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FGOALS-g3 has four component models: the Grid-Point Atmospheric Model of LASG–IAP, version 3 (GAMIL3) for the atmosphere (Li et al., 2013), the LASG–IAP Climate System Ocean Model, version 3 (LICOM3) for the ocean (Lin et al., 2016; Yu et al., 2018; Lin et al., 2020), the Los Alamos sea ice model, version 4 (CICE4) for the sea ice, and the CAS Land Surface Model (CAS-LSM) for the land (Xie et al., 2018). All the components are coupled with version 7 of the flux coupler developed at the National Center for Atmospheric Research (Craig et al., 2005).
GAMIL3 uses a finite difference dynamical core, which conserves mass and effective energy under the standard stratification approximation. The horizontal resolution of GAMIL3 is ~2° (180×80) and the number of vertical layers used in GAMIL3 is 26. The land component is CAS-LSM with the same grid as GAMIL3. With regard to the physical processes of GAMIL3 and CAS-LSM, the details can be found in Li et al. (2020).
The ocean component, LICOM3, has also been extensively improved (Liu et al., 2012; Lin et al., 2016, 2020; Yu et al., 2018; Lin et al., 2020). Its dynamic core with a latitude–longitude grid structure is replaced by arbitrary orthogonal curvilinear coordinates (Yu et al., 2018). Therefore, the tripolar grid from Murray (1996) can be applied in LICOM3 with two North Poles on the Eurasian (65°N, 65°E) and North American (65°N, 115°W) continents. The introduction of the tripolar grid can directly improve the effectiveness of the dynamic core by both enlarging the time steps and removing the zonal filter for momentum and tracers. An Arakawa B-grid is used for the horizontal grid with 360×218 grid points. The eta coordinates with 30 or 80 layers are used in the vertical direction, but only 30 layers are used for the DECK and FAFMIP experiments of CMIP6. With regard to the physical processes, the St. Laurent et al. (2002) internal tidal mixing is introduced into LICOM3 (Yu et al., 2017), and the buoyancy frequency-related thickness diffusivity of Ferreira et al. (2005) is applied in the eddy-induced advection of Gent and McWilliams (1990). In addition, the chlorophyll-a-dependent solar penetration of the Ohlmann (2003) scheme and vertical mixing of Canuto et al. (2002) are inherited from LICOM2. CICE4 is the sea ice component of FGOALS-g3, with the same horizontal resolution as the ocean component.
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Five experiments are carried out in FAFMIP: faf-water, faf-stress, faf-heat, faf-all and faf-passiveheat (Table 1). In the first three experiments, the surface momentum, freshwater and heat flux perturbations are applied, while in faf-all, all three perturbations are applied together. All the forcing data are from Gregory et al. (2016), which are monthly flux anomalies of the ensemble mean of the 61st–80th years 1pctCO2 experiments from 13 CMIP5 models. For comparison between the perturbation and preindustrial control (piControl) experiments, all other conditions of the experiments are the same as those of the setup of the piControl run, including the point to branch the experiment, the concentration of CO2 (280 ppm), etc. The experiments are all 70 years long, and the scale of the CO2 concentration is doubling. The control experiment used in this paper is the faf-passiveheat experiment, which is equivalent to the piControl run but with an extra diagnostic tracer. The details of the five FAFMIP experiments are described as follows:
Name Ocean surface flux perturbation Integration/Year faf-stress Zonal and meridional momentum 70 faf-water Freshwater 70 faf-heat Heat 70 faf-all Zonal and meridional momentum, heat and freshwater 70 faf-passiveheat Heat as in faf-heat, but added as a passive tracer 70 Table 1. Descriptions of the FAFMIP experiments.
In the faf-stress experiment, the perturbations of surface downward fluxes of eastward and northward momentum derived from CMIP5 are applied in the surface zonal and meridional momentum flux. The stress perturbations are directly added to the momentum balance of the seawater but not to the ocean subgrid processes and the momentum balance of the sea ice. Figure 1a shows the annual mean surface momentum flux perturbations for FAFMIP. Its dominant feature is the increase in westerly wind stress in the Southern Ocean, which indicates that large changes in faf-stress will occur in the Southern Ocean.
Figure 1. Surface flux perturbations of (a) momentum (10−3 Pa, color indicates the magnitude of the vector, arrow indicates direction), (b) water (10−6 kg m−2 s−1) and (c) heat (W m−2) from FAFMIP.
In the faf-water experiment, a perturbation of freshwater anomalies is applied to the freshwater flux into the sea surface. The anomalies are the sum of all possible sources in the CMIP5 AOGCM, including precipitation, evaporation, river inflow and water fluxes between floating ice (sea ice and icebergs) and seawater. Figure 1b shows the annual mean surface water flux perturbations for FAFMIP. We find that its pattern is dominated by that of precipitation changes, which are positive near the equator and at mid to high latitudes and negative in the subtropics.
In the faf-heat experiment, a perturbation of the surface downward heat flux in seawater is applied to the heat flux into the sea surface. The anomalies are the sum of all possible sources in the CMIP5 AOGCM, including the net downward radiative fluxes, sensible and latent heat fluxes to the atmosphere, and heat fluxes between sea ice and seawater. In previous studies, we found that there is a negative feedback due to the air–sea interaction at the surface, which will reduce the increase in temperature by approximately 50%. To avoid this effect, a passive tracer, which cannot feel heat perturbation, has been introduced to compute the surface heat flux instead of the sea surface temperature (SST), as proposed by Bouttes et al. (2014). The passive tracer is initialized to the ocean temperature at the start of the experiment and subsequently transported by all the same processes as ocean temperature, except for the heat anomalies. Figure 1c shows the annual mean surface heat flux perturbations for FAFMIP. Large positive anomalies occur in the North Atlantic Ocean and the Southern Ocean.
In the faf-all experiment, the surface flux perturbations of momentum, heat and freshwater are all applied simultaneously into the seawater. The method of computing surface flux uses the same method as that in the faf-heat experiment. The purpose of the faf-all experiment is to quantify the nonlinearities of the effects of the three perturbations. If the ocean response to CO2 forcing may be interpreted as the sum of the effects, the effects of the three perturbations are linear.
In faf-passiveheat, the heat flux perturbation is applied instead to a passive tracer to diagnose the effect of added heat on the ocean temperature through processes other than heat transport due to circulation. The tracer here is initialized to zero and does not affect the processes. Therefore, the faf-passiveheat experiment is the same as the standard piControl but with an additional passive tracer for diagnosis. The results of this experiment are used as a reference in the present paper.
Three additional experiments, faf-heat-NA50pct, faf-heat-NA0pct and faf-antwater-stress for FAFMIP, which were further proposed by the FAFMIP meeting in April 2019, have not been conducted so far. Therefore, they are not included and discussed in the present paper. The former two experiments reduce the double-counted surface heat flux in the North Atlantic due to the change in SST. The latter experiment is to investigate the effects of both wind stress and freshwater in the Southern Ocean. Further details of the implementation of each of the experiments can be found at the following website: http://www.fafmip.org.
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The original model outputs are on a tripolar grid with two poles in the Northern Hemisphere on the continents. The horizontal grid numbers are 360 and 218 in the zonal and meridional directions, respectively. The original grid distribution is kept and the format is slightly changed to Climate Model Output Rewriter (CMOR) file structure as required by FAFMIP. The data have 30 vertical levels, and the original vertical level is not changed on the ESGF note. The first level is at a depth of 5 m with a thickness of 10 m. The variables of Priority 1 for FAFMIP are shown in Table 2.
Name Description zos Sea surface height above geoid zostoga Global average thermosteric sea level thetao Sea water potential temperature thetaoga Global average sea water potential temperature so Sea water salinity msftmz Ocean meridional overturning mass streamfunction hfds Downward heat flux at sea water surface wfo Water flux into sea water pathetao Sea water additional potential temperature prthetao Sea water redistributed potential temperature opottempdiff Tendency of sea water potential temperature expressed as heat content due to parameterized dianeutral mixing opottemppadvect Tendency of sea water potential temperature expressed as heat content due to parameterized eddy advection opottemppmdiff Tendency of sea water potential temperature expressed as heat content due to parameterized mesoscale diffusion opottemprmadvect Tendency of sea water potential temperature expressed as heat content due to residual mean advection opottemptend Tendency of sea water potential temperature expressed as heat content osaltdiff Tendency of sea water salinity expressed as salt content due to parameterized dianeutral mixing osaltpadvect Tendency of sea water salinity expressed as salt content due to parameterized eddy advection osaltpmdiff Tendency of sea water salinity expressed as salt content due to parameterized mesoscale diffusion osaltrmadvect Tendency of sea water salinity expressed as salt content due to residual mean advection osalttend Tendency of sea water salinity expressed as salt content rsdoabsorb Net rate of absorption of shortwave energy in ocean layer Table 2. Descriptions of output variables of Priority 1 for FAFMIP.
Acknowledgments. This study was supported by National Key R&D Program for Developing Basic Sciences (2018YFA0605703), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB42010404) and the National Natural Science Foundation of China (Grants 41976026, 41776030 and 41931183, 41931182). The authors acknowledge the technical support from the National Key Scientific and Technological Infrastructure project "Earth System Science Numerical Simulator Facility" (EarthLab).
Name | Ocean surface flux perturbation | Integration/Year |
faf-stress | Zonal and meridional momentum | 70 |
faf-water | Freshwater | 70 |
faf-heat | Heat | 70 |
faf-all | Zonal and meridional momentum, heat and freshwater | 70 |
faf-passiveheat | Heat as in faf-heat, but added as a passive tracer | 70 |