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\begin{document}

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



\author[1]{Fabrice Ardhuin}%
\author[2]{Matias Alday}%
\author[3]{Maria V. Yurovskaya}%
\affil[1]{Univ. Brest, CNRS, Ifremer, IRD}%
\affil[2]{CNRS, Ifremer}%
\affil[3]{Marine Hydrophysical Institute}%


\vspace{-1em}



  
  \date{June 05, 2024}


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\begin{abstract}
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.%
\end{abstract}%



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