negatives near the Antarctic coast. In general, the meridional temperature gradient is reduced over most of the Southern Ocean, the polar jet is weaker and the sea level pressure rises equatorward of the control ice edge. The high frequency filtered standard deviation of both the sea level pressure and the 300-hPa geopotential height decreases over the southern Pacific and southwestern Atlantic oceans, especially to the north of the ice edge (as prescribed in the control). In contrast, over the Indian Ocean the perturbed simulation exhibits less variability equatorward of about 50°S and increased variability to the south. The zonal averages of the zonal and eddy potential and kinetic energies were evaluated. The effect of removing sea-ice is to diminish the available potential energy of the mean zonal flow, the available potential energy of the perturbations, the kinetic energy of the growing disturbances and the kinetic energy of the mean zonal flow over most of the Southern Ocean. The zonally averaged intensity of the subpolar trough and the rate of the baroclinic energy conversions are also weaker.<br><br><b>Key words. </b>Air-sea interactions · Meteorology and atmospheric dynamics (climatology; ocean · atmosphere interactions)

Moored observations of the current and temperature structure over the continental slope off central California: 2. The energetics of the flow off Point Sur

The data from two current meter moorings off Point Sur, California, were analyzed to examine the energy transfers over the continental slope in the California Current system (CCS). The method used was to calculate terms in the heat equation at intermediate depths between instruments, using the thermal wind relation to estimate the horizontal temperature gradients from the vertical shear. Time series of u, v, and T were collected for 17 months (May 1989 to October 1990) at site P2 on the 800‐m isobath and for 12 months (May 1990 to May 1991) at site P3, 25 km farther offshore on the 1800‐m isobath. Instrument depths were 100, 350, and 500 m at both moorings plus 1000 m at site P3. Mean values of the energy conversion terms were computed at sites P2 and P3 at 225 m depth and at site P2 at 425 m depth. The local change of eddy potential energy (EPE) was approximately balanced (∂EPE/∂t ≈ 0) for each case but the dynamics were quite different. At P2 225, the sources of EPE were advection by the mean flow and baroclinic instability, which converted mean potential energy (MPE) to EPE. The losses came from eddy advection out of the region and the vertical eddy heat flux (VEHF), which converted EPE to eddy kinetic energy (EKE). At P2 425, baroclinic instability provided the major source of EPE. This, plus a small positive contribution from the VEHF, was balanced by advection of EPE out of the region by both the mean flow and the eddies. At P3 225, mean advection and baroclinic instability were negligible, and the downward eddy heat flux (VEHF) was balanced by advection of heat out of the region by the eddies. Time series of terms in the EPE equation were used to examine high energy transfer events at both moorings. Events at P2 involved both horizontal and vertical processes and had longer timescales (several days to weeks) than those at P3, which had much shorter timescales (2–5 days) and were dominated by vertical processes. Compared to the Gulf Stream, the mean baroclinic instability at P2 (both depths) was smaller by at least a factor of 10. A term‐by‐term comparison between the mean values off Point Sur and the Kuroshio Extension showed different processes, with the eddies gaining strength at the expense of the mean flow off Point Sur and the opposite occurring in the Kuroshio Extension.

On the Southward Motion of Mediterranean Salt Lenses

Abstract A new mechanism is proposed to account for the southward motion of salt lenses in the Canary Basin. It relies on the active mixing that is observed at the periphery of such eddies between the warm salty Mediterranean waters of the core and the surrounding fresher and cooler Atlantic waters. Intrusions driven by this thermohaline contrast feed on the eddy potential energy and drain off the eddy volume anomaly. Submitted to the continuous entrainment of its own fluid by the frontal turbulence, the salt lens suffers a velocity divergence at its periphery, induced by a geostrophic adjustment process. This causes in turn a squashing of the density surfaces and the salt lens must then move southward to conserve its potential vorticity, a possibility offered by the spherical nature of the earth. It appears that mixing may load to the coherent latitudinal displacement of an ensemble of fluid parcels on a β plant, while at the same time dissipating the energy of the structure itself.

Energy transformations associated with the synoptic and planetary scales during the evolution of a blocking anticyclone and an upstream explosively-developing cyclone

Energy transformations associated with a wave system that included the development of a blocking anticyclone over the North Atlantic Ocean and the upstream explosive-development of an extratropical cyclone are studied for the period 17–21 January 1979. Included in the investigation are eddy kinetic energy (KE), release of eddy potential energy (CE), generation of eddy kinetic energy (GK), and exchange between eddy and zonal kinetic energy (CK). In addition, these quantities were partitioned into synoptic-scale, planetary-scale, and scale-interaction components. Results indicate that KE, which was about one-half of the total kinetic energy, was maintained by baroclinic conversion of potential to kinetic. Further, as released potential energy was being utilized to generate KE, a portion of the KE was barotropically converted to zonal kinetic energy. These transformations were dominated by the synoptic-scale component, in contrast to the authors' previously-reported height tendency results, which yielded significant contributions from both the synoptic and scale-interaction components. Thus, while changes in the mass field depended not only on the synoptic scale but also on the interactions between the synoptic and planetary scales, the corresponding changes in the eddy motion fields responded largely to synoptic-scale processes. DOI: 10.1034/j.1600-0870.1992.t01-2-00004.x

On the origin of the Azores Current

The Azores Current, south of the Azores Archipelago, is part of the subtropical North Atlantic gyre. Using an international hydrographic data set, we analyze mean and seasonal geostrophic transport fields in the upper 800 m of the ocean in order to determine the origin of the Azores Current in the western basin and seasonal changes in the related flow. Geostrophic currents are obtained by using the method applied by Stramma (1984) in the eastern basin. The Azores Current is found to originate in the area of the Southwest Newfoundland Rise (Figure 10). In winter an almost uniform current connects this region of origin with the Azores Current, while a branching into two current bands is observed in summer, with the southern band forming a marked cyclonic loop. Within the upper 800 m, all of the transport in the northern band and about 70% of the transport in the southern band recirculates in the eastern basin. Additionally, expendable bathythermograph data from the Azores Current region indicate an increase of eddy potential energy from winter to summer.