Assessing Eddying (1/12 Degrees) Ocean Reanalysis Glorys12 Using The 14-Yr Instrumental Record From 59.5 Degrees N Section In The Atlantic

The eddy-resolving (1/12 degrees) global ocean reanalysis GLORYS12 is assessed against 14 years (2002-2015) of independent hydrographic observations collected at 59.5 degrees N in the North Atlantic. Two multi-observations statistical analyses, ISAS-15 and ARMOR-3D, and an eddy-permitting (1/4 degrees) reanalysis, GLORYS025, also contribute to this comparison. The mean thermohaline structure along the 59.5 degrees N section revealed by the observations is well-reproduced by GLORYS12, except for the overflow waters. A good agreement with observations is found for linear trends over the whole period, exhibiting a dipole-like pattern with a cooling/freshening in the main thermocline and a warming/salinization below. However, localized discrepancies with observations suggest the need for improvement in the reanalysis system (especially in the overflows representation and the consistency between the forcing and data assimilation system) and the deep observational array. The reanalysis reliably represents the ocean heat content in the upper 700-m layer but shows significant differences with observations between 700 and 2,000 m. The meridional volume and heat transports across the 59.5 degrees N section are compared for years when ADCP observations were available. The reanalysis does not reproduce the variability observed in the western boundary current but agrees well with observed transports in the other parts of the section. The reanalysis reproduces the major mesoscale eddy features that contribute to the meridional transport and provides the large-scale context of their location. The analysis of time correlation at all measurement points demonstrated that GLORYS12 is the most accurate among the analyzed datasets used in this study to represent and explain the observed ocean characteristics and variability along that section.Plain Language Summary The GLORYS12 reanalysis data set produced recently by Mercator Ocean International forecasting center describes the day-to-day evolution of the worldocean physical properties for 1993-2018. This reanalysis uses many observations of different kinds to constrain the solution of a dynamical ocean forecasting model. GLORYS12 is assessed against 14 years of hydrographic (temperature and salinity) observations, not used in the reanalysis process, collected annually between Scotland and Greenland along 59.5 degrees N. The observed time-mean thermohaline structure and patterns of the temporal trends along 59.5 degrees N are well represented except for the deep waters of Arctic origin that flows along Greenland. Local discrepancies between reanalysis and observations emphasize the need for improvements in the reanalysis system and observations at depth. GLORYS12 provides a reliable assessment of the ocean heat content in the upper 700 m, an important climate index, the agreement which deteriorates at greater depths. The comparison with similar datasets produced by statistical analyses of observations (not using a dynamical model) shows that GLORYS12 is the one that best describes the variability seen in the observations. The reanalysis, because it provides a view of the large-scale circulation context, allows to relate the variability of the meridional heat transport across the section to the presence of mesoscale circulation features.