Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) control the abyssal limb of the global overturning circulation and play a major role in oceanic heat uptake and carbon storage. However, current general circulation models are unable to resolve the observed AABW and NADW formation and transport processes. One key process, that of overflows, motivates the application of overflow parameterisations. We present a sensitivity study of both AABW and NADW properties to three current parameterisations using a z*-coordinate ocean-sea ice model within a realistic-topography sector of the Atlantic Ocean.Overflow parameterisations that affect only tracer equations are compared to a fully dynamical Lagrangian point particle method. An overflow parameterisation involving partial convective mixing of tracers is most efficient at transporting dense NADW water downslope. This parameterisation leads to a maximum mean increase in density in the north of 0.027kgm-3 and a decrease in age of 525years (53%). The relative change in density and age in the south is less than 30% of that in the north for all overflow parameterisations. The reduced response in the south may result from the differing dense water formation and overflow characteristics of AABW compared to NADW. Alternative approaches may be necessary to improve AABW representation in z*-coordinate ocean climate models. ; BMS was supported by the Australian Government Department of the Environment, and CSIRO through the Australian Climate Change Science Programme. AMH was supported by an Australian Research Council Future Fellowship FT120100842. SMH was supported by the ARC Centre of Excellence for Climate System Science (Grant CE110001028)
Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) control the abyssal limb of the global overturning circulation and play a major role in oceanic heat uptake and carbon storage. However, current general circulation models are unable to resolve the observed AABW and NADW formation and transport processes. One key process, that of overflows, motivates the application of overflow parameterisations. We present a sensitivity study of both AABW and NADW properties to three current parameterisations using a z*-coordinate ocean-sea ice model within a realistic-topography sector of the Atlantic Ocean.Overflow parameterisations that affect only tracer equations are compared to a fully dynamical Lagrangian point particle method. An overflow parameterisation involving partial convective mixing of tracers is most efficient at transporting dense NADW water downslope. This parameterisation leads to a maximum mean increase in density in the north of 0.027kgm-3 and a decrease in age of 525years (53%). The relative change in density and age in the south is less than 30% of that in the north for all overflow parameterisations. The reduced response in the south may result from the differing dense water formation and overflow characteristics of AABW compared to NADW. Alternative approaches may be necessary to improve AABW representation in z*-coordinate ocean climate models. ; BMS was supported by the Australian Government Department of the Environment, and CSIRO through the Australian Climate Change Science Programme. AMH was supported by an Australian Research Council Future Fellowship FT120100842. SMH was supported by the ARC Centre of Excellence for Climate System Science (Grant CE110001028)
We present a new framework for global ocean–sea-ice model simulations based on phase 2 of the Ocean Model Intercomparison Project (OMIP-2), making use of the surface dataset based on the Japanese 55-year atmospheric reanalysis for driving ocean–sea-ice models (JRA55-do). We motivate the use of OMIP-2 over the framework for the first phase of OMIP (OMIP-1), previously referred to as the Coordinated Ocean–ice Reference Experiments (COREs), via the evaluation of OMIP-1 and OMIP-2 simulations from 11 state-of-the-science global ocean–sea-ice models. In the present evaluation, multi-model ensemble means and spreads are calculated separately for the OMIP-1 and OMIP-2 simulations and overall performance is assessed considering metrics commonly used by ocean modelers. Both OMIP-1 and OMIP-2 multi-model ensemble ranges capture observations in more than 80 % of the time and region for most metrics, with the multi-model ensemble spread greatly exceeding the difference between the means of the two datasets. Many features, including some climatologically relevant ocean circulation indices, are very similar between OMIP-1 and OMIP-2 simulations, and yet we could also identify key qualitative improvements in transitioning from OMIP-1 to OMIP-2. For example, the sea surface temperatures of the OMIP-2 simulations reproduce the observed global warming during the 1980s and 1990s, as well as the warming slowdown in the 2000s and the more recent accelerated warming, which were absent in OMIP-1, noting that the last feature is part of the design of OMIP-2 because OMIP-1 forcing stopped in 2009. A negative bias in the sea-ice concentration in summer of both hemispheres in OMIP-1 is significantly reduced in OMIP-2. The overall reproducibility of both seasonal and interannual variations in sea surface temperature and sea surface height (dynamic sea level) is improved in OMIP-2. These improvements represent a new capability of the OMIP-2 framework for evaluating process-level responses using simulation results. Regarding the sensitivity of individual models to the change in forcing, the models show well-ordered responses for the metrics that are directly forced, while they show less organized responses for those that require complex model adjustments. Many of the remaining common model biases may be attributed either to errors in representing important processes in ocean–sea-ice models, some of which are expected to be reduced by using finer horizontal and/or vertical resolutions, or to shared biases and limitations in the atmospheric forcing. In particular, further efforts are warranted to resolve remaining issues in OMIP-2 such as the warm bias in the upper layer, the mismatch between the observed and simulated variability of heat content and thermosteric sea level before 1990s, and the erroneous representation of deep and bottom water formations and circulations. We suggest that such problems can be resolved through collaboration between those developing models (including parameterizations) and forcing datasets. Overall, the present assessment justifies our recommendation that future model development and analysis studies use the OMIP-2 framework. ; This research has been supported by the Integrated Research Program for Advancing Climate Models (TOUGOU) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (grant nos. JPMXD0717935457 and JPMXD0717935561), the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) (grant no. 274762653), the Helmholtz Climate Initiative REKLIM (Regional Climate Change) and European Union's Horizon 2020 Research & Innovation program (grant nos. 727862 and 800154), the Research Council of Norway (EVA (grant no. 229771) and INES (grant no. 270061)), the US National Science Foundation (NSF) (grant no. 1852977), the National Natural Science Foundation of China (grant nos. 41931183 and 41976026), NOAA's Science Collaboration Program and administered by UCAR's Cooperative Programs for the Advancement of Earth System Science (CPAESS) (grant nos. NA16NWS4620043 and NA18NWS4620043B), and NOAA (grant no. NA18OAR4320123). ; Peer Reviewed ; Postprint (published version)
