I use data from a global model without tides for my u/v/T/S boundary, and (as noted above) use ADD_FSOBC, so the SSH of the global model is used too.
I want to make a sensitivity study quantifying the effect of tidal forcing on the circulation with ROMS, and I don't use UV_TIDES. I have Chapman-Flather boundary conditions for ssh and u/vbar respectively, and have ADD_FSOBC which adds tidal elevations to zeta boundary prescriptions. The combination (SSH_TIDES && !defined UV_TIDES) causes the exterior barotropic velocity u_{ext} in Flather for u/vbar to be computed via reduced physics.
So the barotropic velocity of the global model is not used in those experiments with tides. So then, how do I define the exterior barotropic velocity u_{ext} for Flather for u/vbar in those runs in which I don't use tidal forcing?
One could use the barotropic velocity of the global model, but it would be inconsistent with the tidal configuration. Would it be logical to use the baroclinic contribution to zeta (read e.g. from a global model) to drive u_{ext} via reduced physics? This would be consistent with the experiments where tidal forcing is included, and is done e.g. if (SSH_TIDES && !defined UV_TIDES) are set, but tidal amplitudes are artifically set to zero (yes?).
But what is the right combination of cpp switches to do this when switching off SSH_TIDES? If I don't set (SSH_TIDES && !defined UV_TIDES), then u_{ext} is automatically read from BOUNDARY(ng)%ubar
What are the problems if u_{ext} is computed from baroclinic contributions to zeta (from a global model without tides or whatever)? I guess it sounds inconsistent to compute a barotropic velocity from baroclinic signals, but if one doesn't have UV_TIDES velocities for the experiments *with* tidal forcing, it does not make sense to use the non-tidal barotropic velocity of the global model for the experiments *without* tidal forcing.