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304 R. Sorgente et al.: Seasonal variability in the Central Mediterranean Sea circulation

                                       (a)A)                        model. The open boundary data are applied to the high reso-
                                                                    lution model by linearly interpolating 10-day averaged con-
      39                                                            secutive fields.

      38                                                            3.5 Surface and bottom boundary conditions

      37North Latitude                                              At the free surface, the climatological atmospheric forcing
                                                                    fields are the same as those used for the coarse model. This
      36                                                            atmospheric forcing data set consists of heat and water flux
                                                                    fields, and wind stress components, on a monthly basis, de-
      35                                                            rived from the European Center for Medium Weather range
                                                                    Forecast 6-hourly Re-Analysis (ERA) data set covering the
      34                                                            period 1979–1993 for the whole Mediterranean Sea (Korres
                                                                    and Lascaratos, 2003). These fields are interpolated into the
      33                                                            model grid using a bi-linear interpolation scheme.

      32 0.1 N/m2                                                      The surface boundary conditions include the momentum,
         9 10 11 12 13 14 15 16 17                                  heat and net volume fluxes. The momentum surface bound-
                                          East Longitude            ary condition adopted is:

                                        (b)B)                            ∂u τ                                            (15)
                                                                    KM ∂z |z=η = ρ0
                                                                    where τ is the wind stress monthly mean climatology and ρ0
      38                                                            is the air density. Figure 3 shows the wind stress fields ob-
                                                                    tained from the ERA data set in February and August. The
      37                                                            structure of the wind stress over the basin is mainly westerly
                                                                    during winter, with a weakening in summer. The annual cy-
      36                                                            cle of the mean east-west and north-south ERA wind stress
                                                                    amplitudes (Fig. 4), used for the perpetual year simulation,
      35                                                            does not contain stochastic components such as from vari-
                                                                    abilities associated with low pressure systems. This is a con-
      34                                                            sequence of the use of a climatological forcing obtained by
                                                                    using a time series with a length of 14 years.
                                                                       The adopted heat flux boundary condition is:
      32 0.1 N/m2
         9 10 11 12 13 14 15 16 17                                      ∂T         Qsol − Qup      + C1     Tz=0 − Tz=η  (16)
                                          East Longitude            KH ∂z |z=η =      ρ0Cp            ρ0Cp               (17)
North Latitude
Fig. 3. The monthly averaged wind stress fields in (a) February and  ,C1  =  ∂Q  =  5W m−2  0 C −1
(b) August derived from the ECMWF Re-Analysis (ERA) data set                ∂T
covering the period from Jan. 1979 – Dec. 1993. One vector every
four grid points is plotted. Units are N/m2.                        where Qsol is the solar radiation monthly mean from ERA,
                                                                    Qup is the upward heat flux computed from the coarse model
                                                                    perpetual year simulation and Cp is the specific heat capac-
                                                                    ity at constant pressure. The annual cycle of these compo-

                                                                    nents is shown in Fig. 5. The second term in Eq. (16) is

                                                                    the flux correction term, where Tz=0 is the monthly averaged
                                                                    climatological sea surface temperature from the Med6 data

                                                                    set and Tz=η is the model first level temperature. Sensitiv-
                                                                    ity studies performed on the model indicate an optimal value
                                                                    for C1 equal to 5 W m−2 0C−1. In this way the heat flux is
                                                                    forced to produce a sea surface temperature consistent with

                                                                    the Med6 monthly climatology. The Med6 monthly mean sea

                                                                    surface temperature and salinity are based on the MEDAT-

                                                                    LAS data set using the MODB analysis technique (Brasseur

                                                                    et al., 1996).

                                                                       Similarly for the salinity flux boundary condition, a cor-

                                                                    rection term is used to ensure that the fresh water flux pro-

                                                                    duces a sea surface salinity field consistent with the ERA
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