Decadal variability in regional and global mean sea level and its causes

Type: Presentation

Venue: GODAE Final Symposium, Nice, France


Ponte, R.M., C. Wunsch, and P. Heimbach, 2008. Decadal variability in regional and global mean sea level and its causes. GODAE Final Symposium, Nice, France, November 2008.

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The determination of regional and global mean sea level change and its causes is a problem of great societal interest, given the potential impact of sea level rise on many densely populated coastal regions. Unfortunately, no published estimate is a true global mean, given the poor data coverage particularly below several hundred meters and in the Southern Ocean, and claimed uncertainties do not account for the likely presence of systematic errors in the data. Similar issues affect the study of regional sea level patterns. A comprehensive attempt to deal with the sea level problem in a climate context is one focus of the Estimating the Circulation and Climate of the Ocean (ECCO) project and its current ECCO-GODAE extension. As part of ECCO-GODAE, 3-dimensional estimates of the time-dependent ocean state are produced by fitting a general circulation model to most available ocean data in a constrained least-squares optimization procedure. For the period analyzed (1992-present), several hundred million ocean observations are used, including all the altimetry and modern hydrography, Argo floats, sea surface temperature, and more recently estimates of the mean oceanic mass provided by space-based gravity measurements. A major effort was devoted to assessing the errors in all data, and which often differ from those commonly accepted. The ECCO-GODAE solutions are dynamically and kinematically consistent with all surface boundary conditions and model equations, and within the expected error bars of model and data; methodology and other details are described in an accompanying paper by Heimbach et al. Regional sea level patterns exhibit decadal variability with magnitudes substantially larger than expected long-term trends in global mean sea level (GMSL) and are associated with oceanic circulation patterns involving both steric and mass changes, with important contributions from temperature and salinity effects throughout the water column, including the abyss. Significant quantitative differences exist between the combined estimates and those based upon partial use of the data base. Quantitative attribution of GMSL changes to heating and freshwater addition, even in the data-rich past 15 years, is still very difficult. In this light, we discuss in detail various possible implementations of data constraints on mean quantities such as global ocean mass, heat content and other relevant variables for GMSL, as well as related model issues on how to best formulate surface freshwater boundary conditions and deal with the volume conservation implicit in the Boussinesq approximation.