A parameterization scheme of vertical mixing due to inertial internal wave breaking in the 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.

The Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the decadal variability of global and regional climate through changing poleward transport of heat. However, realistic simulation of the AMOC, i.e., its strength and spatial structure, remains a challenge for ocean general circulation models (OGCMs) and coupled climate models. Here, we investigate how the simulated AMOC could be affected by improved accuracy of the seawater equation of state (EOS) with an OGCM. Two EOSs used in this study: the UNESCO EOS80, and the “stiffened” EOS derived from the compressibility of sea water and the UNESCO EOS80. Compared to the model using the UNESCO EOS80, the model using the “stiffened” EOS yields stronger deep convection in the Labrador Sea, the Irminger-Iceland-Scotland Basin, and the Greenland-Iceland-Norwegian (GIN) seas, which leads to an improvement in the simulation of the AMOC: Along 26.5°N, the maximum transport is increased from 14.9 to 17.4 Sv and the interface between the upper clockwise cell and lower counterclockwise cell is deepened from 2.8 to 3.3 km, both matching the observations better. Taken the Labrador Sea as an example, the processes, including both direct and indirect causes, that in part responsible for the improved AMOC are as follows. The use of “stiffened” EOS increases the density throughout the water column and weakens the stability of sea water. Moreover, the enhanced cabbeling and thermobaric effect strengthen the vertical advection, intensifying the deep convection and increasing formation of deep water, which eventually improves the simulation of the AMOC. The intensified AMOC, in turn, speeds up the surface return flow, transporting more warm and saline water to the high latitudes in the North Atlantic, which contributes to the densification of surface water. Similar analyses can be applied to the Iceland-Scotland Basin and GIN seas. Thus, the enhanced deep convection and formation of deep water in the Labrador Sea, as well as in the Iceland–Scotland Basin and GIN seas, improve the simulated AMOC.

Daily inter-annual simulations of SST and MLD using atmospherically forced OGCMs: Model evaluation in comparison to buoy time series

Abstract. Ocean general circulation models still have large upper-ocean biases, including in tropical sea surface temperature, that are possibly connected to the representation of vertical mixing. In earlier studies, the ocean vertical mixing parameterization has usually been tuned for a specific site or only within a specific model. We present here a systematic comparison of the effects of changes in the vertical mixing scheme in two different global ocean models, ICON-O and FESOM, run at a horizontal resolution of 10 km in the tropical Atlantic. We test two commonly used vertical mixing schemes: the K-profile parameterization (KPP) and the turbulent kinetic energy (TKE) scheme. Additionally, we vary tuning parameters in both schemes and test the addition of Langmuir turbulence in the TKE scheme. We show that the biases of mean sea surface temperature, subsurface temperature, subsurface currents, and mixed layer depth differ more between the two models than between runs with different mixing scheme settings within each model. For ICON-O, there is a larger difference between TKE and KPP than for FESOM. In both models, varying the tuning parameters hardly affects the pattern and magnitude of the mean state biases. For the representation of smaller-scale variability like the diurnal cycle or inertial waves, the choice of the mixing scheme can matter: the diurnally enhanced penetration of equatorial turbulence below the mixed layer is only simulated with TKE, not with KPP. However, tuning of the parameters within the mixing schemes does not lead to large improvements for these processes. We conclude that a substantial part of the upper-ocean tropical Atlantic biases is not sensitive to details of the vertical mixing scheme.

The salinity boundary condition at the ocean surface plays an important role in the stability of long-term integrations of an oceanic general circulation model (OGCM) and in determining its equilibrium solutions. This study presents a new formulation of the salt flux calculation at the ocean surface based on physical processes of salt exchange at the air-sea interface. The formulation improves the commonly used virtual salt flux with constant reference salinity by allowing for spatial correlations between surface freshwater flux and sea-surface salinity while preserving the conservation of global salinity. The new boundary condition is implemented in the latest version of the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics Climate Ocean Model version 2 (LICOM2.0). The impact of the new boundary condition on the equilibrium simulations of the model is presented. It is shown that the new formulation leads to a stronger Atlantic meridional overturning circulation (AMOC) that is closer to observational estimates. It also slightly improves poleward heat transport by the oceans in both the Atlantic and the global oceans.

Paleo proxy data suggest that the Atlantic meridional overturning circulation (AMOC) was shallower and weaker at the Last Glacial Maximum (LGM) than at present. In this study, we have identified the existence of a thermal threshold of the AMOC which may explain why many coupled climate models fail to simulate the weaker AMOC during the LGM. By using results obtained from a coupled climate model and conducting sensitivity simulations with an ocean general circulation model, we found that the sudden transition from the present‐day AMOC to the weaker glacial AMOC occurs when we gradually change the degree of surface cooling from present‐day to glacial conditions. This result is related to response of deep convection in the northern North Atlantic Ocean; moderate cooling enhances deep convection whereas sufficient cooling results in total covering of sea ice there and suppression of deep convection. The findings indicate the existence of a thermal threshold controlling the AMOC, where the present‐day‐type AMOC suddenly shifts to the weaker glacial AMOC once the surface cooling exceeds this threshold. We also demonstrate that wind stress forcing plays a critical role in controlling the value of the thermal threshold. Our study suggests that slight differences in the degree of surface cooling or wind stress forcing for LGM simulations could lead to the very different response of the AMOC during the LGM as reported in previous LGM simulations.

Characteristics of Coupled Atmosphere-Ocean CO2 Sensitivity Experiments with Different Ocean Formulations

There is large uncertainty in the future regional sea level change under anthropogenic climate change. Our study presents and uses a novel design of ocean general circulation model (OGCM) experiments to investigate the ocean's response to surface buoyancy and momentum flux perturbations without atmosphere‐ocean feedbacks (e.g., without surface restoring or bulk formulae), as part of the Flux‐Anomaly‐Forced Model Intercomparison Project (FAFMIP). In an ensemble of OGCMs forced with identical surface flux perturbations, simulated dynamic sea level (DSL) and ocean heat content (OHC) change demonstrate considerable disagreement. In the North Atlantic, the disagreement in DSL and OHC change between models is mainly due to differences in the residual (resolved and eddy) circulation change, with a large spread in the Atlantic meridional overturning circulation (AMOC) weakening (20–50%). In the western North Pacific, OHC change is similar among the OGCM ensemble, but the contributing physical processes differ. For the Southern Ocean, isopycnal and diapycnal mixing change dominate the spread in OHC change. In addition, a component of the atmosphere‐ocean feedbacks are quantified by comparing coupled, atmosphere‐ocean GCM (AOGCM) and OGCM FAFMIP experiments with consistent ocean models. We find that there is 10% more AMOC weakening in AOGCMs relative to OGCMs, since the extratropical North Atlantic SST cooling due to heat redistribution amplifies the surface heat flux perturbation. This component of the atmosphere‐ocean feedbacks enhances the pattern of North Atlantic OHC and DSL change, with relatively stronger increases and decreases in the tropics and extratropics, respectively.

Previous modeling and observational studies have established that it is possible to accurately monitor the Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N using a coast-to-coast array of instrumented moorings supplemented by direct transport measurements in key boundary regions (the RAPID/MOCHA/WBTS Array). The main sources of observational and structural errors have been identified in a variety of individual studies. Here a unified framework for identifying and quantifying structural errors associated with the RAPID array-based AMOC estimates is established using a high-resolution (eddy resolving at low-mid latitudes, eddy permitting elsewhere) ocean general circulation model, which simulates the ocean state between 1978 and 2010. We define a virtual RAPID array in the model in close analogy to the real RAPID array and compare the AMOC estimate from the virtual array with the true model AMOC. The model analysis suggests that the RAPID method underestimates the mean AMOC by ∼1.5 Sv (1 Sv = 106 m3 s−1) at ∼900 m depth, however it captures the variability to high accuracy. We examine three major contributions to the streamfunction bias: (i) due to the assumption of a single fixed reference level for calculation of geostrophic transports, (ii) due to regions not sampled by the array and (iii) due to ageostrophic transport. A key element in (i) and (iii) is use of the model sea surface height to establish the true (or absolute) geostrophic transport. In the upper 2000 m, we find that the reference level bias is strongest and most variable in time, whereas the bias due to unsampled regions is largest below 3000 m. The ageostrophic transport is significant in the upper 1000 m but shows very little variability. The results establish, for the first time, the uncertainty of the AMOC estimate due to the combined structural errors in the measurement design and suggest ways in which the error could be reduced. Our work has applications to basin-wide circulation measurement arrays at other latitudes and in other basins as well as quantifying systematic errors in ocean model estimates of the AMOC at 26.5°N.

Five non-eddy-resolving oceanic general circulation models driven by atmospheric fluxes derived from the NCEP reanalysis are used to investigate the link between the Gulf Stream (GS) variability, the atmospheric circulation, and the Atlantic meridional overturning circulation (AMOC). Despite the limited model resolution, the temperature at the 200-m depth along the mean GS axis behaves similarly in most models to that observed, and it is also well correlated with the North Atlantic Oscillation (NAO), indicating that a northward (southward) GS shift lags a positive (negative) NAO phase by 0–2 yr. The northward shift is accompanied by an increase in the GS transport, and conversely the southward shift with a decrease in the GS transport. Two dominant time scales appear in the response of the GS transport to the NAO forcing: a fast time scale (less than 1 month) for the barotropic component, and a slower one (about 2 yr) for the baroclinic component. In addition, the two components are weakly coupled. The GS response seems broadly consistent with a linear adjustment to the changes in the wind stress curl, and evidence for baroclinic Rossby wave propagation is found in the southern part of the subtropical gyre. However, the GS shifts are also affected by basin-scale changes in the oceanic conditions, and they are well correlated in most models with the changes in the AMOC. A larger AMOC is found when the GS is stronger and displaced northward, and a higher correlation is found when the observed changes of the GS position are used in the comparison. The relation between the GS and the AMOC could be explained by the inherent coupling between the thermohaline and the wind-driven circulation, or by the NAO variability driving them on similar time scales in the models.

We interrogate the sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to surface heat and freshwater fluxes over the Subpolar Gyre in an ocean general circulation model and its adjoint. Surface heat loss out of the Subpolar Gyre in the winter strengthens the AMOC at a lead time of approximately 6 months. However, the same surface heat flux anomaly in the summer leads to a delayed AMOC weakening that emerges at a lag of 8 months. Under a summer surface cooling perturbation, the AMOC progressively weakens up to a lag of approximately 80 months, and then the negative overturning anomaly persists for years. Compared with the sensitivity to surface heat fluxes, seasonality in the AMOC sensitivity to surface freshwater fluxes is less pronounced, and there is no sign reversal between the response to summer and winter perturbations. We explain the mechanisms behind the large seasonal differences in the AMOC sensitivity to surface heat fluxes and highlight the role of evaporation. Heat flux anomalies over the Subpolar Gyre trigger changes in the rate of evaporation and hence affect the salinity of the mixed layer. Surface cooling gives rise to freshening in the following months, whereas warming leads to salinification. Persistent buoyancy changes due to salinity responses counteract the impact of heat fluxes to a varying extent depending on the seasonal mixed layer depth. On the other hand, air-sea feedback mechanisms exert a positive feedback on the AMOC response to surface freshwater flux perturbations both in the summer and in the winter months.

We use an ocean general circulation model and its adjoint to analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea to variability in the deep branch of the Atlantic meridional overturning circulation (AMOC) on inter-annual timescales. Our study highlights the importance of the North Atlantic Current (NAC) for the north-to-south connectivity in the AMOC and for the meridional transport of Lower North Atlantic Deep Water (LNADW). We identify two mechanisms that allow the Labrador Sea to impact velocities in the LNADW layer. The first mechanism involves a passive advection of surface buoyancy anomalies from the Labrador Sea towards the eastern subpolar gyre by the background NAC. The second mechanism plays a dominant role and involves a dynamical response of the NAC to surface density anomalies originating in the Labrador Sea; the NAC adjustment modifies the northward transport of salt and heat and exerts a strong positive feedback, amplifying the upper ocean buoyancy anomalies. The two mechanisms spin up/down the subpolar gyre on a timescale of years, while boundary trapped waves rapidly communicate this signal to the subtropics and trigger an adjustment of LNADW transport on a timescale of months. The NAC and the eastern subpolar gyre play an essential role in both mechanisms linking the Labrador Sea with LNADW transport variability and the subtropical AMOC. We thus reconcile two apparently contradictory paradigms about AMOC connectivity: (1) Labrador Sea buoyancy anomalies drive AMOC variability; (2) water mass transformation is largest in the eastern subpolar gyre.

The Atlantic Meridional Overturning Circulation (AMOC) is a tipping component of the climate system, with a quasi-global impact. Several numerical and observational studies emphasized two modes of AMOC variability, characterized by two distinct Atlantic sea surface temperature patterns. One is associated with centennial changes, the Trend Mode, and the other with the Atlantic Multidecadal Oscillation (AMO). The origin of the different manifestations of these modes it is not fully understood. Using observational data and an ocean general circulation model we present evidence that, whereas the Trend Mode is mainly linked with deep water formation in the Nordic Seas and with a North Atlantic AMOC cell centered at 50° N, AMO is related with deep water formation in the Labrador and Irminger Seas and with an overturning cell centered at 20° N. In combination with previous studies, these results imply that a main route of increasing atmospheric CO2 concentration influence on AMOC passes through deep water formation in the Nordic Seas and it is reflected in a subpolar North Atlantic meridional cell.

We explore the impact of a latitudinal shift in the westerly wind belt over the Southern Ocean on the Atlantic meridional overturning circulation (AMOC) and on the carbon cycle for Last Glacial Maximum background conditions using a state-of-the-art ocean general circulation model. We find that a southward (northward) shift in the westerly winds leads to an intensification (weakening) of no more than 10% of the AMOC. This response of the ocean physics to shifting winds agrees with other studies starting from preindustrial background climate, but the responsible processes are different. In our setup changes in AMOC seemed to be more pulled by upwelling in the south than pushed by downwelling in the north, opposite to what previous studies with different background climate are suggesting. The net effects of the changes in ocean circulation lead to a rise in atmospheric pCO2 of less than 10 μatm for both northward and southward shift in the winds. For northward shifted winds the zone of upwelling of carbon- and nutrient-rich waters in the Southern Ocean is expanded, leading to more CO2outgassing to the atmosphere but also to an enhanced biological pump in the subpolar region. For southward shifted winds the upwelling region contracts around Antarctica, leading to less nutrient export northward and thus a weakening of the biological pump. These model results do not support the idea that shifts in the westerly wind belt play a dominant role in coupling atmospheric CO2 rise and Antarctic temperature during deglaciation suggested by the ice core data.

Numerical time finite difference schemes in widely used ocean general circulation models are systematically examined to ensure the correct and accurate discretization of the Coriolis terms. Two groups of numerical schemes are categorized. One group is suitable for simulating an inertial wave in the ocean with the necessary condition for stability being F=fAt<l, where f is the Coriolis parameter and At is the integration time step in the model, such as the predictor-corrector Euler scheme, centred difference (leapfrog) scheme, semi-implicit Euler schemes, and leapfrog scheme with a semi-implicit approach. The other group is able to serve as a long-term climate study using a large integration time step which violates F=fAt<l by damping out inertial waves, such as the Cox-Bryan and Oberhuber implicit approaches. Caution should be made in using the Euler forward and other schemes that produce unstable inertial waves; this problem could be serious for a calculation longer than a week. The predictor-corrector scheme is suggested to replace the simple Euler forward scheme. The explicit schemes tend to overestimate the phase frequency, whereas the implicit schemes underestimate it. To better simulate the correct phase frequency (i.e. speed), F<0.1 is suggested.