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Front. Mar. Sci. | doi: 10.3389/fmars.2018.00066

Comparing a multivariate global ocean state estimate with high-resolution in situ data: an anticyclonic intrathermocline eddy near the Canary Islands

  • 1Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria, ULPGC, Spain
  • 2Mediterranean Institute for Advanced Studies (CSIC), Spain
  • 3C.L.S. Space Oceanography Division, France

The provision of high-resolution \textit{in situ} oceanographic data is key for the ongoing verification, validation and assessment of operational products, such as those provided by the Copernicus Marine Core Service (CMEMS).
Here we analyze the ability of ARMOR3D - a multivariate global ocean state estimate that is available from CMEMS - to reconstruct a mesoscale anticyclonic intrathermocline eddy that was previously sampled with high-resolution independent \textit{in situ} observations. ARMOR3D is constructed by merging remote sensing observations with \textit{in situ} vertical profiles of temperature and salinity obtained primarily from the Argo network.
\textit{In situ} data from CTDs and an Acoustic Doppler Current Profiler were obtained during an oceanographic cruise near the Canary Islands (Atlantic ocean). The analysis of the ARMOR3D product using the \textit{in situ} data is done over (i) a high-resolution meridional transect crossing the eddy center and (ii) a three-dimensional grid centered on the eddy center.
An evaluation of the hydrographic eddy signature and derived dynamical variables, namely geostrophic velocity, vertical vorticity and quasi-geostrophic (QG) vertical velocity, demonstrates that the ARMOR3D product is able to reproduce the vertical hydrographic structure of the independently sampled eddy below the seasonal pycnocline, with the caveat that the flow is surface intensified and the seasonal pycnocline remains flat. Maps of ARMOR3D density show the signature of the eddy, and agreement with the elliptical eddy shape seen in the \textit{in situ} data. The major eddy axes are oriented NW-SE in both data sets.
The estimated radius for the \textit{in situ} eddy is $\sim$ 46 km; the ARMOR3D radius is significantly larger at $\sim$ 92 km and is considered an overestimation that is inherited from an across-track altimetry sampling issue.
The ARMOR3D geostrophic flow is underestimated by a factor of 2, with maxima of 0.11 (-0.19) m s$^{-1}$ at the surface, which implies an underestimation of the local Rossby number by a factor of 3.
Both the \textit{in situ} and ARMOR3D eddies have decelerating flows at their northern edges.
The ARMOR3D QG vertical velocity distribution has upwelling/downwelling cells located along the eddy periphery and similar magnitudes to the \textit{in situ}-derived QG vertical velocity.

Keywords: Observation-based product, remote sensing, altimetry, ARGO, Geostrophic flow, vertical velocity, Intrathermocline eddy, eddies, mesoscale eddies

Received: 28 Aug 2017; Accepted: 13 Feb 2018.

Edited by:

Gilles Reverdin, Centre national de la recherche scientifique (CNRS), France

Reviewed by:

Johannes Karstensen, GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
Yves MOREL, UMR5566 Laboratoire d'études en géophysique et océanographie spatiales (LEGOS), France
Louis M. PRIEUR, UMR7093 Laboratoire d'océanographie de Villefranche (LOV), France  

Copyright: © 2018 Barceló-Llull, Pascual, Mason and Mulet. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Ms. Bàrbara Barceló-Llull, Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria, ULPGC, Las Palmas de Gran Canaria, Spain, b.barcelo.llull@gmail.com