Comparing Multiyear Altimetry, Drifter, and Satellite Image Derived Surface Currents in the California Current
Dax Matthews(1) and William Emery(1)
University of Colorado at Boulder - CCAR,
80309 - Boulder,
A 4-year time series, between 1994 and 1997, of sea-surface velocities for the California Current calculated from TOPEX/Poseidon and ERS-1/2 altimetry data, Lagrangian surface drifter data, and data extracted from thermal satellite imagery using the maximum cross-correlation (MCC) technique are compared. The relationship of the MCC velocities to other velocity estimates in the region gives a sense of the type of current MCC is measuring, problems with the MCC method, and insight into the current forcing mechanics in the region. MCC has an advantage over the other data sets of being temporal and spatially denser than altimetry and more widespread than drifting buoy deployments. 30-day spatial fields are created by compositing and optimally interpolating the individual data sets. Strong agreement was found between drifters and MCC in areas of high velocity currents away from the continental shelf. On or near the shelf thermal patterns can be distorted significantly in time, decreasing the accuracy of the MCC method. Velocity magnitudes measured by drifters and MCC were found to have no scale bias, in contrast to previous studies. Altimetry produced high correlations with MCC and drifters near satellite ground tracks, however consistently measured smaller velocities. The closer agreement between MCC and drifters, compared with MCC and altimetry, suggest a role of ageostrophic surface currents in this region, which is likely due in part to Ekman forcing.