Sensitivity of simulated salinities in a three-dimensional ocean general circulation model to vertical mixing of destabilizing surface fluxes

 We present simulations performed with a three dimensional global ocean general circulation model which show that simulated salinities and amounts of convective mixing are very sensitive to vertical mixing of surface buoyancy fluxes. If, as usual, surface buoyancy fluxes are placed entirely in the topmost model level, our model produces excessive convective mixing in the Southern Ocean. This results in poor stimulated salinity in the Southern Ocean. In this simulation, we assume, as usual, that both surface buoyancy forcing and vertical mixing are homogeneous within each grid cell. If, on the other hand, destabilizing surface fluxes are instantaneously mixed into the subsurface ocean, the model produces much less convective mixing and much more realistic salinities. The vertical mixing of surface buoyancy fluxes performed in this simulation is equivalent to assuming that those fluxes affect only a small fraction of each grid cell, and cause vertical mixing only in that limited area. Our interpretation of these results is that the usual assumption that both surface buoyancy forcing and vertical mixing are uniform within each grid cell has a detrimental effect on model results; these results could be significantly improved by good parametrizations which treat the horizontal inhomogeneity of surface buoyancy forcing and of vertical mixing. Received: 25 February 1998 / Accepted: 9 September 1998     

Isopycnal mixing and convective adjustment in an ocean general circulation model

Abstract

Convective adjustment is examined in an ocean general circulation model which uses an isopycnal mixing parametrization. It is found that the use of an explicit convective adjustment scheme is not needed in a variety of equilibria and climate change scenario simulations. A numerical mechanism is proposed to explain this as well as the localized appearance of ‘negative’ diffusion.     

The deep water stratification of ocean general circulation models

Abstract

A central problem in climate and ocean modelling is the accurate simulation of the climatological state of the oceanic density field. A constant vertical diffusivity for heat and salt is frequently employed in ocean general circulation models (OGCMs) and it is usually assigned a value designed to optimize the depth of the pycnocline. One undesired consequence of this choice is a poor representation of the deep water, which is usually insufficiently stratified. In contrast to the uniform diffusivity of many models, some observational studies suggest that the vertical diffusivity is not constant but increases with depth, possibly in inverse proportion to the local buoyancy frequency. Numerical experiments with an OGCM are presented that demonstrate that allowing the vertical diffusivity to increase below the pycnocline substantially increases the stratification of the abyssal water mass of these models without significantly affecting the pycnocline depth, and hence may lead to a better representation of the vertical density structure.     

The influence of numerical advection schemes on the results of ocean general circulation models

The dependence of results from coarse-resolution models of the North Atlantic circulation on the numerical advection algorithm is studied. In particular, the sensitivity of parameters relevant for climate simulations as e.g., meridional transport of mass and heat and main thermocline thickness is investigated. Three algorithms were considered: (a) a central difference scheme with constant values for horizontal and vertical diffusion, (b) an upstream scheme with no explicit diffusion, and (c) a flux-corrected transport (FCT) scheme with constant and strictly isopycnal diffusion. The temporal evolution of the three models on time scales of centuries is markedly different, the upstream scheme resulting in much shorter adjustment time whereas the central difference scheme is slower and controlled by vertical diffusion rather than advection. In the steady state, the main thermocline structure is much less diffusive in the FCT calculation which also has much lower heat transport. Both horizontal circulation and overturning in the meridional-vertical plane are strongest in the upstream-model. The results are discussed in terms of the effective vertical (diapycnal) mixing in the different models. A significant increase in vertical resolution would be required to eliminate the high sensitivity due to the numerical algorithms, and allow physically motivated mixing formulations to become effective.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

A numerical world ocean general circulation model

This paper describes a numerical model of the world ocean based on the fully primitive equations. A “Standard” ocean state is introduced into the equations of the model and the perturbed thermodynamic variables are used in the modle’s calculations. Both a free upper surface and a bottom topography are included in the model and a sigma coordinate is used to normalize the model’s vertical component. The model has four unevenly-spaced layers and 4 × 5 horizontal resolution based on C-grid system. The finite-difference scheme of the model is designed to conserve the gross available energy in order to avoid fictitious energy generation or decay.The model has been tested in response to the annual mean surface wind stress, sea level air pressure and sea level air temperature as a preliminary step to its further improvement and its coupling with a global atmospheric general circulation model. Some of results, including currents, temperature and sea surface elevation simulated by the model are presented.     

Internal secular variability in an ocean general circulation model

We describe results of an experiment in which the Hamburg Large-Scale Geostrophic Ocean General Circulation Model was driven by a spatially correlated white-noise freshwater flux superimposed on the climatological fluxes. In addition to the red-noise character of the oceanic response, the model exhibits pronounced variability in a frequency band around 320 years. The centers of action of this oscillation are the Southern Ocean and the Atlantic.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil.     

The vertical transfer of momentum in the general circulation

Summary A simplified scheme of the transfer of zonal momentum in the general circulation is first summarised. Observational evidence in support of the pattern of vertical transfer is then provided from the relation between the fields of pressure and wind over the oceans in the Trades and westerlies, with large down-gradient motion in the former and very little cross-isobar motion in the latter. In neither case are the concepts of the classical (barotropic) friction layer valid. The vertical gradients of downwind and cross-wind shear stress are evaluated and from them climatological values of eddy viscosity are deduced for the surface layers. Values of eddy viscosity at levels of a few hundred metres are also deduced and found to be greater than at lower levels, consistent with the pattern of momentum transfer advanced. The transfer in the lower troposphere is probably mainly convective above the layer of mechanical turbulence. There is a concluding discussion on the nature of the vertical transfer of momentum in the middle and upper troposphere.

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Zusammenfassung Zunächst wird ein vereinfachtes Schema für den Transport der zonalen Bewegungsgröße in der allgemeinen Zirkulation skizziert. Hierauf werden auf Grund der Beziehungen zwischen Druck- und Windfeld über den Ozeanen in den Gebieten der Passatwinde und der Westströmung auf Beobachtungsmaterial beruhende Beweise für den Vertikaltransport entwickelt; dabei erweist sich der Winkel zwischen Isobaren und Windrichtung in den Passatgebieten als groß, in der Westströmung als klein. In keinem der beiden Fälle haben die Anschauungen über die klassische (barotrope) Reibungsschicht Gültigkeit. Die Vertikalgradienten der Schubspannung in Richtung wie auch senkrecht zum Winde werden berechnet und daraus klimatologische Mittelwerte der scheinbaren Viskosität (Austausch) für bodennahe Schichten abgeleitet. Werte der scheinbaren Viskosität werden auch für Niveaus von einigen 100 m berechnet und größer als in niedrigeren Schichten gefunden, was in Übereinstimmung mit der entwickelten Anschauung über das Schema der Fortpflanzung der Bewegungsgröße steht. Der Austausch in der unteren Troposphäre oberhalb der Schicht mechanischer Turbulenz ist wahrscheinlich in der Hauptsache konvektiv. Zum Schluß wird das Wesen des Vertikalaustausches der Bewegungsgröße in der mittleren und oberen Troposphäre diskutiert.

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Résumé Un schéma simplifié du transport, dans la circulation générale, de la quantité de mouvement zonal est d'abord esquissé. Des arguments en faveur du transport vertical proposé sont alors fournis par les relations des champs de pression et du vent au-dessus des océans dans les zones des alizés et du courant d'ouest; dans la première, l'angle entre l'isobare et la direction du vent est grande, dans la seconde il est petit. Dans aucun des deux cas, la conception de la couche de friction classique (barotropique) n'est valable. Les gradients verticaux des tensions internes sont évalués dans la direction et normalement à la direction du vent et des valeurs climatologiques de la viscosité apparente due à la turbulence en sont déduites pour les couches voisines du sol. Les valeurs de la viscosité apparente calculées pour des niveaux de quelques centaines de mètres sont plus grandes que celles des niveaux bas, concordant ainsi avec le transport vertical proposé. Dans la basse troposphère le transport est probablement surtout convectif au-dessus de la couche de turbulence. En conclusion l'auteur discute la nature du transport vertical de la quantité de mouvement dans la troposphère moyenne et supérieure.

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With 1 Figure.     

Effect of seasonal forcing on global circulation in a world ocean general circulation model

 Effects of the seasonal variation in thermohaline and wind forcing on the abyssal circulation are investigated by using an ocean general circulation model. To isolate effects of the seasonality in the thermohaline forcing from those in the wind forcing, we carry out three experiments with (1) annual-mean wind forcing and perpetual-winter thermohaline forcing, (2) annual-mean wind forcing and seasonal thermohaline forcing, and (3) seasonal wind forcing and seasonal thermohaline forcing. The deep water under the seasonal thermohaline forcing becomes warmer than under the perpetual-winter thermohaline forcing. Although the perpetual-winter thermohaline forcing is widely used and believed to reproduce the deep water better than the annual-mean forcing, the difference between the results of the perpetual-winter and the seasonal thermohaline forcing is significant. The seasonal variation of the Ekman convergence and divergence produces meridional overturning cells extending to the bottom because the period of seasonal cycle is shorter than the adjustment timescale by baroclinic Rossby waves. The heat transport owing to those Ekman flows and temperature anomalies makes the upper water (0–200 m) colder at low to mid-latitudes (40S–40N) and warmer at high latitudes. Also the deep water becomes warmer owing to the warming of the northern North Atlantic, the main source region of North Atlantic Deep Water. The model is also synchronously (i.e., without acceleration) integrated with seasonal forcing for 5400 y. A past study suggested that under seasonal forcing, a sufficient equilibrium state can be achieved after only decades of synchronous integration following more than 10 000 y of accelerated integration. Here, the result so obtained is compared with that of the 5400-y synchronous integration. The difference in the global average temperature is as small as 0.12 °C, and most of the difference is confined to the Southern Ocean. Received: 1 May 1998 / Accepted: 5 January 1999     

Towards a parametrization of river discharges into ocean general circulation models: a closure through energy conservation

Diagnostic methods are defined in order to compare two numerical simulations of ocean dynamics in a region of freshwater influence. The first one is a river plume simulation based on a high resolution numerical configuration of the POM coastal ocean model in which mixing parametrizations have been previously defined. The second one is a simulation based on the NEMO Global Ocean Model used for climate simulations in its half-a-degree configuration in which a river inflow is represented as precipitation on two coastal grid cells. Both simulations are forced with the same freshwater inflows and wind stresses. The divergence of volumetric fluxes above and below the halocline are compared. Results show that when an upwelling wind blows, the two models display similar behavior although the impact of lack of precision can be observed in the NEMO configuration. When a downwelling wind blows, the NEMO Global Ocean configuration can not reproduce the coastally trapped baroclinic dynamics because its grid resolution is too coarse. To find a parametrization to help represent these dynamics in ocean general circulation models, a method based on energy conservation is investigated. This method shows that it is possible to link the energy fluxes provided by river inflows to the divergence of energy fluxes integrated over the grid cells of ocean general circulation models. A parametrization of the dynamics created by freshwater inflows is deduced from this method. This enabled creation of a box model that proved to have the same behavior as the fluxes previously computed from the high resolution configuration.     

Error evolution in the dynamics of an ocean general circulation model

The problem of error propagation is considered for spatially uncorrelated errors of the barotropic stream function in an oceanic general circulation model (OGCM). Such errors typically occur when altimetric data from satellites are assimilated into ocean models. It is shown that the error decays at first due to the dissipation of the smallest scales in the error field. The error then grows exponentially before it saturates at the value corresponding to the difference between independent realizations. A simple analytic formula for the error behavior is derived; it matches the numerical results documented for the present primitive-equation ocean model, and other models in the literature.     

Comparison of vertical mixing schemes embedded in a global ocean GCM. Part II: Large scale circulation and water mass formation

Summary The sensitivity of the circulation and water mass properties in a global ocean circulation model (OGCM) to the stability dependent vertical mixing parameterisations of Pacanowski and Philander (1981) and Henderson-Sellers (1985) is investigated. The work extends a previous study which examined upper ocean charateristics and mixed layer evolution resulting from these schemes incorporated in the OGCM and made a recommendation as to the appropriateness of the latter scheme for global models. Under the assumption of constant vertical eddy coefficients (the control case), the model climatology displays acceptable values of North Atlantic Deep Water formation, Antarctic Circumpolar Current strength, and Indonesian throughflow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that these circulation and water mass properties are sensitive to the choice of parametrisation of vertical mixing and that the two stability dependent schemes are unable to perform satisfactorily over the global domain, instead being better suited to the tropics. Under conditions optimal for representing the tropical current and temperature structure, these schemes result in significant weakening of major currents (particularly, the ACC) and reductions in the rates of deep water formation and poleward heat transports. These deficiencies can only be remedied at the expense of the improvements to the simulation in the tropical part of the domain. The results presented indicate that the suggestions made in the previous study do not extend to situations where the deep ocean, and particularly, the global thermohaline circulation is important.With 8 Figures     

Multi-decadal thermohaline variability in an ocean–atmosphere general circulation model

A century scale integration of a near-global atmosphere–ocean model is used to study the multi-decadal variability of the thermohaline circulation (THC) in the Atlantic. The differences between the coupled and two supplementary ocean-only experiments suggest that a significant component of this variability is controlled by either a collective behavior of the ocean and the atmosphere, particularly in the form of air-sea heat exchange, or sub-monthly random noise present in the coupled system. Possible physical mechanisms giving rise to the mode of this THC variability are discussed. The SST anomaly associated with the THC variability resembles an interdecadal SST pattern extracted from observational data, as well as a pattern associated with the 50–60 year THC variability in the GFDL coupled model. In each case, a warming throughout the subpolar North Atlantic but concentrated along the Gulf Stream and its extension is indicated when the THC is strong. Concomitantly, surface air temperature has positive anomalies over the warmer ocean, with the strongest signal located downwind of the warmest SST anomalies and intruding into the western Eurasian Continent. In addition to the thermal response, there are also changes in the atmospheric flow pattern. More specifically, an anomalous northerly wind develops over the Labrador Sea when the THC is stronger than normal, suggesting a local primacy of the atmospheric forcing in the thermohaline perturbation structure.     

On the influence of frictional parameterization in wind-driven ocean circulation models

In a series of numerical experiments the wind-driven ocean circulation is studied in an idealized, rectangular model ocean, which is forced by steady zonal winds and damped by lateral and/or bottom friction. The problem as described by the barotropic vorticity equation is characterized by a Rossby number (R) and horizontal and/or vertical Ekman numbers (E_L, E_B) only.With free-slip conditions at the boundaries steady solutions for all chosen values of R are obtained, provided the diffusivity is sufficiently large. For both the forms of frictional parameterization a northern boundary current emerges with an eastward penetration scale depending on R. The recirculation pattern in the oceanically relevant ‘intermediate’ range of R is strongly affected by the type of friction. If lateral diffusion dominates bottom friction, a strong recirculating sub-gyre emerges in the northwestern corner of the basin. Its shape resembles the vertically integrated transport fields in recent eddy resolving model (EGCM) studies. The maximum transport is increased to values several times larger than the Sverdrup transport. The increase in transport is coupled with a development of closed contours of potential vorticity, enabling a nearly free inertial flow.This behaviour provides a sharp contrast to the bottom friction case (Veronis) where inertial recirculation only takes place with values of R so large that the eastward jet reaches the eastern boundary. It is shown that the linear friction law puts a strong constraint on the flow by preventing an intense recirculation in a small part of the basin.A reduction of the diffusivity (E_L) in the lateral friction case leads to quasi-steady solutions. The interaction with eddies becomes an integral part of the time mean energetics but does not influence the recirculation character of the flow.The main conclusion of the study is that the horizontal structure of the EGCM-transport fields can be explained in terms of a steady barotropic model where lateral friction represents the dominant dissipation mechanism.     

A global ocean biogeochemistry general circulation model and its simulations

An ocean biogeochemistry model was developed and incorporated into a global ocean general circulation model (LICOM) to form an ocean biogeochemistry general circulation model (OBGCM). The model was used to study the natural carbon cycle and the uptake and storage of anthropogenic CO2 in the ocean. A global export production of 12.5 Pg C yr-1 was obtained. The model estimated that in the pre-industrial era the global equatorial region within 15o of the equator released 0.97 Pg C yr-1 to the atmosphere, which was balanced by the gain of CO2 in other regions. The post-industrial air-sea CO2 flux indicated the oceanic uptake of CO2 emitted by human activities. An increase of 20－50 mol kg-1 for surface dissolved inorganic carbon (DIC) concentrations in the 1990s relative to pre-industrial times was obtained in the simulation, which was consistent with data-based estimates. The model generated a total anthropogenic carbon inventory of 105 Pg C as of 1994, which was within the range of estimates by other researchers. Various transports of both natural and anthropogenic DIC as well as labile dissolved organic carbon (LDOC) were estimated from the simulation. It was realized that the Southern Ocean and the high-latitude region of the North Pacific are important export regions where accumulative air-sea CO2 fluxes are larger than the DIC inventory, whereas the subtropical regions are acceptance regions. The interhemispheric transport of total natural carbon (DIC+LDOC) was found to be northward (0.11 Pg C yr-1), which was just balanced by the gain of carbon from the atmosphere in the Southern Hemisphere.