Comparing a Multivariate Global Ocean State Estimate With High-Resolution in Situ Data: An Anticyclonic Intrathermocline Eddy Near the Canary Islands

Bàrbara Barceló-Llull,Sandrine Mulet,Evan Mason,Ananda Pascual

doi:10.3389/fmars.2018.00066

Bàrbara Barceló-Llull, Sandrine Mulet + Show 2 more

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https://doi.org/10.3389/fmars.2018.00066

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Abstract

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.

Highlights

The growing capacities of new products developed in operational oceanography (Schiller and Brassington, 2011; Bell et al, 2015; Chakraborty et al, 2015; Le Traon et al, 2015; Hernandez et al, 2015; Kaurkin et al, 2016; Sotillo et al, 2016) imply a need for ongoing assessment using independent high-resolution in situ data sets
Five months before the cruise, Barceló-Llull et al (2017b) periodically monitored the signature of the eddies generated by the Canary Islands in sea level anomaly (SLA) maps provided by AVISO in order to select an anticyclonic eddy with a robust signature as the target for their study
The eddy center is located as the minimum in the ARMOR3D geostrophic flow (20.1◦W, 26.1◦N), and the eddy radius is derived from ARMOR3D Absolute Dynamic Topography and density fields giving a value of 92 km

Summary

Introduction

The growing capacities of new products developed in operational oceanography (Schiller and Brassington, 2011; Bell et al, 2015; Chakraborty et al, 2015; Le Traon et al, 2015; Hernandez et al, 2015; Kaurkin et al, 2016; Sotillo et al, 2016) imply a need for ongoing assessment using independent high-resolution in situ data sets. In order to complement satellite observations, sustained observing efforts through profiling and surface drifters, expendable bathythermographs (XBTs), and moorings provide real-time in situ vertical profiles of temperature and salinity on a global, but unstructured, grid. Emerging technology, such as the recently established global underwater glider observing network could, in principle, provide the required high-resolution in situ profile data, but routine mapping of the mesoscale is not yet implemented (Liblik et al, 2016)

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