Ocean Reanalyses: Recent Advances and Unsolved Challenges

• Published in: Frontiers in Marine Science Vol. 6
• Main Authors: Storto, Andrea, Alvera-Azcárate, Aida, Balmaseda, Magdalena A., Barth, Alexander, Chevallier, Matthieu, Counillon, Francois, Domingues, Catia M., Drevillon, Marie, Drillet, Yann, Forget, Gaël, Garric, Gilles, Haines, Keith, Hernandez, Fabrice, Iovino, Doroteaciro, Jackson, Laura C., Lellouche, Jean-Michel, Masina, Simona, Mayer, Michael, Oke, Peter R., Penny, Stephen G., Peterson, K. Andrew, Yang, Chunxue, Zuo, Hao
• Format: Journal Article Web Resource
• Language: English
• Published: Lausanne Frontiers Research Foundation 31.07.2019; Frontiers Media S.A
• Subjects: Annual variations; Atmospheric forcing; Climate; Climate change; Data; Data assimilation; Data collection; Earth; Earth sciences & physical geography; Heat; Historical reanalyses; Inter-comparisons; Observational requirements; Ocean circulation; Physical, chemical, mathematical & earth Sciences; Physique, chimie, mathématiques & sciences de la terre; Sciences de la terre & géographie physique; Uncertainty; Variability; Canada; Italy; United Kingdom > UK; Australia; France; United States > US
• ISSN: 2296-7745; 2296-7745
• DOI: 10.3389/fmars.2019.00418

Ocean reanalyses combine ocean models, atmospheric forcing fluxes, and observations using data assimilation to give a four-dimensional description of the ocean. Metrics assessing their reliability have improved over time, allowing reanalyses to become an important tool in climate services that provide a more complete picture of the changing ocean to end users. Besides climate monitoring and research, ocean reanalyses are used to initialize sub-seasonal to multi-annual predictions, to support observational network monitoring, and to evaluate climate model simulations. These applications demand robust uncertainty estimates and fit-for-purpose assessments, achievable through sustained advances in data assimilation and coordinated inter-comparison activities. Ocean reanalyses face specific challenges: i) dealing with intermittent or discontinued observing networks, ii) reproducing inter-annual variability and trends of integrated diagnostics for climate monitoring, iii) accounting for drift and bias due e.g. to air-sea flux or ocean mixing errors, iv) optimizing initialization and improving performances during periods and in regions with sparse data. Other challenges such as multi-scale data assimilation to reconcile mesoscale and large-scale variability and flow-dependent error characterization for rapidly evolving processes, are amplified in long-term reanalyses. The demand to extend reanalyses backward in time requires tackling all these challenges, especially in the emerging context of earth system reanalyses and coupled data assimilation. This mini-review aims at documenting recent advances from the ocean reanalysis community, discussing unsolved challenges that require sustained activities for maximizing the utility of ocean observations, supporting data rescue and advancing specific research and development requirements for reanalyses.