Baroclinic Tidal Sea Level from Exact-Repeat Mission Altimetry

Abstract

A near-global model for the sea surface expression of the baroclinic tide has been developed using exact-repeat mission altimetry. The methodology used differs in detail from other altimetry-based estimates of the open ocean baroclinic tide, but it leads to estimates that are broadly similar to previous results. It may be used for prediction of the baroclinic sea level anomaly at the frequencies of the main diurnal and semidiurnal tides , , , and , as well as the annual modulates of , denoted and . The tidal predictions are validated by computing variance reduction statistics using independent sea surface height data from the CryoSat-2 altimeter mission. Typical midocean baroclinic tidal signals range from a few millimeters to centimeters of elevation, corresponding to subsurface isopycnal displacements of tens of meters; however, in a few regions, larger signals are present, and it is found that the present model can explain more than 13-cm2 variance at some sites. The predicted tides are also validated by comparison with a database of hourly currents inferred from drogued surface drifters. The database is large enough to permit assessment of a simple model for scattering of the low-mode tide. Results indicate a scattering time scale of approximately 1 day, consistent with a priori estimates of time-variable refraction by the mesoscale circulation.