Evaluating Ocean Tide Models Using Surface Drifters

Accurate models of the ocean tides rely on satellite altimeter data to optimally estimate
tidal water level, in combination with constraints provided by hydrodynamic models.
The quality of ocean tide models, e.g., their forecast skill, can be evaluated by comparison
with independent altimeter, tide gauge, and bottom pressure data; however, the quantity and spatial
distribution of these data are relatively small relative to the task of distinguishing the
differences between tide models. Evaluating the quality of tide models near the sparsely-sampled
coastline and on shallow shelves remains a challenge.

This work explores the usefulness of surface current data from the NOAA Global Drifter Program for
evaluating and comparing ocean tide models. Comparisons are restricted to those tide models which
permit predictions of both currents and elevations, namely, the FES2014 and TPXO9 barotropic
tide models, and the HRET baroclinic tide model. Comparisons of surface currents exhibit geographic
differences in model performance which deviate in some cases from the comparisons based on
water level (via altimetry). Comparisons of predictions for different tidal frequencies (M2, S2, etc.)
follow the relative signal-to-noise of the tidal estimates -- larger tides are known with greater accuracy
than smaller tides -- and this is reflected in the variance reduction statistics of surface currents.
Because the predicted currents depend on the bathymetry and mixing parmeterizations used in the tide
models, the comparisons with drifter data indicate particular regions where improvement of these aspects
of the models may be beneficial.