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\subsection {Information Exchanged with the Coupler}

When CSIM5 is run coupled, it sends and receives forcing information from the other
components via a flux coupler.  Message passing between the ice model and
the coupler is accomplished using MPI.  Fluxes computed within the ice 
model and used by other components are sent to the flux coupler for distribution.
Although CSIM5 contains an ice thickness distribution in each grid cell,
and the ice state variables and fluxes depend on the ice thickness, only
aggregate quantities of each grid cell are passed to the coupler.

The coupler requires that the fluxes it receives from the ice model be 
divided by the total ice area in each grid cell, since the coupler multiplies
these fluxes by the ice area.  This is done in subroutine {\it scale\_fluxes}
just before the call to {\it to\_coupler}.  These fluxes have units of
"per unit ice area".

The forcing information received by the ice model from the coupler at the
top of the timestep is listed in Table \ref{table:flux_to_seaice}. By convention,
directional fluxes are positive downward.  The symbols in the first column 
correspond to those in the equations found in the Scientific Description 
document.  These are the forcing variables required by the ice model for running
coupled or uncoupled.  The information calculated by the ice model and sent to
the coupler at mid-timestep is listed in Table \ref{table:flux_to_coupler}.

\begin{table}
  \begin{center}
  \caption{Fluxes and state variables received by the sea ice model from the coupler}
  \label{table:flux_to_seaice}
  \begin{tabular}{llll} \hline
         Symbol   & Variable Name      & Description                     & Units     \\ \hline \hline
                     &              & Atmospheric Variables              &  \\  \hline
         $z_a$       & {\tt zlvl}   &  Reference height                  &  m    \\
         $u_a$       & {\tt uatm}   &  Zonal wind speed at $z_a$         &  m s$^{-1}$    \\
         $v_a$       & {\tt vatm}   &  Meridional wind speed at $z_a$    &  m s$^{-1}$    \\
         $\theta_a$  & {\tt potT}   &  Potential temperature at $z_a$    &  K             \\
         $T_a$       & {\tt Tair}   &  Air temperature at $z_a$          &  K             \\
         $q_a$       & {\tt Qa}     &  Specific humidity at $z_a$        &  kg kg$^{-1}$  \\
         $\rho_a$    & {\tt rhoa}   &  Air density at $z_a$              &  kg m$^{-3}$   \\   \hline
                        & Atmosphere $\Rightarrow$ ice fluxes            &  \\  \hline
         $F_{SWvdr}$ & {\tt swvdr}  &  Direct, visible downwelling shortwave         &  W m$^{-2}$  \\
         $F_{SWvdf}$ & {\tt swvdf}  &  Diffuse, visible downwelling shortwave        &  W m$^{-2}$  \\
         $F_{SWndr}$ & {\tt swidr}  &  Direct, near infrared downwelling shortwave   &  W m$^{-2}$  \\
         $F_{SWndf}$ & {\tt swidf}  &  Diffuse, near infrared downwelling shortwave  &  W m$^{-2}$  \\
         $F_{LWDN}$  & {\tt flw}    &  Downwelling longwave                          &  W m$^{-2}$  \\
         $F_{RN}$    & {\tt frain}  &  Freshwater flux due to rain                   &  kg m$^{-2}$ s$^{-1}$  \\
         $F_{SNW}$   & {\tt fsnow}  &  Freshwater flux due to snow (liquid)          &  kg m$^{-2}$ s$^{-1}$  \\ \hline
                        & Ocean Variables                                 &  \\  \hline
         $T_o$       & {\tt sst}      &  Sea surface temperature            &  K            \\
         $S_o$       & {\tt sss}      &  Sea surface salinity               &  ppt          \\
         $u_o$       & {\tt uocn}     &  Surface ocean current              &  m s$^{-1}$         \\
         $v_o$       & {\tt vocn}     &  Surface ocean current              &  m s$^{-1}$         \\
         $H_{ox}$    & {\tt ss\_tltx} &  Sea surface slope                  &  m m$^{-1}$         \\
         $H_{oy}$    & {\tt ss\_tlty} &  Sea surface slope                  &  m m$^{-1}$         \\ \hline
                        & Ocean $\Rightarrow$ ice fluxes             &  \\  \hline
         $F_{Qoi}$   & {\tt frzmlt}   &  Freezing/melting potential         &  W m$^{-2}$         \\ \hline
  \end{tabular}
  \end{center}
\end{table}


\begin{table}
  \begin{center}
  \caption{Fluxes and state variables sent from sea ice model to coupler}
  \label{table:flux_to_coupler}
  \begin{tabular}{llll} \hline
         Symbol         & Variable Name     & Description                 & Units        \\ \hline \hline
         $T_{ref}$      & {\tt Tref}   &  Atmospheric reference temperature (2 m) &  K  \\
         $Q_{ref}$      & {\tt Qref}   &  Atmospheric specific humidity (2 m)     &  kg kg$^{-1}$ \\ \hline
                        &           & Ice Variables                             &  \\  \hline
         $A$            & {\tt ailohi} &  Ice concentration                     &      \\
         $T_s$          & {\tt Tsfc}   &  Surface temperature                   &  K    \\
         $\alpha_{vdr}$ & {\tt alvdr}  &  Albedo (visible, direct)              &       \\
         $\alpha_{ndr}$ & {\tt alidr}  &  Albedo (near infrared, direct)        &       \\
         $\alpha_{vdf}$ & {\tt alvdf}  &  Albedo (visible, diffuse)             &       \\
         $\alpha_{ndf}$ & {\tt alidf}  &  Albedo (near infrared, diffuse)       &       \\  \hline
                        & Ice $\Rightarrow$ atmosphere fluxes           &  \\  \hline
         $F_{LH}$       & {\tt flat}   &  Latent heat flux                      &  W m$^{-2}$    \\
         $F_{SH}$       & {\tt fsens}  &  Sensible heat flux                    &  W m$^{-2}$    \\
         $F_{LWUP}$     & {\tt flwout} &  Upwelling longwave                    &  W m$^{-2}$    \\
         $F_{EVAP}$     & {\tt evap}   &  Evaporated water                      &  kg m$^{-2}$ s$^{-1}$    \\
         $\tau_{ax}$    & {\tt tauxa}  &  Atmosphere-ice stress, zonal          &  N m$^{-2}$    \\
         $\tau_{ay}$    & {\tt tauya}  &  Atmosphere-ice stress, meridional     &  N m$^{-2}$    \\  \hline
                        & Ice $\Rightarrow$ ocean fluxes           &  \\  \hline
         $F_{SWo}$      & {\tt fswthru} &  Shortwave transmitted to ocean       &  W m$^{-2}$    \\
         $F_{Qio}$      & {\tt fhnet}   &  Net heat flux to ocean               &  W m$^{-2}$    \\
         $F_{Wo}$       & {\tt fresh}   &  Fresh water flux                     &  kg m$^{-2}$ s$^{-1}$    \\
         $F_{Sa}$       & {\tt fsalt}   &  Salt flux                            &  kg m$^{-2}$ s$^{-1}$    \\
         $\tau_{ox}$    & {\tt tauxo}   &  Ice-ocean stress, zonal              &  N m$^{-2}$    \\
         $\tau_{oy}$    & {\tt tauyo}   &  Ice-ocean stress, meridional         &  N m$^{-2}$    \\  \hline
  \end{tabular}
  \end{center}
\end{table}

