An empirical parameterization for the salinity of subsurface water entrained into the ocean mixed layer (S_e) in the tropical Pacific

An empirical parameterization for S_e is proposed and tested in an intermediate ocean model (IOM) of the Tropical Pacific Ocean. An inverse modeling approach is first adopted to estimate S_e from a sea surface salinity (SSS) anomaly (SSSA) model using observed in-situ SSS measurements, simulated upper ocean currents, and freshwater flux (evaporation minus precipitation, E-P) data. A relationship between S_e and sea level (SL) anomalies is then obtained by utilizing an empirical orthogonal function (EOF) technique. This empirical scheme is able to estimate S_e anomalies reasonably well in the equatorial Pacific Ocean and can be used to parameterize S_e fields in terms of SL anomalies for use in SSSA calculations. An optimized S_e parameterization naturally leads to a balanced depiction of the subsurface effect on SSS variability in association with entrainment and vertical mixing. As a result, SSSA simulations can be potentially improved in the Tropical Pacific.