Seasonal to decadal variability of the global overturning circulation inferred from the ECCO-GODAE ocean state estimate

Author: P. Heimbach, Rui M. Ponte, C. Wunsch, G. Forget, C. Hill and J.M. Campin
November 5, 2008
GODAE Final Symposium, Nice, France

Heimbach, P., R.M. Ponte, C. Wunsch, G. Forget, J.M. Campin, and C. Hill, 2008. Seasonal to decadal variability of the global overturning circulation inferred from the ECCO-GODAE ocean state estimate. GODAE Final Symposium, Nice, France, November 2008.

With its provision of the first continuous oceanographic data set of quasi-global coverage, satellite altimetry was the main driver for embarking on the Estimating the Circulation and Climate of the Ocean (ECCO) project. The goal was, and remains, to combine as many satellite and in-situ observations as practical with a state-of-the-art general circulation model (GCM) to produce a best estimate of the time-evolving three-dimensional state of the ocean, being the dynamically consistent solution to a free-running GCM. Since its start, the adjoint-based branch of ECCO has gone through several product versions. The latest of these, version 3, is currently being produced as part of what is formally ECCOGODAE. Improvements lie with the underlying model, the inclusion of new and novel types of observations, and the refinement of (still poorly known) prior uncertainty estimates in every observational element whose magnitude determines the least-squares solution.

In terms of scientific analysis the focus is on global three-dimensional aspects of oceanic changes, in particular the global meridional overturning circulation (MOC). This recognizes the fact that the Atlantic MOC which has been the subject of much debate is merely a component of the larger general circulation, and attempts to detect secular changes ought to be directed at the global problem. Changes in any region propagate through the global ocean over long time scales, with (multi-decadal) adjustment through baroclinic Rossby waves providing a lower bound of such adjustments. Analysis of the 15-year solution which spans the recent period of unprecedented sampling reveals most of the variability in the annual and semi-annual cycles and contained mainly in the vast bodies of the tropical Pacific and the Southern Ocean water masses. However, robust trend estimates remain elusive, with only some weak spatially complex signatures, and the poor data sampling before the 1990s render problematic reliable trend estimates over longer periods.

All of these inferences have strong implications for long-running circulation observation strategies, including the need to avoid aliasing of the strong seasonal cycles, distinguishing purely local effects from larger-scale ones, and the long duration of the observations that will be required for documentation and understanding.
Work on a next-generation estimation system is in progress to address some remaining gap in the current product. This includes a fully global grid representing the Arctic, and with an extended control space to account for model parameter uncertainties. Technical hurdles include the extension of the adjoint infrastructure to generalized grid topologies, and the provision of stable adjoint sea ice sensitivities. Another focus is the continued extension of the observational backbone, such as the use of time-varying satellite gravity measurements from GRACE.