assimilate TSLA into the restart fields of my ROMS model

[Fu_Zhao]

I'm trying to use EnKF to assimilate TSLA(along-track Sea Level Anomaly) data into the restart fields of my ROMS model. I have defined the PERFECT_RESTART option while compling ROMS. And my assimilation is offline. My CPP options associated with momentum equations is UV_U3HADVECTION, UV_C4VADVECTION. Because the variable zeta in the restart file has been saved three time steps information with this form zeta(ocean_time, three, eta_rho, xi_rho) in the restart file. My question is which time step of zeta in the restart fields should be assimilated? In other word, once I get the increment fields of zeta from the EnKF, which time step of zeta shoud be added the increments? Thanks very much.