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.     

Dimethylsulphide production in the subantarctic southern ocean under enhanced greenhouse conditions

Dimethylsulphide (DMS) is an important sulphur‐containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP), which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the micro‐physical properties of clouds relevant to climate change are sensitive to CCN concentration in air, it has been postulated that marine sulphur emissions may play a rôle in climate regulation. The Subantarctic Southern Ocean (41–53°S) is relatively free of anthropogenic sulphur emissions, thus sulphate aerosols will be mainly derived from the biogenic source of DMS, making it an ideal region in which to evaluate the DMS‐climate regulation hypothesis. We have extended a previous modelling analysis of the DMS cycle in this region by employing a coupled general circulation model (CGCM) which has been run in transient mode to provide a more realistic climate scenario. The CGCM output provided meteorological data under the IPCC/IS92a radiative forcing scenario. A DMS production model has been forced with the CGCM climate data to simulate the trend in the sea‐to‐air DMS flux for the period 1960 to 2080, corresponding to equivalent CO₂ tripling relative to pre‐industrial levels. The results confirm a minor but non‐negligible increase in DMS flux in this region, in the range +1% to +6% predicted over the period simulated. Uncertainty analysis of the DMS model predictions have confirmed the positive sign for the change in DMS flux, that is a negative DMS feedback on warming.     

On the theory of a steady circulation in a baroclinic ocean

Some results are presented of numerical experiments associated with calculations of stationary fields of current velocity and temperature in the ocean. Results obtained by two different approaches to the theory of ocean circulation are compared: without regard for horizontal exchange under conditions of bottom sticking and with due regard for horizontal exchange in case of bottom slipping.     

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.     

An unsteady wind-driven ocean circulation model

This model of a continuously stratified ocean, consisting of surface Ekman and thermal layers and a deep abyssal region, is driven by an unsteady surface wind stress. The model yields a nonlinear planetary wave equation for the surface temperature which is solved by the method of characteristics. Horizontal distributions of the surface temperature, the heat content and the potential energy are computed. For a model zonal wind stress relevant to the North Atlantic, a parameter sensitivity study is made.Long time scale observations of the heat content in the North Atlantic north of 45°N show that periods of heating and cooling of a few decades duration frequently occur. Using a zonal wind stress consisting of the superposition of a mean and two time harmonic components, secular variations in the surface temperature field manifest themselves as a beat period, provided the time harmonic frequencies are close together. Finally, to investigate the effects on the circulation produced by the changing intensity of the westerly winds, numerical results are presented for the case when the wind stress distribution oscillates in the north-south direction with time. Results from the study show that the heating and cooling periods of a few decades duration in the North Atlantic can be reproduced.     

Steering of upper ocean currents and fronts by the topographically constrained abyssal circulation

A two-layer theory is used to investigate (1) the steering of upper ocean current pathways by topographically constrained abyssal currents that do not impinge on the bottom topography and (2) its application to upper ocean – topographic coupling via flow instabilities where topographically constrained eddy-driven deep mean flows in turn steer the mean pathways of upper ocean currents and associated fronts. In earlier studies the two-layer theory was applied to ocean models with low vertical resolution (2–6 layers). Here we investigate its relevance to complex ocean general circulation models (OGCMs) with high vertical resolution that are designed to simulate a wide range of ocean processes. The theory can be easily applied to models ranging from idealized to complex OGCMs, provided it is valid for the application. It can also be used in understanding some persistent features seen in observed ocean frontal pathways (over deep water) derived from satellite imagery and other data. To facilitate its application, a more thorough explanation of the theory is presented that emphasizes its range of validity. Three regions of the world ocean are used to investigate its application to eddy-resolving ocean models with high vertical resolution, including one where an assumption of the two-layer theory is violated. Results from the OGCMs with high vertical resolution are compared to those from models with low vertical resolution and to observations. In the Kuroshio region upper ocean – topographic coupling via flow instabilities and a modest seamount complex are used to explain the observed northward mean meander east of Japan where the Kuroshio separates from the coast. The Japan/East Sea (JES) is used to demonstrate the impact of upper ocean – topographic coupling in a relatively weak flow regime. East of South Island, New Zealand, the Southland Current is an observed western boundary current that flows in a direction counter to the demands of Sverdrup flow and counter to the direction simulated in nonlinear global flat bottom and reduced gravity models. A model with high vertical resolution (and topography extending through any number of layers) and a model with low vertical resolution (and vertically compressed but otherwise realistic topography confined to the lowest layer) both simulate a Southland Current in the observed direction with dynamics depending on the configuration of the regional seafloor. However, the dynamics of these simulations are very different because the Campbell Plateau and Chatham Rise east and southeast of New Zealand are rare features of the world ocean where the topography intrudes into the stratified water column over a relatively broad area but lies deeper than the nominal 200 m depth of the continental shelf break, violating a limitation of the two-layer theory. Observations confirm the results from the high vertical resolution model. Overall, the model simulations show increasingly widespread upper ocean – topographic coupling via flow instabilities as the horizontal resolution of the ocean models is increased, but fine resolution of mesoscale variability and the associated flow instabilities are required to obtain sufficient coupling. As a result, this type of coupling is critical in distinguishing between eddy-resolving and eddy-permitting ocean models in regions where it occurs.     

Steering of upper ocean currents and fronts by the topographically constrained abyssal circulation

A two-layer theory is used to investigate (1) the steering of upper ocean current pathways by topographically constrained abyssal currents that do not impinge on the bottom topography and (2) its application to upper ocean – topographic coupling via flow instabilities where topographically constrained eddy-driven deep mean flows in turn steer the mean pathways of upper ocean currents and associated fronts. In earlier studies the two-layer theory was applied to ocean models with low vertical resolution (2–6 layers). Here we investigate its relevance to complex ocean general circulation models (OGCMs) with high vertical resolution that are designed to simulate a wide range of ocean processes. The theory can be easily applied to models ranging from idealized to complex OGCMs, provided it is valid for the application. It can also be used in understanding some persistent features seen in observed ocean frontal pathways (over deep water) derived from satellite imagery and other data. To facilitate its application, a more thorough explanation of the theory is presented that emphasizes its range of validity. Three regions of the world ocean are used to investigate its application to eddy-resolving ocean models with high vertical resolution, including one where an assumption of the two-layer theory is violated. Results from the OGCMs with high vertical resolution are compared to those from models with low vertical resolution and to observations. In the Kuroshio region upper ocean – topographic coupling via flow instabilities and a modest seamount complex are used to explain the observed northward mean meander east of Japan where the Kuroshio separates from the coast. The Japan/East Sea (JES) is used to demonstrate the impact of upper ocean – topographic coupling in a relatively weak flow regime. East of South Island, New Zealand, the Southland Current is an observed western boundary current that flows in a direction counter to the demands of Sverdrup flow and counter to the direction simulated in nonlinear global flat bottom and reduced gravity models. A model with high vertical resolution (and topography extending through any number of layers) and a model with low vertical resolution (and vertically compressed but otherwise realistic topography confined to the lowest layer) both simulate a Southland Current in the observed direction with dynamics depending on the configuration of the regional seafloor. However, the dynamics of these simulations are very different because the Campbell Plateau and Chatham Rise east and southeast of New Zealand are rare features of the world ocean where the topography intrudes into the stratified water column over a relatively broad area but lies deeper than the nominal 200 m depth of the continental shelf break, violating a limitation of the two-layer theory. Observations confirm the results from the high vertical resolution model. Overall, the model simulations show increasingly widespread upper ocean – topographic coupling via flow instabilities as the horizontal resolution of the ocean models is increased, but fine resolution of mesoscale variability and the associated flow instabilities are required to obtain sufficient coupling. As a result, this type of coupling is critical in distinguishing between eddy-resolving and eddy-permitting ocean models in regions where it occurs.     

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.     

Wind-driven ocean circulation transition to barotropic instability

This article deals with the ocean circulation driven by steady zonal winds, and damped by bottom and biharmonic friction, when represented by the simple barotropic vorticity equation. A double gyre antisymmetrical wind stress pattern in a square basin is considered. Wind forcing and dissipation parameters are chosen within the ranges of what has been used in previous studies. The flow characteristics for both steady and unsteady situations are tentatively described as functions of model external parameters through the analysis of a large set of numerical experiments. Functional relations are derived for the mid-latitude jet parameters (length, width and transport) on the basis of scaling arguments. With the diagrams established for these quantities in forcing and dissipation parameter relations allow quantitative predictions of model response to a wide range of parameter choices to be made. The transition to barotropic instability is interpreted by analysing and comparing the spin-up phase of different numerical experiments leading either to stable or unstable solutions. Two major types of destabilization are identified, namely through meandering of the mid-latitude eastward jet and Rossby wave radiation from the westward return flow. The characteristics of the flows are shown to be highly sensitive to the external parameter changes. Competition between eddy kinetic energy level and eastward jet extension appears to consttitute the key point of this class of solutions, controlling in particular the intensity of transport in the inner gyres, driven by the eddy field on the two sides of the mid-basin jet, in a very similar manner to that of the more complex multilayered EGCMs.     

Future ocean uptake of CO2: interaction between ocean circulation and biology

We discuss the potential variations of the biological pump that can be expected from a change in the oceanic circulation in the ongoing global warming. The biogeochemical model is based on the assumption of a perfect stoichiometric composition (Redfield ratios) of organic material. Upwelling nutrients are transformed into organic particles that sink to the deep ocean according to observed profiles. The physical circulation model is driven by the warming pattern as derived from scenario computations of a fully coupled ocean-atmosphere model. The amplitude of the warming is determined from the varying concentration of atmospheric CO₂. The model predicts a pronounced weakening of the thermohaline overturning. This is connected with a reduction of the transient uptake capacity of the ocean. It yields also a more effective removal of organic material from the surface which partly compensates the physical effects of solubility. Both effects are rather marginal for the evolution of atmospheric pCO₂. Running climate models and carbon cycle models separately seems to be justified. Received: 9 August 1995 / Accepted: 22 April 1996     

海洋环流的长期变化和预估

IPCC第六次评估报告(AR6)于2021年8月在IPCC第一工作组第14次联合大会上得到审议通过,并得到了IPCC第54届全会接受和批准.文中主要对该报告第九章"海洋、冰冻圈和海平面"中与海洋环流的相关评估内容进行解读.与以前的IPCC报告相比,AR6进一步确认人类活动对海洋环流的影响,并基于最新的数值模式给出对未来...     

Wind forcing of the ocean and the Atlantic meridional overturning circulation

Wind forcing of the oceans is analysed for a millennium-length period of a simulation with a global coupled model. The time mean value of the zonal wind energy input to the oceans was found to be 0.97?TW, similar to other estimates, with maximum inputs in the Southern Ocean and Equatorial Pacific Ocean. The meridional wind energy input was also evaluated. The time series of the zonal wind energy input consisted of white noise, with marked multiannual and multidecadal variability, and had a range of 1.6 between minimum and maximum values. Seasonal variations in the wind energy input were most marked in the Pacific Ocean. Composites of the various climatic terms involved in the wind energy input, for maximum and minimum values of the input time series, identified distinct differences in anomaly values, particularly over the Southern Ocean. The temporal variability of the Pacific equatorial wind energy input was clearly identified with ENSO events. The model reproduced the observed structure of the Atlantic meridional overturning circulation surprisingly well. The time series of this circulation exhibited interannual to centennial variability. Correlations, both instantaneous and lagged, failed to identify any meaningful relationship between the temporal variability of the circulation and the wind energy input to the Southern Ocean. The optimum correlation was found between time smoothed versions of the time series for this circulation and the heat input to the North Atlantic Ocean, implying, as noted elsewhere, that this heat input is the principal driver of the temporal variability of the circulation.     

Global ocean circulation and equator-pole heat transport as a function of ocean GCM resolution

To determine whether resolution of smaller scales is necessary to simulate large-scale ocean climate correctly, I examine results from a global ocean GCM run with different horizontal grid spacings. The horizontal grid spacings span a range from coarse resolutions traditionally used in climate modeling to nearly the highest resolution attained with today's computers. The experiments include four cases employing 4°, 2°, 1° and 1/2° spacing in latitude and longitude, which were run with minimal differences among them, i.e., in a controlled experiment. Two additional cases, 1/2° spacing with a more scale-selective sub-gridscale mixing of heat and momentum, and approximate 1/2° spacing, are also included. The 1/2° run resolves most of the observed mesoscale eddy energy in the ocean. Artificial constraints on the model tend to minimize differences among the different resolution cases. Nevertheless, the simulations show significant changes as resolution increases. These changes generally but not always bring the model into better agreement with observations. Differences are typically more noticeable when comparing the 4° and 2° runs than when comparing the 2° and 1° runs or the 1° and 1/2° runs. A reasonable conclusion to draw for current studies with coupled ocean-atmosphere GCMs is that the ocean grid spacing could be set to about 1° to accrue the benefits of enhanced resolution without paying an excessively steep price in computer-time cost. The model's poleward heat transport at 1/2° grid spacing peaks at about 1 × 10¹⁵ W in the Northern Hemisphere and 0.5 × 10¹⁵ W in the Southern Hemisphere. These values are significantly below observations, a problem typical of ocean GCMs even when they are less constrained than in the present study. This present problem is alleviated somewhat in the 1/2° run. In this case, however, the eddies resolved by the model generally act to counter rather than to reinforce the heat transport of the mean flow. Improved heat transport may result less from enhanced resolution than from other changes made in this version of the model, such as more accurate wind forcing.     

An intermediate model for large-scale ocean circulation studies

The design and purposes of an intermediate model are discussed along with fundamentals of the model and results of numerical experiments. The main purposes of the model are reconstructions of the schemes of the ocean large-scale circulation and paleocirculation. For these problems numerical effectiveness is the key factor. A novel feature is the parameterization of the side wall Ekman boundary layers which was introduced to enable the use of geostrophy for calculating baroclinic velocity. This approach of connecting the side frictional layers with a non-viscous interior is not model-specific and can be used in any model employing geostrophy in the interior. The method can facilitate the no-flux and no-slip boundary conditions at the side walls in such models. Preliminary numerical experiments with simple basin geometry and idealized forcing aimed at a comparison with primitive equation and planetary geostrophic models are carried out. A direct comparison with the Geophysical Fluid Dynamics Laboratory (GFDL) primitive equation model was performed for a quantitative test of the proposed model. The results of the experiments are discussed in the context of the applicability of intermediate models for studying ocean climate dynamics.     

Sea ice induced changes in ocean circulation during the Eemian

We argue that Arctic sea ice played an important role during early stages of the last glacial inception. Two simulations of the Institut Pierre Simon Laplace coupled model 4 are analyzed, one for the time of maximum high latitude summer insolation during the last interglacial, the Eemian, and a second one for the subsequent summer insolation minimum, at the last glacial inception. During the inception, increased Arctic freshwater export by sea ice shuts down Labrador Sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15%, respectively. A positive feedback of the Atlantic subpolar gyre enhances the initial freshening by sea ice. The reorganization of the subpolar surface circulation, however, makes the Atlantic inflow more saline and thereby maintains deep convection in the Nordic Seas. These results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes.     

Sensitivity of Atlantic meridional overturning circulation to the dynamical framework in an ocean general circulation model

The horizontal coordinate systems commonly used in most global ocean models are the spherical latitude–longitude grid and displaced poles, such as a tripolar grid. The effect of the horizontal coordinate system on Atlantic meridional overturning circulation (AMOC) is evaluated by using an OGCM (ocean general circulation model). Two experiments are conducted with the model—one using a latitude–longitude grid (referred to as Lat_1) and the other using a tripolar grid (referred to as Tri). The results show that Tri simulates a stronger North Atlantic deep water (NADW) than Lat_1, as more saline water masses enter the Greenland–Iceland–Norwegian (GIN) seas in Tri. The stronger NADW can be attributed to two factors. One is the removal of the zonal filter in Tri, which leads to an increasing of the zonal gradient of temperature and salinity, thus strengthening the north geostrophic flow. In turn, it decreases the positive subsurface temperature and salinity biases in the subtropical regions. The other may be associated with topography at the North Pole, because realistic topography is applied in the tripolar grid while the latitude–longitude grid employs an artificial island around the North Pole. In order to evaluate the effect of the filter on AMOC, three enhanced filter experiments are carried out. Compared to Lat_1, an enhanced filter can also augment NADW formation, since more saline water is suppressed in the GIN seas, but accumulated in the Labrador Sea, especially in experiment Lat_2_S, which is the experiment with an enhanced filter on salinity.