Clim Dyn (2017) 49:813–841 DOI 10.1007/s00382-015-2728-5 An ensemble of eddy-permitting global ocean reanalyses from the MyOcean project Simona Masina1,2 · Andrea Storto1 · Nicolas Ferry3 · Maria Valdivieso4 · Keith Haines4 · Magdalena Balmaseda5 · Hao Zuo5 · Marie Drevillon3 · Laurent Parent3 Received: 20 February 2015 / Accepted: 23 June 2015 / Published online: 10 July 2015 © The Author(s) 2015. This article is published with open access at Springerlink.com Abstract A set of four eddy-permitting global ocean level, transports across pre-defned sections, and sea ice reanalyses produced in the framework of the MyOcean parameters. The eddy-permitting nature of the global project have been compared over the altimetry period reanalyses allows also to estimate eddy kinetic energy. 1993–2011. The main differences among the reanaly- The results show that in general there is a good consist- ses used here come from the data assimilation scheme ency between the different reanalyses. An intercom- implemented to control the ocean state by inserting parison against experiments without data assimilation reprocessed observations of sea surface temperature was done during the MyOcean project and we conclude (SST), in situ temperature and salinity profles, sea level that data assimilation is crucial for correctly simulat- anomaly and sea-ice concentration. A frst objective of ing some quantities such as regional trends of sea level this work includes assessing the interannual variabil- as well as the eddy kinetic energy. A second objective ity and trends for a series of parameters, usually con- is to show that the ensemble mean of reanalyses can be sidered in the community as essential ocean variables: evaluated as one single system regarding its reliability SST, sea surface salinity, temperature and salinity aver- in reproducing the climate signals, where both variabil- aged over meaningful layers of the water column, sea ity and uncertainties are assessed through the ensemble spread and signal-to-noise ratio. The main advantage of having access to several reanalyses differing in the way data assimilation is performed is that it becomes possi- This paper is a contribution to the special issue on Ocean ble to assess part of the total uncertainty. Given the fact estimation from an ensemble of global ocean reanalyses, that we use very similar ocean models and atmospheric consisting of papers from the Ocean Reanalyses Intercomparison forcing, we can conclude that the spread of the ensem- Project (ORAIP), coordinated by CLIVAR–GSOP and GODAE OceanView. The special issue also contains specifc studies using ble of reanalyses is mainly representative of our ability single reanalysis systems. to gauge uncertainty in the assimilation methods. This uncertainty changes a lot from one ocean parameter to * Simona Masina another, especially in global indices. However, despite [email protected] several caveats in the design of the multi-system ensem- 1 Centro Euro-Mediterraneo sui Cambiamenti Climatici ble, the main conclusion from this study is that an eddy- (CMCC), Bologna, Italy permitting multi-system ensemble approach has become 2 Istituto Nazionale di Geofsica e Vulcanologia (INGV), mature and our results provide a frst step towards a Bologna, Italy systematic comparison of eddy-permitting global ocean 3 Mercator Océan, Ramonville Saint‑Agne, France reanalyses aimed at providing robust conclusions on the recent evolution of the oceanic state. 4 Department of Meteorology, National Centre for Earth Observation (NCEO), University of Reading (U-Reading), Reading, UK Keywords Global ocean reanalyses · Data assimilation · 5 European Centre for Medium-Range Weather Forecasts Eddy-permitting models · Essential ocean variables · (ECMWF), Reading, UK Ensemble mean · Ensemble spread 1 3 814 S. Masina et al. 1 Introduction the climate community but also to fsheries and off- shore industry, and foster intermediate and downstream There is an increasing need for estimating the present and services for the beneft of agencies with environmental past three-dimensional state of the ocean in the context assessment responsibilities and monitoring duties. of ocean monitoring, climate variability assessments and predictability purposes, such as the initialization and vali- This work is intended to provide a description of the dation of long-range (i.e. seasonal and decadal) forecasts. state-of-the-art of eddy-permitting global ocean REAs However, the oceans remain seriously under-sampled and produced in the framework of the MyOcean project using, observational time series are often of limited usefulness to wherever possible, an ensemble approach. The work also generate the required ocean estimates and ocean change illustrates examples of possible validation strategies with indicators due to the short periods of coverage and sparse the purpose of showing the applicability of these products geographical distributions. On the other hand, over the for a wide range of scientifc investigations, and to other course of the past few decades, considerable advancements relevant communities interested in the assessment of oce- have been made in the development of ocean data assimila- anic conditions at global and regional scales over recent tion techniques which combine ocean models, atmospheric decades. The coordinated European MyOcean effort on the forcing fuxes and ocean observations, and a number of global REAs intercomparison aimed at providing recom- ocean data assimilation systems have been developed to mendations for future REAs production by identifying the estimate the time-evolving, three-dimensional state of the weaknesses of existing individual systems and the suitabil- ocean. These combinations are known as ocean reanaly- ity of an ensemble approach. Furthermore, it was intended ses (REAs) and their production is a recent activity that to give feedback on how to improve the ocean observing started approximately at the beginning of year 2000. Since system, assimilation methods, models and surface fuxes, then considerable progress has been made and today REA and how to promote interaction with the user community production is an established reality in several research and and to encourage the archive of individual reanalysis prod- operational centers where REAs use advanced multivariate ucts in public data repositories freely available to all users data assimilation schemes that allow assimilation of most (www.myocean.eu). of the available types of observation. There are low reso- Thanks to satellite altimetry, sea level displacements lution REAs (about 1°), spanning long time periods (typi- associated with ocean eddies have been observed with a cally 50 years), as well as higher resolution products (about few centimeter accuracy for more than two decades, and 1°/4°), which exhibit eddy permitting capabilities and are there is evidence both from observations and modeling available for shorter records (usually the altimeter period studies that eddies play a role in the meridional transport of 1993-onwards). heat (e.g. Souza et al. 2011; Smith et al. 2000; Valdivieso In particular, two communities are devoting effort in et al. 2014). There is also evidence that ocean mesoscale exploiting existing and new ocean REAs for a variety of features have an impact on the atmospheric winds (e.g. purposes such as quantifying improvements in quality and Chelton et al. 2004; Maloney and Chelton 2006). That is uncertainty, and defning indices for ocean monitoring: why including mesoscale features in REAs with constantly increasing resolution is an important issue and contrib- 1. The Ocean Re-analyses Intercomparison Projects utes to improving our understanding of ocean and climate (ORA-IP), undertaken by the GOV and CLIVAR/ variability. GSOP communities (Balmaseda et al. 2014). Rather The next generation of operational climate prediction than following a fxed protocol, ORA-IP exploits the systems will implement eddy-permitting ocean models, and existing reanalysis products, taking advantage of the it is therefore urgent to assess the capability of the global diversity to gain insight on how robust our knowledge ocean REAs to be able to provide good quality initial con- of the ocean is. As part of this effort, a large suite of ditions for such systems. The eddy variability is still poorly indices and diagnostic quantities obtained from various represented in global ocean models, despite its acknowl- ocean reanalysis products are compared and evaluated edged important contribution to oceanic variability and using observations where available. Many papers on its expected impact on climate variability in the upcom- the ORA-IP are included in this Special Issue. ing generation of coupled models which include eddying 2. The EU funded MyOcean project (www.myocean.eu), oceans. among several goals, aimed at providing a series of Nevertheless, one has to keep in mind that subsurface validated eddy-permitting and/or eddy-resolving global ocean observations are scarce before the 2000s (i.e. prior and European Seas REAs covering the recent “altime- to the full deployment of Argo foats) and that large uncer- try era” (namely 1 January 1993-onwards) (Ferry et al. tainties may exist in the ocean REAs, making a robust esti- 2012). These products are not only targeted towards mation of the ocean history with reliable error bars still a 1 3 An ensemble of eddy-permitting global ocean reanalyses from the MyOcean project In Table forusers( arefreelyavailable work used inthis The numerical products