Evolution equation of zeta

[aryapaul]

I am planning to assimilate ( using Ensemble Kalman Filter technique ) sea level anomaly using satellite observations. I am new to ROMS and have gone through the documentation ( not in a detailed manner though ). I would like to ask how zeta is computed in ROMS. Is there a dynamical equation for zeta like that for the velocities and tracers ? Is there any other group who has assimilated or has been trying to assimilate ROMS output using EnKF ?