Eddy-resolving General Circulation Model Research Articles

A Seasonal Undercurrent Along the Northwest Coast of Australia

In the North West Shelf region of Australia is a surface current (Holloway Current), which flows southwestward along the shelf break. This paper describes a seasonal undercurrent below the Holloway Current. A 5-day climatology is constructed from the output of an eddy-resolving oceanic general circulation model (OGCM). A seasonal northeastward-flowing undercurrent is found on the upper continental slope during the climatological April–May. This undercurrent reverses during February–March. During its annual cycle, the phase of the undercurrent tends to propagate southwestward and upward. The annual frequency dominates, but the positive and negative phases of the undercurrent are not symmetric in the yearly cycle because of the contributions from the semi-annual and 1/3-annual components. We propose a hypothesis that this undercurrent is a beam of coastal trapped wave (CTW). As an initial attempt to assess the plausibility of this hypothesis, we construct an idealized linear coastal-trapped wave (CTW) solution driven by an idealized harmonic meridional winds at the annual frequency. The solution takes the form of a beam originating from the forcing region on the continental shelf and propagating offshore and southward. When it emerges on to the continental slope, it takes the form of an undercurrent. This idealized solution shares several properties with the undercurrent in the OGCM despite several discrepancies.

Atlantic–Pacific asymmetry of subsurface temperature change and frontal response of the Antarctic Circumpolar Current for the recent three decades

For the 32-year period from 1979 to 2010, trends of surface and subsurface temperature and meridional motion of the current system in the Antarctic Circumpolar Current (ACC) region are studied with in situ observations and an eddy-resolving general circulation model. The observed and simulated surface temperature shows a similar pattern between the Atlantic and Pacific: warming to the north of the Subantarctic/Subtropical Fronts in the Atlantic and of the Subtropical Front in the Pacific and cooling to the south of those fronts. The subsurface temperature trend, again from both observation and model, reveals an asymmetric pattern between the Atlantic and Pacific: subsurface warming is dominant over the whole ACC region in the Atlantic, while both warming and cooling are significant in the Pacific, the former located to the north of the Subantarctic Front and the latter to the south. The model reveals that the ACC has generally shifted poleward in the Atlantic, while it has shifted equatorward around Subantarctic Front and Polar Front in the Pacific. The ACC shift is consistent with the overall subsurface temperature trend. The basin-scale difference of the ACC response can be related to the different regime of the trend in meridional gradient of the zonal wind stress to the north and south of 50–55°S and suggests a coupling of the ACC and overlying westerly on the multi-decadal time scale.

Subthermocline Eddies in the Western Equatorial Pacific as Shown by an Eddy-Resolving OGCM*

Abstract Sporadic in situ observations have shown evidence that subthermocline eddies exist off the Mindanao coast. These subthermocline eddies are believed to play an important role in the heat, freshwater, and other ocean property transports of the region, but their characteristics and in particular their pathway and source of energy are poorly explored because of the lack of long-term observations. Analysis of results from an eddy-resolving general ocean circulation model has revealed that most subthermocline eddies off the Mindanao coast originate from the equatorial South Pacific Ocean to the west of the Ninigo Group. These eddies propagate northward along the New Guinea coast, cross the equator in the far western Pacific, and reach the Mindanao coast at a typical propagation speed of ~0.12 m s−1. The dominant time scales of these eddies range between 50 and 60 days.

The Adiabatic Pole-to-Pole Overturning Circulation

Abstract The adiabatic pole-to-pole cell of the residual overturning circulation (ROC) is studied in a two-hemisphere, semienclosed basin, with a zonally reentrant channel occupying the southernmost eighth of the domain. Three different models of increasing complexity are used: a simple, analytically tractable zonally averaged model; a coarse-resolution numerical model with parameterized eddies; and an eddy-resolving general circulation model. Two elements are found to be necessary for the existence of an adiabatic pole-to-pole cell: 1) a thermally indirect, wind-driven overturning circulation in the zonally reentrant channel, analogous to the Deacon cell in the Antarctic Circumpolar Current (ACC) region, and 2) a set of outcropping isopycnals shared between the channel and the semienclosed region of the Northern Hemisphere. These points are supported by several computations varying the domain geometry, the surface buoyancy distribution, and the wind forcing. All three models give results that are qualitatively very similar, indicating that the two requirements above are general and robust. The zonally averaged model parameterizes the streamfunction associated with adiabatic buoyancy fluxes as downgradient diffusion of buoyancy thickness, with a diffusivity in the semienclosed region of the Northern Hemisphere much larger than that in the ACC region. In the simple model, the disparity in diffusivities is necessary to obtain a substantial pole-to-pole ROC. The simple model also illustrates how the geometry of the isopycnals is shaped by the interhemispheric ROC, leading to three major thermostads, which the authors identify with the major water masses of the Atlantic: that is, North Atlantic Deep Water, Antarctic Intermediate Water, and Antarctic Bottom Water.

What Sets the Strength of the Middepth Stratification and Overturning Circulation in Eddying Ocean Models?

Abstract The processes maintaining stratification in the oceanic middepth (between approximately 1000 and 3000 m) are explored using an eddy-resolving general circulation model composed of a two-hemisphere, semienclosed basin with a zonal reentrant channel in the southernmost eighth of the domain. The middepth region lies below the wind-driven main thermocline but above the diffusively driven abyssal ocean. Here, it is argued that middepth stratification is determined primarily in the model’s Antarctic Circumpolar Current. Competition between mean and eddy overturning in the channel leads to steeper isotherms and thus deeper stratification throughout the basin than would exist without the channel. Isotherms that outcrop only in the channel are nearly horizontal in the semienclosed portion of the domain, whereas isotherms that also outcrop in the Northern Hemisphere deviate from horizontal and are accompanied by geostrophically balanced meridional transport. A northern source of deep water (water with temperatures in the range of those in the channel) leads to the formation of a thick middepth thermostad. Changes in wind forcing over the channel influence the stratification throughout the domain. Since the middepth stratification is controlled by adiabatic dynamics in the channel, it becomes independent of the interior diffusivity κ as κ → 0. The meridional overturning circulation (MOC), as diagnosed by the mean meridional volume transport, also shows a tendency to become independent of κ as κ → 0, whereas the MOC diagnosed by water mass transport shows a continuing dependence on κ as κ → 0. A nonlocal scaling for MOC is developed that relates the strength of the northern MOC to the depth of isotherms in the southern channel. The results of this paper compare favorably to observations of large-scale neutral density in the World Ocean.

The cyclonic circulation in the Australian–Antarctic basin simulated by an eddy-resolving general circulation model

Flow structure in the Australian–Antarctic basin is investigated using an eddy-resolving general ocean circulation model and validated with iceberg and middepth float trajectories. A cyclonic circulation system between the Antarctic Circumpolar Current and Antarctic Slope Current consists of a large-scale gyre in the west (80–110° E) and a series of eddies in the east (120–150° E). The western gyre has an annual mean westward transport of 22 Sv in the southern limb. Extending west through the Princess Elizabeth Trough, 5 Sv of the gyre recirculates off Prydz Bay and joins the western boundary current off the Kerguelen Plateau. Iceberg trajectories from QuickScat and ERS-1/2 support this recirculation and the overall structure of the Antarctic Slope Current against isobath in the model. Argo float trajectories also reveal a consistent structure of the deep westward slope current. This study indicates the presence of a large cyclonic circulation in this basin, which is comparable to the Weddell and Ross gyres.

Contribution of oxygen minimum zone waters to the coastal upwelling off Mauritania

The coastal upwelling off Mauritania and its connection with the oxygen minimum zone (OMZ) in the tropical Atlantic is investigated in an eddy-resolving general circulation model. Two main supply routes for the upwelling are identified. First a southern eastward pathway crossing 23°W between 3°N and 10°N related to the equatorial zonal current system supplies up to 50% of the water upwelled in winter, and about 30% in summer. Second, another eastward pathway crossing 23°W further north between 28°N and 38°N supplies 35% of the upwelled water in spring compared to 25% during the rest of the year. Most of the water of the northern pathway is entrained into the mixed layer already before reaching the upwelling region. Only the southern pathway contributes not recently ventilated waters to the upwelling. The connection with the OMZ is very weak, only about 1% of the upwelling waters originate here. On the other hand, if water from the OMZ reaches the surface mixed layer within 6 years, this mostly (71%) happens in the upwelling region.

The zonal momentum balance in an eddy-resolving general-circulation model of the Southern Ocean

The momentum balance in the zonally unbounded region of the Southern Ocean is examined using an eddy-resolving ocean general-circulation model (namely FRAM). Momentum, which is input at the surface and accelerates the Antarctic Circumpolar Current, is transferred down the water column and removed by topographic form stress. Bottom friction and lateral eddy viscosity are found to be negligible. the poleward flux of eastward momentum has a small effect in redistributing momentum. In spite of this, below the wind-driven surface layer and above the level of topography, the poleward momentum-flux divergence provides the main balance along with the ageostrophic flux of planetary vorticity (although the magnitude of these terms is an order of magnitude smaller than the wind stress). Below the Ekman layer, standing eddies produce a drag on the flow whilst transient eddies accelerate the flow. However, the impact of transient eddies is smaller. the downward transfer of momentum is achieved by interfacial form stress. This can be understood in terms of a poleward density (heat) flux. the main contribution comes from standing eddies, with a smaller contribution from transient eddies. Both contributions assist the transfer. the flux of density (heat) from the neighbouring oceans (to the north and south of the Drake Passage latitudes) influences the depth penetration of zonal momentum, particularly in the upper 1000 m. the Johnson-Bryden theory is generalized to give an additional term which is proportional to the stream function for the residual circulation associated with Eliassen-Palm cross-sections.

Comparison of the Semtner and Chervin eddy-resolving global ocean model with LUCIE and satellite observations in the Leeuwin Current region

Output from the Semtner and Chervin eddy-resolving global ocean general circulation model is compared with observations from the Leeuwin Current Interdisciplinary Experiment (LUCIE) and satellite data for the coastal waters of Western Australia. The model output is a snapshot over the domain 9-43�S, 90-120�E for a day in mid July 1987, which is during the season that the Leeuwin Current is expected to be well established along the western and southern coasts of Western Australia. Maximum Leeuwin Current velocities in the model are of the order of 60 cm s-1 and are found in the southern part of the current on the western coast and around into the Great Australian Bight. At depths below about 200 m, and centred near 400 m, there is an equatorward-flowing undercurrent with maximum velocity of order 25 cm s-1. Comparison of temperature and salinity cross-sections with LUCIE observations reveals that the model output for this day exhibits many realistic features. In particular, the model fields display a number of prominent meanders and eddies on the Leeuwin Current as well as further offshore. Consistent with observations, mesoscale features associated with the Leeuwin Current are concentrated between 25�S and the Cape Mentelle region; the flow in the northern part of the Leeuwin Current and the North West Shelf may be too weak to induce eddy-generating instabilities. Prominent in the model output are two large meanders on the Leeuwin Current between 25�S and 29�S and two anticyclonic eddies further downstream; features similar to these are evident in satellite data during winter 1987.

Performance of the Capacitance Matrix Method for Solving Helmhotz-Type Equations in Ocean Modelling

A Capacitance Matrix Method has been implemented in a quasi-geostrophic eddy-resolving general circulation model, for applications to real oceanic basins. This method allows the use of very fast direct solvers to treat problems on irregular regions. The aim of the present study is to test the performance of this method in terms of CPU requirements and accuracy.

Use of satellite scatterometer winds to force an ocean general circulation model

The present study is an attempt to evaluate the capacity of scatterometer wind data to be provided by the forthcoming European Remote Sensing 1 (ERS 1) and NASA Scatterometer (NSCAT) satellite missions, to successfully force basin scale ocean circulations. The mid‐latitude ocean is simulated with a three‐layer, quasi‐geostrophic, eddy‐resolving model in a square box geometry. The variable wind stress, which is used to simulate scatterometer winds, is derived from European Centre for Medium‐Range Weather Forecast (ECMWF) daily analyses of the wind stress over the North Atlantic. Its curl is applied as a forcing at every grid point of the model to drive a circulation of reference. The model is forced in real time under the swath with the curl derived from the raw scatterometer winds, with a condition of persistence that keeps the forcing constant between two consecutive passages of the satellite. The winds of ERS 1 (at 3‐ and 35‐day periods) produce circulations which globally resemble that of the reference experiment. A 3‐day repeat orbit appears to be the most suitable for producing raw winds capable of realistically driving a numerical ocean model. NSCAT (3‐day period) reproduces the main features of the reference large scale circulation with more success than ERS 1, and it is clear that NSCAT raw winds are far more suitable for driving an ocean Eddy‐Resolving General Circulation Model (EGCM) than ERS 1 winds because of their denser spatial coverage. ERS 1 and NSCAT raw wind stress curl fields are then objectively analyzed to produce smoothed “interpolated” wind stress curl fields having continuous values at every model grid point. Simulations with ERS 1 and NSCAT interpolated forcings show an improvement in the structure of the mean flow, but discrepancies in the energetics lead us to conclude that our interpolated winds are not significantly better than the raw winds. A «combined» forcing is constructed consisting of daily maps containing NSCAT raw data under the swaths and the corresponding objectively analyzed field outside the swaths. The circulation driven by the combined forcing reproduces remarkably well the main features and the energetics of the reference circulation. The patchy resolution of the combined forcing appeared to have little effect on the results of the simulations. Our conclusion is that, within the framework of the present study, scatterometer winds (and particularly those of NSCAT) seem appropriate for providing a forcing function to mid‐latitude ocean models, and for that purpose, these satellite data may not require prior assimilation in meteorological models, despite their patchiness and irregular resolution.

Potential Vorticity Sources and Sinks in a Quasi-geostrophic Ocean: beyond Western Boundary Currents

Abstract Potential vorticity dynamics for a quasi-geostrophic eddy-resolving general circulation model (EGCM) are studied in order to determine the effects of mesoscale variability on the potential vorticity distribution of a wind-driven ocean. The study employs both Eulerian and Lagrangian analyses in the effort to describe the potential vorticity gain/loss cycle along the path of a particle. While the mean wind stress curl is the dominant potential vorticity source for the interior of the upper layer, a redistribution of eddy potential vorticity creates sources of potential vorticity for the multiple gyres in the lower layers. This redistribution is a result of the local generation of eddies via baroclinic instabilities. These eddies are advected by the western boundary current into the midlatitude jet where they are responsible for a cross-gyre potential vorticity exchange. This exchange is concentrated at the entrance to the eastward jets where northward and southward boundary currents converge. From ...

An Intercomparison of Inverse Methods Using an Eddy-Resolving General Circulation Model

Abstract Three inverse methods (the Bernoulli, beta-spiral, and box inverse methods) are used on mean data from an eddy-resolving oceanic general circulation model, in an attempt to reconstruct the observed mean flow field. Inversions are performed in the Gulf Stream extension, a quiet region which is relatively eddy-free, the center of the region of homogenized potential vorticity, and a near-equatorial area, together with an inversion of the flow across a transoceanic sector. Resolutions for the inversions of ⅓°, 1° and 2° are used. Numerical estimates of geostrophy using the wider resolutions can give top-to-bottom thermal wind shears in error by up to 1 cm s−1 in a flow change of around 8 cm s−1. Two “scores” for the methods are created, one which tests pointwise accuracy (the “global” score) and one which tests fluxes of mass through a section (the “flux” score). The Bernoulli method yields accurate global scores except in the homogenized region; the box inverse method yields fairly accurate global s...

A Stratified Model of the Inertial Recirculation

An inertial gyre with characteristics very similar to the recirculation observed in eddy-resolving general circulation models is obtained with a simple, analytically tractable, two-layer model. The recirculating gyre is contained in a box of simple geometry, which isolates it from the sverdrup interior. The gyre is forced by prescribing anomalous values of potential vorticity northward in the subtropical gyre or can be though of as a rough parametrization of diabatic forcing. In both cases the forcing is confined to the water above the thermocline, which is represented by the upper layer and is transmitted to the abyssal ocean through interfacial friction. The condition for the abyssal water to be set in motion, is derived and for oceanic values the recirculation goes all the way to the bottom. When this occurs the center of the gyre is dominated by a barotropic flow, while the baroclinic flow is confined to the edges of the gyre. The width and strength of the gyre can be easily calculated in the limit of long, narrow gyres. The meridional scale of the gyre is directly proportional to the vorticity anomaly injected at the northern boundary, and the barotropic part of the transport is proportional to the cube of the abyssal gyre width, in close analogy with the results found by Cessi, Ierley and Young in a one layer model.

A Model of the Inertial Recirculation Driven by Potential Vorticity Anomalies

Some essential features of a recirculating inertial gyre (the “recirculation”) can be analyzed with a very simple, analytically tractable model. In wind-driven eddy-resolving general circulation models the recirculation appears as a strong sub-basin-scale inertial flow with homogeneous potential vorticity. The constant value of potential vorticity decreases with increasing forcing/dissipation ratio while the size and the strength of the recirculating gyre increases. In the subtropical gyre the recirculating gyre might be driven by anomalous values of low potential vorticity carried northward by the western boundary current. We have modeled this process using a barotropic model and prescribing the values of potential vorticity at the edge of the gyre. Our model gyre is contained in a rectangular box in an attempt to simplify the geometry as much as possible and to isolate the processes occurring in the recirculating region. With weak diffusion the prescribed boundary forcing induces a flow with constant potential vorticity. We show how to calculate the homogenized value of potential vorticity in the interior without explicitly solving for the flow. We also numerically solve our model and so obtain explicit solutions. Two distinct cases arise: 1) For strong boundary forcing the gyre fills the whole box. Therefore the homogenized value of potential vorticity can be determined but the extent of the recirculation is prescribed. 2) For weak boundary forcing the recirculation fills only part of the basin and the size of the gyre must be determined as well as the homogenized value of potential vorticity within it. The latter case is the most relevant to the wind-driven, numerical experiments, because in these calculations the recirculating flow is confined to a sub-basin-scale region. Also in this case the homogenized value of potential vorticity decreases with increasing forcing, while the size and the strength of the gyre increase.

Open-ocean response and normal mode excitation in an eddy-resolving general circulation model

Abstract Analysis of a two-layer, flat-bottom, steady-wind driven, eddy-resolving general circulation model reveals a distinct separation in frequency of baroclinic and barotropic motion in the region distant from the model Gulf Stream. The far-field motions at periods less (greater) than about 100 days are predominantly barotropic (baroclinic), unlike the near-field, eddy-generating, free-jet region which contains barotropic and baroclinic energy throughout the modei frequency range. The far-field barotropic energy produces a peak in the model sea-level spectra between 25 and 50 days with a magnitude comparable to energy levels observed in spectra of sea level from oceanic island tide gauges. The far-field barotropic motion is clearly composed of large-scale, resonant, barotropic normal modes drive by mesoscale activity of the turbulent, free-jet region. Oceanic mesoscale turbulence may therefore provide for planetary normal modes an excitation mechanism distinct from atmospheric forcing. The open-ocean, barotropic, model response is very similar to that of a fluctuating-wind driven model, which suggests that atmospheric and intrinsic forcing of mid-ocean eddies may be of comparable importance.

Open-ocean response and normal mode excitation in an eddy-resolving general circulation model : Miller, A.J., W.R. Holland and M.C. Hendershott, 1987. Geophys. astrophys. Fluid Dynam., 37(4): 253–278

Open-ocean response and normal mode excitation in an eddy-resolving general circulation model : Miller, A.J., W.R. Holland and M.C. Hendershott, 1987. Geophys. astrophys. Fluid Dynam., 37(4): 253–278

The nonlinear spin-up of a stratified ocean

Abstract The adjustment of a nonlinear, quasigeostrophic, stratified ocean to an impulsively applied wind stress is investigated under the assumption that barotropic advection of vortex tube length is the most important nonlinearity. The present study complements the steady state theories which have recently appeared, and extends earlier, dissipationless, linear models. In terms of Sverdrup transport, the equation for baroclinic evolution is a forced advection-diffusion equation. Solutions of this equation subject to a “tilted disk” Ekman divergence are obtained analytically for the case of no diffusion and numerically otherwise. The similarity between the present equation and that of a forced barotropic fluid with bottom topography is shown. Barotropic flow, which is assumed to mature instantly, can reverse the tendency for westward propagation, and thus produce regions of closed geostrophic contours. Inside these regions, dissipation, or equivalently the eddy field, plays a central role. We assume that eddy mixing effects a lateral, down-gradient diffusion of potential vorticity; hence, within the closed geostrophic contours, our model approaches a state of uniform potential vorticity. The solutions also extend the steady-state theories, which require weak diffusion, by demonstrating that homogenization occurs for moderately strong diffusion. The evoiution of potential vorticity and the thermocline are examined, and it is shown that the adjustment time of the model is governed by dissipation, rather than baroclinic wave propagation as in linear theories. If dissipation is weak, spin-up of a nonlinear ocean may take several times that predicted by linear models, which agrees with analyses of eddy-resolving general circulation models. The inclusion of a western boundary current may accelerate this process, although dissipation will still play a central role.

The Stability of Ocean Currents in Eddy-Resolving General Circulation Models

The stability of currents generated in an oceanic eddy-resolving general circulation model EGCM (Holland, 1978) is investigated by solving the eigenvalue problem associated with the finite-difference quasi-geostrophic vorticity equations which govern the flow. In general, both barotropically and baroclinically unstable waves are shown to exist for instantaneous currents found in the EGCM. Although these simulated flows are not always quasi-steady in the sense required by the theory and are themselves modified by the presence of the finite-amplitude eddies, many characteristics of the eddy field and its interaction with the time-mean circulation can nevertheless be deduced by the linear stability analysis. In particular, these investigations show that linear stability considerations correctly identify regions of instability in the ocean circulation model and accurately predict the low-order statistical features of the eddy field such as wavelength, period and phase speed. The effects of weakly unstable regions which are masked by global diagnostic techniques can be studied with the local stability model. The linear stability analysis also predicts, with some success, higher order statistics such as the sign and structure of intra-eddy energy fluxes that are important indicators of the dynamics of the unstable regions.