Mean Eddy Research Articles

Eddy–Mean Flow Interaction in the Kuroshio Extension Region

Abstract The authors use data collected by a line of tall current meter moorings deployed across the axis of the Kuroshio Extension (KE) jet at the location of maximum time-mean eddy kinetic energy to characterize the mean jet structure, the eddy variability, and the nature of eddy–mean flow interactions observed during the Kuroshio Extension System Study (KESS). A picture of the 2-yr record mean jet structure is presented in both geographical and stream coordinates, revealing important contrasts in jet strength, width, vertical structure, and flanking recirculation structure. Eddy variability observed is discussed in the context of some of its various sources: jet meandering, rings, waves, and jet instability. Finally, various scenarios for eddy–mean flow interaction consistent with the observations are explored. It is shown that the observed cross-jet distributions of Reynolds stresses at the KESS location are consistent with wave radiation away from the jet, with the sense of the eddy feedback effect on the mean consistent with eddy driving of the observed recirculations. The authors consider these results in the context of a broader description of eddy–mean flow interactions in the larger KE region using KESS data in combination with in situ measurements from past programs in the region and satellite altimetry. This demonstrates important consistencies in the along-stream development of time-mean and eddy properties in the KE with features of an idealized model of a western boundary current (WBC) jet used to understand the nature and importance of eddy–mean flow interactions in WBC jet systems.

Intraseasonal Modulation of the North Pacific Storm Track by Tropical Convection in Boreal Winter

Abstract The modulation of the North Pacific storm track by tropical convection on intraseasonal time scales (30–90 days) in boreal winter (December–March) is investigated using the NCEP–NCAR reanalysis and NOAA satellite outgoing longwave radiation (OLR) data. Multivariate empirical orthogonal function (MEOF) analysis and case compositing based upon the principal components (PCs) of the EOFs reveal substantial changes in the structure and intensity of the Pacific storm track quantified by vertically (925–200 mb) averaged synoptic eddy kinetic energy (SEKE) during the course of a typical Madden–Julian oscillation (MJO) event. The storm-track response is characterized by an amplitude-varying dipole propagating northeastward as the center of the anomalous tropical convection moves eastward across the eastern Indian Ocean and the western-central Pacific. A diagnosis of the SEKE budget indicates that the storm-track anomaly is induced primarily by changes in the convergence of energy flux, baroclinic conversion, and energy generation due to the interaction between synoptic eddies and intraseasonal flow anomalies. This demonstrates the important roles played by eddy–mean flow interaction and eddy–eddy interaction in the development of the extratropical response to MJO variability. The feedback of synoptic eddy to intraseasonal flow anomalies is pronounced: when the center of the enhanced tropical convection is located over the Maritime Continent (western Pacific), the anomalous synoptic eddy forcing partly drives an upper-tropospheric anticyclonic (cyclonic) and, to its south, a cyclonic (anticyclonic) circulation anomaly over the North Pacific. Associated with the storm-track anomaly, a three-band (dry–wet–dry) anomaly in both precipitable water and surface precipitation propagates poleward over the eastern North Pacific and induces intraseasonal variations in the winter hydroclimate over western North America.

Zonal structure of the mean flow and eddies in the Azores Current system

[1] The surface circulation and eddy field from the Azores Current system are studied here by analyzing surface drifters records and altimetry maps collected over more than 16 years. Clear differences in mean flow and eddy characteristics allow for a classification of the Azores Current in three zonal sectors: west of 30°W between the Mid-Atlantic Ridge and the Hyeres-Atlantis seamount system, between 30°W and the longitude of Madeira, and east of Madeira. A detailed and quantitative characterization of each sector is given. The transition between the western and central parts is controlled by the Hyeres-Atlantis ridge. In the transition to the eastern sector there is a change in the dynamics and the flow is forced by the cyclonic recirculation driven by the Mediterranean outflow. There is no clear expression of a continuous surface northern counterflow, but three distinct westward flows are seen: one along the southeastern border of the Azores plateau, another one southwest of Madeira, and a third one along 31°N west of the Hyeres seamount. No clear tilt in the axis of the Azores Current is found in the calculated mean surface flow from surface drifters. Observations from both drifters and altimetry data show that anticyclonic features dominate in the northern region, whereas cyclonic ones dominate in the southern region. Overall, cyclones are more numerous, constituting 60% of large eddies. The eddy features detected in the west tend to be larger than those in the east, both in size and sea level anomaly. There is indication of different mean propagation speeds east and west of 25°W–30°W, with the mean speeds consistently increasing by 25%–45% in the west. Estimates for the rate of formation of strong eddies range from 1.4 to 2.4 year−1 for cyclones, and 1.2 to 1.7 year−1 for anticyclones.

Estimate of eddy energy generation/dissipation rate in the world ocean from altimetry data

Assuming eddy kinetic energy is equally partitioned between the barotropic mode and the first baroclinic mode and using the weekly TOPEX/ERS merged data for the period of 1993~2007, the mean eddy kinetic energy and eddy available gravitational potential energy in the world oceans are estimated at 0.157 and 0.224 EJ; the annual mean generation/dissipation rate of eddy kinetic energy and available gravitational potential energy in the world oceans is estimated at 0.203 TW. Scaling and data analysis indicate that eddy available gravitational potential energy and its generation/dissipation rate are larger than those of eddy kinetic energy. High rate of eddy energy generation/dissipation is primarily concentrated in eddy-rich regions, such as the Antarctic Circumpolar Current and the western boundary current extensions. Outside of these regimes of intense current, the energy generation/dissipation rate is two to four orders of magnitude lower than the peak values; however, along the eastern boundaries and in the region where complicated topography and current interact the eddy energy generation/dissipation rate is several times larger than those in background.

Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-Scale Disturbances over the Western North Pacific. Part II: Apparent Heat and Moisture Sources and Eddy Momentum Transport*

Abstract The interactions between the boreal summer intraseasonal oscillation (ISO) and synoptic-scale variability (SSV) are investigated by diagnosing the atmospheric apparent heat source (Q1), apparent moisture sink (Q2), and eddy momentum transport. It is found that the synoptic Q1 and Q2 heating (cooling) anomalies are in phase with cyclonic (anticyclonic) vorticity disturbances, aligned in a southeast–northwest-oriented wave train pattern over the western North Pacific (WNP). The wave train is well organized and strengthened (loosely organized and weakened) during the ISO active (suppressed) phase. The nonlinearly rectified Q1 and Q2 fields due to the eddy–mean flow interaction account for 10%–30% of the total intraseasonal Q1 and Q2 variabilities over the WNP. During the ISO active (suppressed) phase, the nonlinearly rectified intraseasonal Q1 and Q2 heating (cooling) appear to the northwest of the ISO enhanced (suppressed) convection center, favoring the northwestward propagation of the ISO. A diagnosis of the zonal momentum budget shows that the eddy momentum flux convergence forces an intraseasonal westerly (easterly) tendency to the north of the ISO westerly (easterly) center during the ISO active (suppressed) phase. As a result, the eddy momentum transport may contribute to the northward propagation of the boreal summer ISO over the WNP.

Finite-Amplitude Wave Activity and Mean Flow Adjustments in the Atmospheric General Circulation. Part I: Quasigeostrophic Theory and Analysis

Abstract A diagnostic relationship between finite-amplitude wave activity and the associated adiabatic adjustments to the zonal-mean zonal wind and temperature is developed in the quasigeostrophic (QG) framework and is applied to a 23-yr segment (1979–2001) of the 40-yr ECMWF Re-Analysis (ERA-40) data. Wave activity is defined in terms of an instantaneous areal displacement of QG potential vorticity (PV) from zonal symmetry. Unlike previous forms, the tendency of wave activity equals exactly the negative of the eddy PV flux (Eliassen–Palm flux divergence) in the conservative limit, even at finite amplitude. This allows one to integrate the transformed Eulerian mean (TEM) theory in time and quantify the departure (adiabatic adjustment) of the zonal-mean state from an eddy-free reference state in terms of the observed wave activity. The structure of wave activity identifies synoptic eddies in the extratropics and planetary waves in the high latitudes of winter-to-spring stratosphere. In addition, a thin layer of high wave activity is found at the top of the lowermost stratosphere (∼17 km) in the summer extratropics. The reference state is constructed by “zonalizing” the PV contours conservatively (preserving area) on the isobaric surface and by inverting the resultant PV gradient for the mean flow. The adjustment associated with wave activity depends on the assumed surface boundary condition for the reference state. With a no-slip condition, the observed zonal-mean temperature is on average ∼33 (90) K higher than the reference state in the troposphere (stratosphere) of the Arctic winter, while the zonal-mean zonal wind is ∼30 m s−1 slower in the upper stratosphere. Since the reference state filters out the advective eddy–mean flow interaction, it fluctuates less than the zonal-mean state, potentially improving the signal-to-noise ratio for climate diagnosis.

The role of linear wave refraction in the transient eddy–mean flow response to tropical Pacific SST anomalies

AbstractThe midlatitude response to tropical Pacific SST anomalies involves changes in transient eddy propagation, but the processes leading to the transient eddy changes are still not clear. In a recent study, we used a series of controlled general circulation model (GCM) experiments in which an imposed tropical Pacific sea‐surface temperature (SST) anomaly is turned on abruptly and the response is analyzed in terms of its high‐ and low‐frequency parts, to show that the changes in transient eddies induced by El Niño Southern Oscillation (ENSO) arise from changes in wave refraction on the altered mean flow. In this work, we use a quasi‐geostrophic linear model and a linear stationary wave model, to interpret the GCM experiments and obtain the sequence of events that lead from a tropical SST anomaly to the quasi‐equilibrium change in the mean and transient atmospheric circulation. The initial direct response of the mean flow is confined to the tropical and subtropical Pacific, similar to what is obtained from a stationary wave model. This tropical–subtropical mean flow change initiates a transient eddy response, which induces a midlatitude mean flow anomaly. The wave–mean flow system evolves towards a state in which the eddy anomalies maintain the mean flow anomalies, allowing them to persist. It is further shown that, while eddy momentum fluxes persistently accelerate and decelerate the subtropical and midlatitude mean flow, the eddy heat flux effect on the zonal mean flow is much more variable, and only marginally significant. The linear quasi‐geostrophic model calculations capture the evolution of eddy momentum flux anomalies equatorwards of 60°N quite well, suggesting linear wave refraction can explain the midlatitude ENSO anomalies. However, other processes, like stationary waves or changes in the nonlinear stage of eddy life cycles, are needed to explain the ENSO‐related anomalies at high latitudes, polewards of around 60°N. Copyright © 2010 Royal Meteorological Society

Finite-Amplitude Wave Activity and Diffusive Flux of Potential Vorticity in Eddy–Mean Flow Interaction

Abstract An exact diagnostic formalism for finite-amplitude eddy–mean flow interaction is developed for barotropic and quasigeostrophic baroclinic flows on the beta plane. Based on the advection–diffusion–reaction equation for potential vorticity (PV), the formalism quantifies both advective and diffusive contributions to the mean flow modification by eddies, of which the latter is the focus of the present article. The present theory adopts a hybrid Eulerian–Lagrangian-mean description of the flow and defines finite-amplitude wave activity in terms of the areal displacement of PV contours from zonal symmetry. Unlike previous formalisms, wave activity is readily calculable from data and the local Eliassen–Palm relation does not involve cubic or higher-order terms in eddy amplitude. This leads to a natural finite-amplitude extension to the local nonacceleration theorem, as well as the global stability theorems, in the inviscid and unforced limit. The formalism incorporates mixing with effective diffusivity of PV, and the diffusive flux of PV is shown to be a sink of wave activity. The relationship between the advective and diffusive fluxes of PV and its implications for parameterization are discussed in the context of wave activity budget. If all momentum associated with wave activity were returned to the zonal-mean flow, a balanced eddy-free flow would ensue. It is shown that this hypothetical flow uREF is unaffected by the advective PV flux and is driven solely by the diffusive PV flux and forcing. For this reason, uREF, rather than the zonal-mean flow, is proposed as a diagnostic for the diffusive mean-flow modification. The formalism is applied to a freely decaying beta-plane turbulence to evaluate the contribution of the diffusive PV flux to the jet formation.

Eddy-Induced Instability for Low-Frequency Variability

Abstract Synoptic eddy–mean flow interaction has been recognized as one of the key sources for extratropical low-frequency variability. In this paper, the underlying dynamics of this interaction are examined from the perspective of a synoptic eddy-induced dynamic instability. To delineate this instability, a barotropic model is used that is linearized with respect to a stochastic basic flow prescribed with both climatologic-mean flow and synoptic eddy statistics. This linear model captures the dynamics of feedback between synoptic eddy and low-frequency flow through a dynamic closure that relates the anomalous eddy vorticity forcing to low-frequency flow anomalies. After reducing this dynamic closure to its fundamental components, this stability is elucidated with analytical results under the most idealized consideration of basic flow. It is shown that through systematic alteration of the synoptic eddy structures in the basic flow, a low-frequency planetary-scale perturbation generates anomalous eddy vorticity forcing positively proportional to the vorticity of the perturbation. Such a perturbation amplifies itself; the energy source for its growth comes from the reservoir residing in the basic synoptic eddy activity. Thus, the growth rate of the synoptic eddy-induced dynamic instability depends primarily on the kinetic energy level of the basic synoptic eddy activity. Moreover, this instability is scale selective with preference for zonal symmetric and asymmetric planetary-scale modes, whose meridional and zonal scales are roughly in the range of those of the observed leading low-frequency patterns. Analysis of this synoptic eddy-induced instability provides insight into the origin of extratropical low-frequency variability.

Regime Transitions of Steady and Time-Dependent Hadley Circulations: Comparison of Axisymmetric and Eddy-Permitting Simulations

Abstract Steady-state and time-dependent Hadley circulations are investigated with an idealized dry GCM, in which thermal forcing is represented as relaxation of temperatures toward a radiative-equilibrium state. The latitude ϕ0 of maximum radiative-equilibrium temperature is progressively displaced off the equator or varied in time to study how the Hadley circulation responds to seasonally varying forcing; axisymmetric simulations are compared with eddy-permitting simulations. In axisymmetric steady-state simulations, the Hadley circulations for all ϕ0 approach the nearly inviscid, angular-momentum-conserving limit, despite the presence of finite vertical diffusion of momentum and dry static energy. In contrast, in corresponding eddy-permitting simulations, the Hadley circulations undergo a regime transition as ϕ0 is increased, from an equinox regime (small ϕ0) in which eddy momentum fluxes strongly influence both Hadley cells to a solstice regime (large ϕ0) in which the cross-equatorial winter Hadley cell more closely approaches the angular-momentum-conserving limit. In axisymmetric time-dependent simulations, the Hadley cells undergo transitions between a linear equinox regime and a nonlinear, nearly angular-momentum-conserving solstice regime. Unlike in the eddy-permitting simulations, time tendencies of the zonal wind play a role in the dynamics of the transitions in the axisymmetric simulation. Nonetheless, the axisymmetric transitions are similar to those in the eddy-permitting simulations in that the role of the nonlinear mean momentum flux divergence in the zonal momentum budget shifts from marginal in the equinox regime to dominant in the solstice regime. As in the eddy-permitting simulations, a mean-flow feedback—involving the upper-level zonal winds, the lower-level temperature gradient, and the poleward boundary of the cross-equatorial Hadley cell—makes it possible for the circulation fields to change at the transition more rapidly than can be explained by the steady-state response to the thermal forcing. However, the regime transitions in the axisymmetric simulations are less sharp than those in the eddy-permitting simulations because eddy–mean flow feedbacks in the eddy-permitting simulations additionally sharpen the transitions.

A general circulation model ensemble study of the atmospheric circulation of Venus

The response of three numerical model dynamical cores to Venus‐like forcing and friction is described in this paper. Each dynamical core simulates a super‐rotating atmospheric circulation with equatorial winds of 35 ± 10 m/s, maintained by horizontally propagating eddies leaving the equatorial region and inducing a momentum convergence there. We discuss the balance between the mean circulation and eddies with reference to the production of a super‐rotating equatorial flow. The balance between the horizontal eddies and vertical eddies in the polar region is discussed and shown to produce an indirect overturning circulation above the jet. The indirect overturning may be related to the observed region of the polar dipole in the Venus atmosphere. Reservoirs of energy and momentum are calculated for each dynamical core and explicit sources and sinks are diagnosed from the general circulation model (GCM). The effect of a strong “sponge layer” damping to rest is compared with eddy damping and found to change significantly the momentum balance within the top “sponge layer” but does not significantly affect the super‐rotation of the bulk of the atmosphere. The Lorenz (1955) energy cycle is calculated and the circulation is shown to be dominated by energy conversion between the mean potential energy and mean kinetic energy reservoirs, with barotropic energy conversion between the mean kinetic energy and eddy kinetic energy reservoirs. We suggest modifications to the GCM parameterizations on the basis of our analysis of the atmospheric circulation and discuss the effect of numerical parameterizations on the simulated atmosphere.

Mean mesoscale global ocean currents from geodetic pre‐GOCE MDTs with a synthesis of the North Pacific circulation

A new milestone in satellite geodesy is about to be completed with the expected availability of the high‐resolution data from the already in‐orbit Global Ocean Circulation Explorer (GOCE) satellite. There are still some open questions related to capability of recovering mean mesoscale (50 km plus) geostrophic circulation (MGC) patterns from observational Gravity Recovery and Climate Experiment (GRACE)‐based geodetic Mean Dynamic Topography (MDTs), which we examine here in detail. A new set of global geodetic MDTs with high grid resolutions was computed (0.1° and 0.25°) based on the newest EGM08 and GRACE GGM02 geoid models. The high resolution coupled to good mesoscale feature discrimination was derived using efficient filtering procedure based on singular spectrum analysis. The new solutions were contrasted with five important MDTs, including the hybrid and the geodetic MDTs, through empirical orthogonal functions analysis, which permit the determination of the mutual correlation among the MDTs, and establishment of error bounds. The methodology made viable a detailed and robust analysis of the North Pacific Ocean circulation based on these MGCs including comparisons with known results from other studies. We prove the superiority of the geodetic EGM08 (as opposed to hybrid) MDT models by obtaining less noisy and more intense and slender western boundary currents and their recirculations, the bifurcations at the Shatsky Rise, and the known mean eddies in the low‐latitude shear region (NEC‐NECC).

Low-Frequency Variability in a Turbulent Baroclinic Jet: Eddy–Mean Flow Interactions in a Two-Level Model

Abstract The origin of low-frequency variability in the midlatitude jet is investigated using a two-level baroclinic channel model. The model state fields are separated into slow and fast components using intermediate time- scale averaging. In the equation for the fast variables the nonlinear wave–wave interactions are parameterized as a stochastic excitation. The slowly varying ensemble mean eddy fluxes obtained from the resulting stochastic turbulence model are coupled with the slowly varying mean flow dynamics. This forms a coupled set of deterministic equations on the slow time scale that governs the dynamics of the eddy–mean flow interaction. The equilibria of this coupled system are found as a function of the excitation strength, which controls the level of turbulence. At low levels of turbulence the equilibrated flow with zonally symmetric mean forcing remains zonally symmetric, but as excitation increases it undergoes zonal symmetry-breaking bifurcations. Time-dependent flows arising from these bifurcations take the form of westward-propagating wavelike structures resembling blocking patterns. Features of these waves are characteristic of blocking in both observations and atmospheric general circulation model simulations including retrogression, eddy variance concentrated upstream of the waves, and eddy momentum flux forcing the waves.

Proper Orthogonal Decomposition Analysis of Separated and Reattached Pressure Gradient Flows

are discussed. Significant differences are observed in the reconstructed Reynolds stress profiles at the center of the separated shear layer and those close to the reattachment point. The results also indicate that contribution of the energetic structures to the Reynolds stresses is higher at the upstream section of the ribs than in the separated layer. Thiswasattributedtoamoreuniformenergydistributionamongtheproperorthogonaldecompositionmodesinthe separated shear layer than in the upstream section. The distributions of the mean velocity, mixing length, and eddy viscosityatselectedlocationswerealsoanalyzed.Themixing-lengthprofilesintheinnerregionoftheseparatedshear layer do not follow the well-documented linear profile reported for simple near-wall turbulent flows.

A Theory of Baroclinic Turbulence

Abstract Understanding the physical mechanism maintaining fluid turbulence remains a fundamental theoretical problem. The two-layer model is an analytically and computationally simple system in which the dynamics of turbulence can be conveniently studied; in this work, a maximally simplified model of the statistically steady turbulent state in this system is constructed to isolate and identify the essential mechanism of turbulence. In this minimally complex turbulence model the effects of nonlinearity are parameterized using an energetically consistent stochastic process that is white in both space and time, turbulent fluxes are obtained using a stochastic turbulence model (STM), and statistically steady turbulent states are identified using stochastic structural stability theory (SSST). These turbulent states are the fixed-point equilibria of the nonlinear SSST system. For parameter values typical of the midlatitude atmosphere, these equilibria predict the emergence of marginally stable eddy-driven baroclinic jets. The eddy variances and fluxes associated with these jets and the power-law scaling of eddy variances and fluxes are consistent with observations and simulations of baroclinic turbulence. This optimally simple model isolates the essential physics of baroclinic turbulence: maintenance of variance by transient perturbation growth, replenishment of the transiently growing subspace by nonlinear energetically conservative eddy–eddy scattering, and equilibration to a statistically steady state of marginal stability by a combination of nonlinear eddy-induced mean jet modification and eddy dissipation. These statistical equilibrium states provide a theory for the general circulation of baroclinically turbulent planetary atmospheres.

Eddy activity in the four major upwelling systems from satellite altimetry (1992–2007)

Eddy activity in the four major eastern boundary upwelling systems (EBUS) is investigated using 15 years of satellite altimetry data. Based on the analysis of more than 4000 long-lived eddy trajectories in every EBUS, we show that mesoscale structures are mainly generated along the continental coasts and south of the main archipelagos and propagate westward with velocities increasing toward the equator. These mesoscale eddies, having radii of 70–160 km, are then frequently observed along the coastal transition zones and frontal regions and some large oceanic areas are preferentially populated by cyclonic or anticyclonic eddies. Temporal variations of the number of newly-formed eddies and the associated eddy activity index, defined as the mean eddy energy density, are finally examined at seasonal and interannual scales. The strongest seasonal (interannual, respectively) variations are observed in the California (Benguela) upwelling systems. The proposed indices also exhibit contrasted long-term trends in each EBUS, which suggests that eddy activity might be sensitive to a warming climate.

Eddy diffusivities estimated from observations in the Labrador Sea

Eddy diffusivities in the Labrador Sea (LS) are estimated from deep eddy resolving float trajectories, moored current meter records, and satellite altimetry. A mean residence time of 248 days in the central LS is observed with several floats staying for more than 2 years. By applying a simple random walk diffusion model, the observed distribution of float residence times in the central LS could be explained by a mean eddy diffusivity of about 300 m2 s−1. Estimates from float trajectories themselves and from moored current meter records yield significantly higher eddy diffusivities in the central LS of 950–1100 m2 s−1. This discrepancy can be explained by an inhomogeneity of the eddy diffusivity at middepth with high/low values in the central LS/region between central LS and deep Labrador Current, which could be conjectured from the mean altimetric eddy kinetic energy (EKE) distribution. The different diffusivities explain both (1) a fast lateral homogenization of water masses in the central LS and (2) a weak exchange between central LS and boundary current. The mean Lagrangian length scale of 11.5 ± 0.7 km as estimated from deep float trajectories is only slightly larger than the mean Rossby radius of deformation (8.8 km). Largest eddy diffusivities within the central LS are associated with strong eddy drifts, rather than with large swirl velocities and associated large EKE.

Interannual and Decadal Variations in Cross-Shelf Transport in the Gulf of Alaska

Abstract The marine ecosystem of the Gulf of Alaska (GOA) is one of the richest on the planet. The center of the GOA is characterized by high-nutrient and low-chlorophyll-a concentration. Recent observational studies suggest that advection of iron-rich coastal water is the primary mechanism controlling open ocean productivity. Specifically, there is evidence that mesoscale eddies along the coastal GOA entrain iron-rich coastal waters into the ocean interior. This study investigates the cross-shelf transport statistics in the GOA using a free-surface, hydrostatic, eddy-resolving primitive equation model over the period 1965–2004. The statistics of coastal water transport are computed using a model passive tracer, which is continuously released at the coast. The passive tracer can thus be considered a proxy for coastal biogeochemical quantities such as silicate, nitrate, iron, or oxygen, which are critical for explaining the GOA ecosystem dynamics. On average along the Alaska Current, it has been shown that at the surface while the advection of tracers by the average flow is directed toward the coast consistent with the dominant downwelling regime of the GOA, it is the mean eddy fluxes that contribute to offshore advection into the gyre interior. South of the Alaskan Peninsula, both the advection of tracers by the average flow and the mean eddy fluxes contribute to the mean offshore advection. On interannual and longer time scales, the offshore transport of the passive tracer in the Alaskan Stream does not correlate with large-scale atmospheric forcing, nor with local winds. In contrast in the Alaska Current region, stronger offshore transport of the passive tracer coincides with periods of stronger downwelling (in particular during positive phases of the Pacific decadal oscillation), which trigger the development of stronger eddies.

Mean dynamic topography and eddy kinetic energy in the Mediterranean Sea: Comparison between altimetry and a 1/16 degree ocean circulation model

A high-resolution (1/16 degree) ocean general circulation model is used to estimate the mean dynamic topography (MDT) and surface eddy kinetic energy (EKE) in the Mediterranean Sea. Model outputs based on a 12-year run with a perpetual annual atmospheric forcing are compared with the MDT and EKE derived from satellite altimetry. The model MDT is consistent with previous estimates and reproduces most of the major Mediterranean currents and eddies. In contrast, the model MDT derived from temperature/salinity climatology misses many circulation features in the Central and Levantine basins. The model EKE reproduces well the magnitudes and spatial distributions of the altimeter derived EKE, and is a significant improvement over the previous 1/8 degree model studies. However, the use of perpetual atmospheric forcing may cause some eddy features to appear more stable than characterized with high interannual variability in the altimeter EKE. The study highlights the importance of large mesoscale variability in transport processes, especially along the southern areas of the Mediterranean. The generally good agreement suggests that model simulations can be used in combination with in situ and altimeter observations to better understand the dynamics and make forecasts of the mesoscale eddies in the Mediterranean Sea.

Statistical dynamical subgrid-scale parameterizations for geophysical flows

Simulations of both atmospheric and oceanic circulations at given finite resolutions are strongly dependent on the form and strengths of the dynamical subgrid-scale parameterizations (SSPs) and in particular are sensitive to subgrid-scale transient eddies interacting with the retained scale topography and the mean flow. In this paper, we present numerical results for SSPs of the eddy–topographic force, stochastic backscatter, eddy viscosity and eddy–mean field interaction using an inhomogeneous statistical turbulence model based on a quasi-diagonal direct interaction approximation (QDIA). Although the theoretical description on which our model is based is for general barotropic flows, we specifically focus on global atmospheric flows where large-scale Rossby waves are present. We compare and contrast the closure-based results with an important earlier heuristic SSP of the eddy–topographic force, based on maximum entropy or statistical canonical equilibrium arguments, developed specifically for general ocean circulation models (Holloway 1992 J. Phys. Oceanogr. 22 1033–46). Our results demonstrate that where strong zonal flows and Rossby waves are present, such as in the atmosphere, maximum entropy arguments are insufficient to accurately parameterize the subgrid contributions due to eddy–eddy, eddy–topographic and eddy–mean field interactions. We contrast our atmospheric results with findings for the oceans. Our study identifies subgrid-scale interactions that are currently not parameterized in numerical atmospheric climate models, which may lead to systematic defects in the simulated circulations.