DYNAMIC OCEAN TOPOGRAPHY

The mean dynamic ocean topography (DOT) is the difference between the time-averaged sea surface and the geoid (the equipotential surface of the Earth's gravity field that best fits the mean sea surface). All geoid slopes are 'horizontal'. A tilt of the the sea surface relative to the horizontal measures the strength of surface 'geostrophic' currents. The DOT measures the long-term-averaged strength of ocean currents, the 'steady-state' circulation. One example is the Gulf Stream, whose position averaged over any one year now is about the same as in 1786, when Benjamin Franklin and Timothy Folger charted it (Richardson, 1980). The North-South (meridional) gradient of the DOT is proportional to the East-West (zonal) geostrophic component of ocean surface current velocities (the rest is the wind-driven Ekman current); the zonal gradient of the DOT is proportional to the meridional velocity.

The DOT can be constructed from geodetic data: an altimetric mean sea surface (from over a decade of radar altimetry), and an accurate geoid.

The DOT can also be constructed by combining in-situ oceanoraphic data (temperature and salinity of seawater, direct measurements of current velocity, etc). A third way is by combining the geodetic estimate (altimetry and geoid) with the traditional oceanographic estimate (Niiler et al, 2003).

Here we offer a recent (Jul-Aug 2008), purely geodetic estimate. It was prepared by Don Chambers (U. Texas-Austin), from the Mean Sea Surface constructed by O. Andersen, P. Knudsen and colleagues et at at the Danish National Space Center (Mean Sea Surface (Knudsen, Vest, Andersen) ) and the geoid comstructed by Nikos Pavlis and colleagues(EGM-2008 Geoid) based on GRACE data, other space and situ gravity data.

---

DATA ACCESS
DYNAMIC OCEAN TOPOGRAPHY IMAGE (large) DATA (ASCII)
EASTWARD GEOSTROPHIC VELOCITY IMAGE (large) DATA (ASCII)
NORTHWARD GEOSTROPHIC VELOCITY IMAGE (large) DATA (ASCII)

---

References:

Chambers (2004): Powerpoint presentation (Power Point, 2.15 MB) presents an earlier version of this work, using earlier MSS and Geoid estimates..

Niiler, P. P., N. A. Maximenko, and J. C.McWilliams (2003): Dynamically balanced absolute sea level of the global ocean derived from near-surface velocity observations, Geophys. Res. Lett., 30(22), 2164, doi: 10.1029/2003GL018628.

Richardson, P.L. (1980): The Benjamin Franklin and Timothy Folger Charts of the Gulf Stream, In: Oceanography, the Past, edited by M. Sears and D. Merriman. Springer Verlag, Inc, NY.

Pavlis, N. et al.(2008): An Earth Gravitational Model to Degree 2160: EGM2008.
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/NPavlis&al_EGU2008.ppt

Knudsen, P., Vest, A.L., and Andersen, O (2004) Evaluating mean dynamic topography models within the GOCINA project. In Proceedings of the 2004 Envisat & ERS Symposium, 6–10 September 2004, Salzburg, Austria (ESA SP– 572, April 2005). Published and distributed by: ESA Publications Division.

Citation:

When using these data, please acknowledge this website, and cite:

Tapley B.D., D.P. Chambers, S. Bettadpur and J.C. Ries, 2003: Large scale ocean circulation from the GRACE GGM01 Geoid. Geophys. Res. Letters 30 (22):doi:10.1029/2003GL018622. (The DOT and velocities presented here are updates of what this paper presented).