Study of the ocean response to climate changes in XX

2. Description of the Coupled Model and Numerical Experiments

[7] The coupled atmosphere-ocean general circulation model developed at the INM RAS was used to simulate climate changes. The previous model is described in detail in [Diansky and Volodin, 2002] and its modifications are described in [Dymnikov et al., 2005; Volodin and Diansky, 2004]. Here, we will give its short description.

[8] A complete description of the atmospheric general circulation model is given by V. A. Alekseev et al. (preprint, 1998). The spatial resolution for the atmospheric block is 5^o in longitude, 4^o in latitude, and 21 levels in the vertical in the s -coordinates. The equations of hydro-thermodynamics are solved on a "C'' grid according to the Arakawa classification with the use of the finite-difference method described in [Galin et al, 2003]. The model involves the parameterizations of radiation processes [Galin, 1998], convection, and condensation [Betts, 1986]; the processes occurring in soil and on the dry-land surface [Volodin and Lykosov, 1998]; and gravity-wave resistance [Hines, 1997; Palmer et al., 1986].

[9] A detailed description of the ocean general circulation model and its features is given in [Diansky et al., 2002; Sarkisyan et al., 2005]. The model is based on the ocean primitive equations in the Boussinesq, hydrostatic, and rigid-lid approximations written in a spherical isobathic s -coordinate system. Its resolution is 2.5^o times 2^o in longitude and latitude, respectively, and 33 non-uniform levels in depth. The discretization of dynamics equations was performed on a C grid with low dissipation. The use of the C grid at the resolution 2.5^o times 2^o allows one to approximate the calculation region for straits. For example, it is possible to describe the salt exchange between the Atlantic and the Mediterranean Sea through the Strait of Gibraltar, a procedure that is very important for the description of a thermohaline circulation in the North Atlantic, and the exchanges through the Bering Strait as well. In implementing the model numerically, the method of splitting up [Marchuk, 1975] in physical processes and spatial coordinates is used. This method allows one to efficiently realize implicit methods of time integration. The vertical viscosity and diffusion are parameterized according to [Pacanovsky and Philander, 1981]. The model also involves a block of sea-ice calculation [Yakovlev, 2003], in which the thermodynamics of ice is taken into account but its motion is disregarded.

[10] Data exchange between the atmosphere and ocean models occurs every 6 h of model time. This interval is also used as a numerical step of the ocean model. Within this interval, the calculated (with the atmosphere model with a step of 1 h) fluxes of sensible and latent heat, freshwater, momentum, and total (incoming and outgoing) long- and shortwave radiation at the ocean surface are accumulated and then averaged. The fields are converted from the atmospheric spatial grid to the oceanic grid with the aid of linear interpolation. Since the spatial oceanic grid is denser than the atmospheric grid, the spatial weight averaging procedure was used to convert the ocean surface temperature (OST) to the atmospheric grid. The latter implies that the OST at all oceanic grid points falling within a mesh of the atmospheric grid is taken with a weight proportional to the area of intersection of the corresponding oceanic mesh with the mesh of the atmospheric grid. No correction of the heat and momentum fluxes is used in adjusting the atmosphere and ocean models. The correction of the freshwater flux is used only for the Greenland, Norwegian, Barents, and Kara seas. Such a correction makes it possible to reproduce a thermohaline circulation in the northern Atlantic more adequately and does not directly affect the hydrologic-cycle elements, such as precipitation, river runoff, etc. In calculating fluxes at the ocean surface in the atmosphere model, the OST is taken from the uppermost calculation level in the ocean model.

[11] The coupled atmosphere-ocean general circulation model is implemented in a 16-processor cluster (Intel Itanium) at the INM RAS. Calculations with eight processors for 10 model years take 24 h.

[12] The reproduction of the climate of the late 20th century with this coupled model is described in [Dymnikov et al., 2005].

[13] In the experiments, in addition to the coupled atmosphere-ocean general circulation model, an atmosphere model coupled with a simple balance model of the heat content of a homogeneous 50-m ocean layer is used. Additionally, this model also involves the calculation of sea ice and uses the correction of heat fluxes at the ocean surface. Such a simplified coupled model allows prompt obtainment of an equilibrium response to a prescribed external forcing, for example, to an increase in the CO₂ concentration.

[14] The following experiments with the coupled model were carried out in the CMIP framework.

Figure 1

[15] 1. Experiment on simulation of the climate of the 20th century (experiment 20). The initial state for this experiment was obtained in the following way. First, in the ocean model, temperature and salinity were specified according to the observational data given in [Steele et al., 2001], flow velocities were zero, the amount of sea ice was equal to zero, and the atmospheric condition was taken from the integration of the atmosphere model with a specified OST distribution. From this initial state, the model was integrated for 160 years; in this case, the concentrations of all atmospheric components corresponded to those of the end of the 20th century. Then, the calculations were performed for 80 years, and, in this case, the concentrations of all atmospheric constituents corresponded to those of the year 1871. The state of the model at the end of the calculations was starting for experiment 20. In the course of this experiment, real time variations in the atmospheric concentrations of carbon dioxide (CO₂ ), methane (CH₄ ), and nitrogen monoxide (N₂ O) - major greenhouse gases associated with man's activities - were specified. These gases were assumed to be well mixed, and their concentrations did not depend on the spatial coordinates. Real time variations in the sulfate-aerosol concentration depending on latitude and longitude, the volcanic-aerosol optical depth depending on latitude, and the solar constant were also specified. In the model, only the direct effect of sulfate aerosol was taken into account. In the INM model, the direct radiation effect of variations in the concentration of sulfate aerosol during the 20th century was approximately 1.5 times larger than its mean value given in [Solomon et al., 2007] and was equal to -0.75 W m^-2. Figure 1 gives the temporal behaviors of all external forcings used. The data were taken from the Internet at http://wwwpcmdi.llnl.gov/ipcc.climate_forcing.php. The duration of experiment 20 was 130 years; i.e., the calculation was performed to the end of the year 2000.

[16] 2. Experiments on simulation of the climate of the 21st-22nd centuries. In these experiments, the initial data were taken from the model state obtained at the end of experiment 20, which corresponded to the beginning of the year 2001. The atmospheric concentrations of carbon dioxide, methane, nitrogen monoxide, and sulfate aerosol in the 21st century corresponded to those from IPCC scenarios A1B, A2, and B1 (Figure 1). Therefore, these experiments will be denoted according to the scenario abbreviations. The solar constant and the concentrations of volcanic aerosols were set constant and equal to their values observed in 2000. During the 22nd century, the concentrations of all gas constituents corresponded to their values for the year 2100. The duration of each of these experiments was 200 years.

[17] 3. Experiment 21 on the simulation of the climate of the 21st century under the assumption that the concentrations of all gases corresponded to their values observed in 2000. The model state for the end of experiment 20 was used as the initial state. The duration of the experiment was 100 years.

[18] 4. Control experiment, in which the concentrations of all gases and the solar constant corresponded to their values for the year 1871. The same initial state was used as that for experiment 20. The duration of the experiment was 330 years.

[19] 5. Experiment 2CO₂, in which the CO₂ concentration was increased at a rate of 1% per year up to its doubling (during 70 years), plus 150 calculation years with doubled CO₂. The concentrations of the rest of the gases were set constant. The same initial state was used as for experiment 20. The duration was 220 years.

[20] 6. Experiment 4CO₂, in which the CO₂ concentration was increased at a rate of 1% per year up to its quadrupling (during 140 years), plus 150 years of calculation with quadruple CO₂. The same initial state was used as for experiment 20. The duration was 290 years.

[21] 7. Experiment SLC on simulation of the climate of the late 20th century via a model with a 50-m ocean. The duration of the experiment was 60 years, excluding 20 years during which an equilibrium climate was reached.

[22] 8. Experiment different from the SLC experiment by doubled CO ₂. The duration of the experiment was 60 years, excluding 20 years during which an equilibrium climate was reached.