Estimates of sea surface height and near-surface alongshore coastal currents from combinations of altimeters and tide gauges

Present methods used to retrieve altimeter data do not provide reliable estimates of sea surface height (SSH) in the nearshore region, resulting in a measurement gap of 25–50 km next to the coast. In the present work, gridded SSH fields produced by Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) in the offshore region are combined with coastal tide gauge time series of SSH to improve estimation in that gap along the west coast of the United States in the northern California Current System between 40° and 45°N and 123.8° and 126°W. To assess the increase in skill provided by this procedure, the geostrophic alongshore currents, calculated from the new SSH fields in the gap region, are compared to three in situ, nearshore current measurements, resulting in correlation coefficients of 0.73–0.83 and standard deviations of the differences of 11.6–12.6 cm/s, substantially improved from the AVISO-only results. When the Ekman current components are estimated and added to the geostrophic currents, comparisons to the 10 m deep acoustic Doppler current profiler velocities are only slightly improved. The Ekman components make a more significant contribution when compared to HF radar surface current measurements, providing correlations of 0.94 and standard deviations of the differences of 6.4–9.5 cm/s. These results represent a dramatic improvement in the quality of the SSH fields and estimated alongshore currents when additional, realistic SSH data from the coastal region are added. Here we use coastal tide gauges to provide the additional SSH data but also discuss more general approaches for altimeter SSH retrievals in coastal regions where tide gauge data are not available.