Abstract
In 1999, the consortium for Estimating the Circulation and Climate of the Ocean (ECCO) set out to synthesize the hydrographic data collected by the World Ocean Circulation Experiment (WOCE) and satellite sea surface height measurements into a complete and coherent description of the ocean afforded by an ocean general circulation model. Twenty years later, the versatility of ECCO's estimation framework enables production of global and regional ocean and sea-ice state estimates that incorporate not only the initial suite of data and its successors, but nearly all data streams available today. New observations include measurements from Argo floats, marine mammal-based hydrography, satellite retrievals of ocean bottom pressure and sea surface salinity, and ice-tethered profiler data in polar regions. The framework also produces improved estimates of uncertain inputs, including initial conditions, surface atmospheric state variables, and mixing parameters. The freely available state estimates and related efforts are property-conserving, allowing closed budget calculations that are a requisite to detect, quantify, and understand the evolution of climate-relevant signals as mandated by the Coupled Model Intercomparison Project Phase 6 (CMIP6) protocol. The solutions can be reproduced by users through provision of the underlying modeling and assimilation machinery. Regional efforts have spun off that offer increased spatial resolution to better resolve relevant processes. Emerging foci of ECCO are on global sea level change, in particular contributions from polar ice sheets, and the increased use of biogeochemical and ecosystem data to constrain global cycles of carbon, nitrogen and oxygen. Challenges in the coming decade include provision of uncertainties, informing observing system design, globally increased resolution, and moving toward coupled Earth system estimation with consistent momentum, heat and freshwater fluxes between the ocean, atmosphere, cryosphere and land.
Highlights
The central goal of the ECCO consortium is the production of global ocean state and parameter estimates in support of climate research
The analysis reveals negative volume anomalies during the winters of 2009/10 and 2010/11
A natural extension of ECCO consists in the coupled oceanatmosphere estimation problem, an avenue pursued by many reanalysis groups today
Summary
The central goal of the ECCO consortium is the production of global ocean state and parameter estimates in support of climate research. Avoiding shortcomings identified in atmospheric reanalysis (e.g., Bengtsson et al, 2004, 2007) and making optimal use of the sparse observational coverage calls for the use of smoothing methods from optimal estimation theory (Wunsch and Heimbach, 2007, 2013; Stammer et al, 2016). The latest ECCO solution can be used to produce climatologies, based on most data available from the global observing system since the early 1990s, for temperature and salinity, but which provides consistent three-dimensional flow fields and connected dynamical variables (e.g., sea level and bottom pressure), consistent surface forcing fields, and property budgets to explore the underlying dynamics (e.g., Ekman and Sverdrup transports, mixing, and vorticity fluxes) (Fukumori et al, 2018). Self-consistency among the range of state variables is invaluable for depictions of the global ocean, e.g., in terms of its overturning circulation (Cessi, 2019)
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