Total Surface Current Vector and Shear from a Sequence of Satellite images: Effect of Waves in Opposite Directions

The Total Surface Current Velocity (TSCV) - the horizontal vector quantity that advects seawater - is an Essential Climate Variable, with few observations available today. The TSCV can be derived from the phase speed of surface gravity waves, and the estimates of the phase speeds of different wavelengths could give a measure of the vertical shear. Here we combine 10-m resolution Level-1C of the Sentinel 2 Multispectral Instrument, acquired with time lags up to 1s, and numerical simulation of these images. Retrieving the near surface shear requires a specific attention to waves in opposing direction when estimating a single phase speed from the phase difference in an image pair. Opposing waves lead to errors in phase speeds that are most frequent for shorter wavelengths. We propose an alternative method using a least-square fit of the current speed and amplitudes of waves in opposing directions to the observed complex amplitudes of a sequence of 3 images. When applied to Sentinel 2, this method generally provides more moisy estimate of the current. A byproduct of this analysis is the "opposition spectrum" that is a key quantity in the sources of microseisms and microbaroms. For future possible sensors, the retrieval of TSCV and shear can benefit from increased time lags, resolution and exposure time of acquisition. These findings should allow new investigations of near-surface ocean processes including regions of freshwater influence or internal waves, using existing satellite missions such as Sentinel 2, and provide a basis for the design of future optical instruments