Abstract:
The primary focus of this project is to use Jason-1, TOPEX/POSEIDON and other altimetric data (including those from the GEOSAT series, and the ERS-n series) to determine the general circulation of the ocean, on a global continuous basis. The central vehicle for this exploitation is a global general circulation model and its adjoint which we have developed into a system for global data assimilation and analysis [Marotzke et al., 1999]. Results from these efforts are being used to understand the oceanic heat and fresh water fluxes, their divergences, their dynamical causes and effects, as well as a variety of related issues connecting the oceanic circulation to climate variability.
Statistical descriptions
As part of our work on statistical descriptions Topex/Poseidon (T/P) data have been analyzed on a global scale to produce estimates of the averaged frequency, wavenumber, and frequency/wavenumber spectra as well as their regional forms [Wunsch and Stammer, 1995; Stammer, 1997; Wunsch, 1997; Stammer, 1998]. Those studies include especially the estimation of eddy fluxes and eddy parameterization, spectral representations, and response to atmospheric loading. We extended that work by including all available in situ information about ocean variability to estimate the first global spectral description of the ocean's eddy variability on space scale from about 100 km to basin scale, and from several weeks to a few years [Zang and Wunsch, 2001]. As part of this activity we also exploit the recently-demonstrated, qualitatively-important covariance of interannual altimetric signals with sea surface temperature anomalies and its implication for studying ocean currents.
As Jason-1 data become available, we will continue the monitoring activity by applying the new data to ocean dynamics studies. Any inconsistency in the data will show up as spurious signals in the analyses that will be inconsistent with our knowledge about the ocean.
Global data assimilation
Based on T/P data we have demonstrated that it is now possible to produce full three-dimensional, time-varying estimates of the oceanic circulation in a mode of operation somewhat like that of numerical weather prediction. Using the general circulation model of Marshall et al. [1997] and its adjoint, estimates we have made of the errors in the altimetry, in the wind, and buoyancy fluxes from NCEP daily analyses and in the Levitus temperature and salinity climatologies, we have succeeded in demonstrating the feasibility of a complete three-dimensional self-consistent oceanic estimate of the absolute time-varying circulation over a 6-year period from 1992 through 1997. Results are being used to study ocean transports of heat, freshwater and volume and to understand their divergences and interactions with the atmosphere. A detailed discussion of the analysis is provided in Stammer et al. [2001].
A few representative results are summarized in figures 1 and 2.
The estimated mean sea surface height field and mean velocity fields at 27 and 1542 m depth (figure 1) shows all major current systems. But with the low model resolution, they are necessarily overly smooth. The mean net surface heat and freshwater flux fields as they result from the optimization are displayed in figure 2a. Their mean change relative to the prior NCEP fields are provided in figure 2b. Ocean transports of all quantities are very energetic and variable. The estimated time-varying model state, model transports and consistent surface flux fields will be the basis for a wide variety of climate and societal applications. Many interdisciplinary applications are already under way, including studies of the ocean's impact on the Earth angular momentum budget [Ponte et al., 2000]. Results are likewise being exploited to estimate the ocean uptake and distribution of CFS and ultimately carbon.
It is anticipated that, in two to three years, the project will be able to address the U.S. CLIVAR and GODAE related objective of depicting the time-evolving ocean state with spatial resolution up to 1/4 degree globally and with substantially higher resolution in nested regional approaches which are required for quantitative studies of the ocean circulation. Complementary to this, a 50-year long reanalysis experiment is anticipated but with only a 1-degree spatial resolution that coincides with the NCEP/NCAR reanalysis period.
References
Marotzke J., R. Giering, Q.K. Zhang, D. Stammer, C.N. Hill, T. Lee, 1999: Construction of the adjoint MIT ocean general circulation model and application to Atlantic heat transport sensitivity. J. Geophys. Res., 104, 29,529 - 29,548.
Marshall J., A. Adcroft, C. Hill, L. Perelman, C. Heisey, 1997a: A finite-volume, incompressible navier-stokes model for studies of the ocean on parallel computers. J. Geophys. Res., 5753-5766.
Ponte R.M., D. Stammer, C. Wunsch, 2001: Improved ocean angular momentum estimates using an ocean model constrained by large-scale data. Geophys. Res. Lett. (in press).
Stammer D., 1997: Global characteristics of ocean variability from regional TOPEX/POSEIDON altimeter measurements. J. Phys. Oceanogr., 27, 1743-1769.
Stammer D., 1998: On eddy characteristics, eddy transports and mean flow properties. J. Phys. Oceanogr., 28, 727-739.
Stammer D., C. Wunsch, R. Ponte, 2000: De-Aliasing of global high frequency barotropic motions in altimeter observations. Geophys. Res. Lett., 27, 1175-1178.
Stammer D., C. Wunsch, R. Giering, C. Eckert, P. Heimbach, J. Marotzke, A. Adcroft, C.N. Hill, J. Marshall, 2001, The global ocean circulation and transports during 1992 - 1997, estimated from ocean observations and a general circulation model. (to be submitted).
Wunsch C., 1997: The vertical partition of oceanic horizontal kinetic energy. J. Phys. Oceanogr., 27, 1770-1794.
Wunsch C., D. Stammer, 1995: The global frequency-wavenumber spectrum of oceanic variability estimated from TOPEX/POSEIDON altimetric measurements. J. Geophys. Res., 100, 24,895-24,910.
Zang X., C. Wunsch, 2001: Spectral description of low frequency oceanic variability. J. Phys. Oceanogr. (in press).
