ALTIMETER-DERIVED VARIABILITY OF SURFACE VELOCITIES IN THE CALIFORNIACURRENT SYSTEM .1. EVALUATION OF TOPEX ALTIMETER VELOCITY RESOLUTION

In this paper, we evaluate the temporal and horizontal resolution of g eostrophic surface velocities calculated from TOPEX satellite altimete r heights. Moored velocities (from vector-averaging current meters and an acoustic Doppler current profiler) at depths below the Ekman layer are used to estimate the temporal evolution and accuracy of altimeter geostrophic surface velocities at a point. Surface temperature gradie nts from satellite fields are used to determine the altimeter's horizo ntal resolution of features in the velocity field. The results indicat e that the altimeter resolves horizontal scales of 50-80 km in the alo ng-track direction. The rms differences between the altimeter and curr ent meters are 7-8 cm s(-1), much of which comes from small-scale vari ability in the oceanic currents. We estimate the error in the altimete r velocities to have an rms magnitude of 3-5 cm s(-1) or less. Uncerta inties in the eddy momentum fluxes at crossovers are more difficult to evaluate and may be affected by aliasing of fluctuations with frequen cies higher than the altimeter's Nyquist frequency of 0.05 cycles d(-1 ), as indicated by spectra from subsampled current meter data. The edd y statistics that are in best agreement are the velocity variances, ed dy kinetic energy and the major axis of the:variance ellipses. Spatial averaging of the current meter velocities produces greater agreement with all altimeter statistics and increases our confidence that the al timeter's momentum fluxes and the orientation of its variance ellipses (the statistics differing the most with single moorings) represent we ll the statistics of spatially averaged currents (scales of 50-100 km) in the ocean. Besides evaluating altimeter performance, the study rev eals several properties of the circulation in the California Current S ystem: (1) velocity components are not isotropic but are polarized, st rongly so at some locations, (2) there are instances of strong and per sistent small-scale variability in the velocity, and (3) the energetic region of the California Current is isolated and surrounded by a regi on of lower energy starting 500-700 km offshore. This suggests that th e source of the high eddy energy within 500 km of the coast is the sea sonal jet that develops each spring and moves offshore to the central region of the California Current, rather than a deep-ocean eddy field approaching the coast from farther offshore.