CLIVAR Working Group on Ocean Model Development

Co-ordinated Ocean-Ice Reference Experiments

To investigate the climatological ocean and sea-ice states realised through multi-centennial simulations forced by idealised repeating normal year forcing that has been derived from 43 years of interannually varying forcing, retaining synoptic variability with a seamless transition from 31 December to 1 January (Large and Yeager, 2004).

After being initially proposed in 2004, CORE-I has now reached a critical mass with a community-wide proof of concept approval and seven ocean-ice models (see Table 1) that have been run for 500 years with the repeat 'normal year forcing' of Large and Yeager (2004). This is a significant step forward from 2005 when only three geopotential modelling groups were involved. The experiment has yielded a wide variety of results, with more questions raised than answered. Broad comparison projects such as this achieve much in this way by raising questions, which then motivate further research. Without such a comparison, questions would remain unasked, and thus unanswered. A peer-review paper with 24 authors (Griffies et al., 2008) has been submitted to Ocean Modelling.

Table 1: Models that have taken part in CORE-I:

Results from 500yr CORE-I integrations

Many models performed similarly in tropics (though with notable outliers). Otherwise, there are major differences, which mainly point to differences in model configurations and algorithms. Figures 1 and 2 illustrate some important results from the CORE-I integrations on the simulation of the Atlantic thermohaline circulation. A stable meridional overturning circulation with a realistic transport strength and structure is important for maintaining a realistic ocean climate. Figure 1 shows the different behavior exhibited by different models, with some stabilizing relatively quickly (eg KNMI-MICOM, though to an extremely weak, almost absent overturning circulation) while others taking over 500 years to stabilize (GFDL-MOM, which was confirmed to have stabilized when the integration was extended to 600 years); and the GFDL-HIM simulation possess multi-centennial variability which is too long to assess with a 500 year run.

Figure 2 shows the global meridional overturning streamfunction averaged over model years 491-500. The FSU-HYCOM streamfunction has been interpolated to depth based coordinates, leading to noisy results. The GFDL-HIM and KNMI-MICOM streamfunction is kept on its native potential density coordinates and the figures are split into upper and lower plots to distinguish the Ekman-driven cells in the upper layer and the overturning cells in the deep ocean. The NADW cell is almost absent in the KNMI-MICOM simulation. All the z-level models have a realistic AABW cells of 5-10Sv, while the isopycnal models show much stronger overturning in the South.

Figure 1: Time series of the annual mean Atlantic meridional overturning streamfunction (Sv = 10⁶m³s^-1). The index is computed as the maximum Atlantic MOC streamfunction at 45^oN in the region beneath the Ekman layer (Griffies et al, 2008).

Figure 2: The global meridional overturning streamfunction (Sv = 10⁶m³s^-1), time averaged over the model years 491-500. The GFDL-HIM and NMI-MICOM results are plotted on the original potential density coordinates referenced to 2000db (1035-rho₂₀₀₀) (Griffies et al, 2008).

Difficulties in establishing a stable climate simulation with reasonable biases using the MICOM code, KNMI investigators decided to transfer their efforts to the French geopotential model OPA, also represented by Kiel in the CORE-I study.  This code conversion represents a major decision for the KNMI team, and is a nontrivial outcome of the CORE-I project.

Some groups were not able to maintain a quasi-stable MOC for the CORE-I multi-centennial simulations without applying a non-trivial salinity restoring, also necessary to damp drifts in deep water mass properties. The participating groups were given the freedom to choose their own salinity restoring depending on each model's sensitivity. For example, CCSM maintained a globally weak salinity forcing with a piston velocity of 50m/4yrs; GFDL-MOM strengthened the restoring to 50m/300days; and KNMI and ORCA using stronger restoring in certain high latitude regions. Details are given in Griffies t al. (2008).

Some participating groups ran extra simulations to examine the sensitivity to the choice of salinity restoring. Figure 3 shows the annual mean volume transports across 45^oN in the North Atlantic and across the Drake Passage for two MPI and GFDL-MOM simulation, one with weaker restoring (50m/4yr) and one with stronger restoring (50m/300days). The GFDL-MOM-A weak restoring simulation (left, black line) develops a series of growing amplitude multi-decadal oscillations after 250 year, demonstrating the need for multi-centennial simulations to evaluate the stability of the overturning circulation. This sensitivity led to the strong restoring experiment (GFDL-MOM-B) being chosen as the standard experiment that contributed to the CORE-I evaluation. The MPI model is less sensitive to the choice of salinity restoring. Even though the weaker restoring experiment (MPI-A) exhibited greater amplitude variability in the Drake Passage, this was the preferred experiment for the rest of the CORE-I evaluation.

Figure 3: Timeseries of the annual mean volume transports (Sv=10⁶ m³ s^-1) from two MPI (left) and two GFDL-MOM (right) simulations. The black lines are from ocean-ice (MPI-A and GFDL-MOM-A) simulations with a weak salinity restoring of 50m/4yr piston velocity and the blue lines are for simulations with a stronger salinity restoring of 50m/300days piston velocity (MPI-B and GFDL-MOM-B). The top panels show the maximum meridional overturning streamfunction at 45^oN and the bottom panels show the eastward transport through the Drake Passage (Griffies et al, 2008).

Results from other additional Kiel-ORCA simulations indicate that the MOC solution is dependent on the drift in overflow density (not convection in the Labrador Sea). The drift in transport arises after about 200 years of integration and is related to a progressive freshening of intermediate waters in the Nordic Seas. This dependence will therefore be closely related to the choice of precipitation and river run-off, as well the choice of SSS restoring.

Hypotheses for model sensitivity to salinity forcing:

The Large and Yeager (2004) dataset has overly strong precipitation in the Arctic and high latitude Atlantic.
Annual mean river input over full year should be switched to more realistic seasonal cycle.
Stable models (CCSM and MPI, with displaced pole placing fine resolution near Lab Sea) may resolve Atlantic currents better, reducing the ability of fresh Arctic water from halting the THC by advecting fresh water faster through convection regions.
Weak SSS restoring places some models to one side of the mixed boundary condition instability, and other models to the other side.
Full answer may be a combination of the above.

Initial results on the sensitivity of the MOC solution to model resolution indicate that models with a finer horizontal resolution (in the North Atlantic Ocean) are less sensitive to SSS restoring and can have a stable MOC with weak restoring. R. Greatbatch points out that the models are existing close to instability related to mixed boundary conditions. CORE-I experiments, perhaps initially of 200 years, could be run at different resolutions to test the robustness of the solutions.

In conclusion, the CORE-I results have highlighted the limitations of forcing ocean-ice models with a non-responsive atmosphere. This experimental setup led to an exaggeration of the effect of positive feedback mechanisms on the MOC prompting the use of stronger SSS restoring by most groups.

References

Griffies, S. M. et al., 2008: Coordinated Ocean-ice Reference Experiments (COREs). Ocean Modelling, under review.

Griffies, S.M., Böning, C., and Treguier, A. M., 2007: Design Considerations for Coordinated Ocean-Ice Reference Experiments, Flux News, 3, 3-5.

Large, W.G. and S.G. Yeager, 2008: The Global Climatology of an Interannually Varying Air Sea Flux Data Set, Climate Dynamics, submitted (draft available here)

Large, W. and S. Yeager, 2004: Diurnal to decadal global forcing for ocean and sea-ice models: the datasets and flux climatologies. NCAR Technical Note: NCAR/TN-460+STR, CGD Division of the National Centre for Atmospheric Research.

last updated Thu, Jul 10, 2008 by Anna Pirani