Coarse-resolution Ocean General Circulation Research Articles

Parameterized and resolved Southern Ocean eddy compensation

The ability to parameterize Southern Ocean eddy effects in a forced coarse resolution ocean general circulation model is assessed. The transient model response to a suite of different Southern Ocean wind stress forcing perturbations is presented and compared to identical experiments performed with the same model in 0.1° eddy-resolving resolution. With forcing of present-day wind stress magnitude and a thickness diffusivity formulated in terms of the local stratification, it is shown that the Southern Ocean residual meridional overturning circulation in the two models is different in structure and magnitude. It is found that the difference in the upper overturning cell is primarily explained by an overly strong subsurface flow in the parameterized eddy-induced circulation while the difference in the lower cell is mainly ascribed to the mean-flow overturning. With a zonally constant decrease of the zonal wind stress by 50% we show that the absolute decrease in the overturning circulation is insensitive to model resolution, and that the meridional isopycnal slope is relaxed in both models. The agreement between the models is not reproduced by a 50% wind stress increase, where the high resolution overturning decreases by 20%, but increases by 100% in the coarse resolution model. It is demonstrated that this difference is explained by changes in surface buoyancy forcing due to a reduced Antarctic sea ice cover, which strongly modulate the overturning response and ocean stratification. We conclude that the parameterized eddies are able to mimic the transient response to altered wind stress in the high resolution model, but partly misrepresent the unperturbed Southern Ocean meridional overturning circulation and associated heat transports.

Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model

Abstract. Direct observations, satellite measurements and paleo records reveal strong variability in the Atlantic subpolar gyre on various time scales. Here we show that variations of comparable amplitude can only be simulated in a coupled climate model in the proximity of a dynamical threshold. The threshold and the associated dynamic response is due to a positive feedback involving increased salt transport in the subpolar gyre and enhanced deep convection in its centre. A series of sensitivity experiments is performed with a coarse resolution ocean general circulation model coupled to a statistical-dynamical atmosphere model which in itself does not produce atmospheric variability. To simulate the impact of atmospheric variability, the model system is perturbed with freshwater forcing of varying, but small amplitude and multi-decadal to centennial periodicities and observational variations in wind stress. While both freshwater and wind-stress-forcing have a small direct effect on the strength of the subpolar gyre, the magnitude of the gyre's response is strongly increased in the vicinity of the threshold. Our results indicate that baroclinic self-amplification in the North Atlantic ocean can play an important role in presently observed SPG variability and thereby North Atlantic climate variability on multi-decadal scales.

The relation of meridional pressure gradients to North Atlantic deep water volume transport in an ocean general circulation model

We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic.

Decoupling of iron and phosphate in the global ocean

We formulate a mechanistic model of the coupled oceanic iron and phosphorus cycles. The iron parameterization includes scavenging onto sinking particles, complexation with an organic ligand, and a prescribed aeolian source. Export production is limited by the availability of light, phosphate, and iron. We implement this biogeochemical scheme in a coarse resolution ocean general circulation model using scavenging rates and conditional stability constants guided by laboratory studies and a suite of box model sensitivity studies. The model is able to reproduce the broad regional patterns of iron and phosphorus. In particular, the high macronutrient concentrations of the Southern Ocean, tropical Pacific, and subarctic Pacific emerge from the explicit iron limitation of the model. In addition, the model also qualitatively reproduces the observed interbasin gradients of deep, dissolved iron with the lowest values in the Southern Ocean. The ubiquitous presence of significant amounts of free ligand is also explicitly captured. We define a tracer, Fe* which quantifies the degree to which a water mass is iron limited, relative to phosphorus. Surface waters in high‐nutrient, low‐chlorophyll regions have negative Fe* values, indicating Fe limitation. The extent of the decoupling of iron and phosphorus is determined by the availability and binding strength of the ligand relative to the scavenging by particulate. Global iron concentrations are sensitive to changes in scavenging rate and physical forcing. Decreasing the scavenging rate 40% results in ∼0.1 nM increase in dissolved iron in deep waters. Forcing the model with weaker wind stresses leads to a decrease in surface [PO4] and [Fe] in the Southern Ocean due to a reduction in the upwelling strength.

Modeling ocean deep convection

The goal of this study is to assess models for Deep Convection with special emphasis on their use in coarse resolution ocean general circulation models. A model for deep convection must contain both vertical transport and lateral advection by mesoscale eddies generated by baroclinic instabilities. The first process operates mostly in the initial phases while the second dominates the final stages. Here, the emphasis is on models for vertical mixing. When mesoscales are not resolved, they are treated with the Gent and McWilliams parameterization. The model results are tested against the measurements of Lavender, Davis and Owens, 2002 (LDO) in the Labrador Sea. Specifically, we shall inquire whether the models are able to reproduce the region of “ deepest convection,” which we shall refer to as DC (mixed layer depths 800–1300 m). The region where it was measured by Lavender et al. (2002) will be referred to as the LDO region. The main results of this study can be summarized as follows. (1) 3° × 3° resolution. A GFDL-type OGCM with the GISS vertical mixing model predicts DC in the LDO region where the vertical heat diffusivity is found to be 10 m 2 s −1, a value that is quite close to the one suggested by heuristic studies. No parameter was changed from the original GISS model. However, the GISS model also predicts some DC in a region to the east of the LDO region. (2) 3° × 3° resolution. A GFDL-type OGCM with the KPP model (everything else being the same) does not predict DC in the LDO region where the vertical heat diffusivity is found to be 0.5 × 10 −4 m 2 s −1 which is the background value. The KPP model yields DC only to the east of the LDO region. (3) 1° × 1° resolution. In this case, a MY2.5 mixing scheme predicts DC in the LDO region. However, it also predicts DC to the west, north and south of it, where it is not observed. The behavior of the KPP and MY models are somewhat anti-symmetric. The MY models yield too low a mixing in stably stratified flows since they predict a critical Richardson number Ri(cr)=0.19 which is five times smaller than the value Ri(cr)=O(1) needed to obtain realistic ML depths. However, as discussed above, in unstable stratifications the MY models yield better results. On the other hand, the KPP model, which was motivated primarily by the need to overcome the MY “too low mixing” in stable stratification, yields at coarse resolution, no DC in the LDO region. In this respect, the GISS model, yields both a correct Ri(cr)=O(1) in stable stratification and correct results in the unstable configuration in the LDO region. (4) 1/3° × 1/3° resolution. In this case, KPP predicts mixed layer depths up to 1.7 km inside the LDO region where at coarse resolution none existed. However, the model still produces DC at locations outside the LDO region where it is not observed. However, since these regions are intermingled with very shallow mixed layer depths, the resulting mean mixed layer depths turn out to be less than 800 m almost everywhere outside the LDO region. (5) 1/12° × 1/12° resolution. In this case, KPP predicts mixed layer depths up to 3 km both inside and outside the LDO region. These regions are, here too, intermingled with very shallow mixed layer depths with resulting mean mixed depths greater than 800 m both inside and outside the LDO region. In conclusion, as for a model for deep convection to be used in coarse resolution, these results indicate that the GISS mixing model fares well with observations in both stable and unstable stratifications but overestimates its geographical extent. This leads to the problem of future improvements of the model. It must be generalized to include the following physically important features: (a) rotation that becomes important in the later phases of deep convection when it acts to slow down the rate of mixed layer deepening, (b) non-locality, in particular skewness which is large (negative) in the initial phases of deep convection and becomes small in the final stages, and finally, (c) a new model to treat lateral advection by baroclinic eddies that in the final stages of deep convection dominates over vertical transport.

Intermediate water mass production controlled by southern hemisphere winds

It is demonstrated that the production of intermediate water in a coarse resolution ocean general circulation model is controlled by Southern Hemisphere winds. Results from four equilibrium experiments using simplified topography and surface forcing are presented. The first experiment was carried out with no wind forcing, subsequent experiments employed annual mean surface stresses, which were amplified using factors of 0.5, 1, and 2.0 south of 30°S. In all experiments, the salinity minimum characteristic for intermediate water is reproduced. Volume transports are directly proportional to the applied Southern Hemisphere surface stresses. These force an increased export of intermediate water and heat into the South Pacific Ocean northward across 30°S and through Drake Passage into the South Atlantic Ocean. It results in a warming of the South Pacific Ocean, which is at a maximum in the intermediate water density range.

Effects of locally enhanced vertical diffusivity over rough bathymetry on the world ocean circulation

Observations indicate that vertical diffusivity in the deep ocean is considerably enhanced over rough bathymetry and is very small elsewhere. Here we investigate effects of locally enhanced vertical diffusivity over rough bottom topography on the world ocean circulation by use of a coarse resolution ocean general circulation model. Vertical diffusivity is enhanced in the model, taking into account effects of internal tide breaking. For comparison, two cases of an experiment are carried out, where vertical diffusivity increases with depth without horizontal inhomogeneity. Horizontal distribution of deep upwelling is qualitatively different, depending on whether or not vertical diffusivity is horizontally inhomogeneous. Upwelling of Circumpolar Deep Water in the Pacific and Antarctic Bottom Water in the Atlantic is confined where vertical diffusivity is enhanced in the case of horizontally inhomogeneous vertical diffusivity, whereas it is horizontally uniform in the other cases. This difference leads to different three‐dimensional structure of the deep circulation.

Sensitivity of a coarse resolution ocean general circulation model under climate change forcing

Sensitivity of a coarse resolution ocean general circulation model under climate change forcing

Sensitivity of a coarse resolution ocean general circulation model under climate change forcing

Tunable parameters in an ocean general circulation model are assessed as modifiers to the model response to several climate change scenarios. For the current climate scenario it is found that vertical diffusivity plays a dominant roˆ le as expected. An analysis of the horizontal diffusivity variation suggests that use of the peak value of the overturning streamfunction is a misleading indicator of model flow strength. For the warming and cooling scenarios vertical and horizontal diffusivity variations dominated the equilibria response. However, these tunable parameters are of minor significance in the transient response in the cooling scenarios. The diffusively dominant warming scenarios, in contrast, very much depend on the magnitude of the vertical diffusivity. This has important implications for the use of coarse resolution models in coupled atmosphereocean model climate change simulations.

Mixed boundary conditions in ocean general circulation models and their influence on the stability of the model's conveyor belt

When driven under “mixed boundary conditions”, coarse resolution ocean general circulation models (OGCMs) generally show a high sensitivity of the present‐day thermohaline circulation against perturbations. We will show that an alternative formulation of the boundary condition for temperature, a mixture of prescribed heat fluxes and additional restoring of the sea surface temperature to a climatological boundary temperature with a longer time constant, drastically alters the stability of the modes of the thermohaline circulation. The results from simulations with the Hamburg large‐scale geostrophic OGCM indicate that the stability of the mode of the thermohaline circulation with formation of North Atlantic deepwater increases, if the damping of sea surface temperature anomalies is reduced, whereas the opposite is true for the mode without North Atlantic deep water formation. It turns out that the formulation of the temperature boundary condition also affects the variability of the model.

Circulation driven by observed surface thermohaline fields in a coarse resolution ocean general circulation model

This paper reports the results from a series of mechanistic studies on the global ocean circulation by assimilating a set of observed surface thermohaline data into a low‐resolution Geophysical Fluid Dynamics Laboratory (GFDL) global ocean general circulation model (OGCM). In most of the experiments the surface wind stress is set to zero. The surface thermohaline forcing features a strong relaxation of surface temperature and salinity towards observed values. The ocean circulation resulting from the assimilation of surface thermohaline fields depends heavily upon the various parameterizations of vertical mixing processes. Consistent with previous studies, a greater vertical diffusivity results in a greater meridional overturning stream function and a greater meridional heat transport. The vertical structure of stratification is particularly well reproduced if the vertical diffusivity is set to be buoyancy dependent. Even in the absence of explicit wind forcing, the major circulation features are reproduced. In particular, water mass properties are realistic, and the Antarctic Circumpolar Current (ACC) and the Gulf Stream have significant barotropic components (especially the ACC). The feature of realistic water masses can be attributed to the reproduction of convection‐dominated heat and freshwater fluxes in the polar and subpolar regions, where these water masses are formed. In the absence of explicit wind, the barotropic transports are driven by bottom pressure torque, which is associated with variable bottom topography and upslope/downslope bottom flows. Convective processes play an important role in redistributing the water mass, generating the barotropic transport, and maintaining the thermohaline structure. In the absence of convective adjustment, the global circulation is characterized by a thermocline reversal in polar and subpolar regions. Deep convection also leads to larger bottom flows, and hence indirectly play a role in generating the barotropic transports. In the low‐resolution model, the barotropic ACC is broad; this seals the Indian Ocean from the Atlantic Ocean, and prevents both the barotropic and baroclinic interbasin exchange between the Indian and the Atlantic oceans. In this situation, the North Atlantic Deep Water (NADW) outflow is compensated by the cool and fresh South Pacific water from the east of Drake Passage (cool water route). When convection is used as a device to suppress ACC, and to ensure that the NADW is the only deep water source, the NADW formation is seen to cause a large Indonesian throughflow. The teleconnection between the throughflow and the NADW is via the Agulhas leakage water. In the absence of ACC, the NADW outflow is compensated by the warm and salty Indian water through the leakage (warm water route). The results show that the realization of warm water route depends on the structure of the modeled ACC.