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Satellite observations have revolutionized oceanography, capturing diverse phenomena over much of the globe. However, it remains to understand how surface fields, like sea surface height, reflect the motion occurring at depth. The vertical structure of ocean eddies is often expressed in terms of "baroclinic modes," which are basis functions derived assuming a flat ocean bottom. Bathymetry alters the modes though, weakening the bottom velocities. Using analytical solutions, we demonstrate that with realistic bathymetry and/or bottom friction, the bottom velocities are nearly zero. The resulting " surface modes" should be ubiquitous in the ocean. This would explain the dominant mode of variability obtained from globally distributed current meter data and is consistent with energy spectra derived from sea surface height data. The results yield a simple way to infer subsurface velocities from satellite data and suggest that ocean analyses should be made in terms of surface modes and topographic waves.

Plain Language Summary Observations of the sea surface from satellites have revolutionized oceanography. But it is important to understand how the surface fields, such as sea height and surface temperature, reflect the motion occurring at depth. Traditionally, oceanographers have represented the vertical structure of ocean currents using "modes." One of these, for example, does not vary with depth. The present work demonstrates how these modes change when realistic bottom topography is taken into account. The results suggest a simple way to predict subsurface velocities and have implications for our understanding of ocean dynamics.