Absolute Dynamic Topography from Altimetry: Status and Prospects in the Upcoming GOCE Era

Abstract

The absolute dynamic topography, from which the ocean’s currents can be derived by geostrophy, is obtained, either by subtracting the geoid height from the altimetric Mean Sea Surface height above the reference ellipsoid, or by estimating the ocean Mean Dynamic Topography and adding it to the altimetric sea level anomalies, which are known with centimeric accuracy. Huge improvements have been made since the launch of the first satellite altimeters, for the estimation of both the geoid and the Mean Dynamic Topography. At the present time, satellite-only geoid models based on GRACE data are useful for oceanographic studies at scales greater than 300 km. In the meanwhile, a number of methods have been developed to estimate the shortest scales of the ocean’s Mean Dynamic Topography. Most recent MDT solutions have a global \({\raise0.7ex\hbox{1} \mathord{\left/ {\vphantom {1 4}}\right.\kern-\nulldelimiterspace}\lower0.7ex\hbox{4}}^ \circ \) resolution. Consequently, an increasing number of scientific studies have been recently published, based on the use of the absolute dynamic topography signal, covering a number of topics, from the assimilation of altimetric data into operational ocean forecasting systems, the study of eddy-mean interactions, the computation of volume transport, the monitoring of ocean currents. Finally, further improvements are expected from the future exploitation of GOCE data, from which the geoid will be mapped at 100 km resolution and 1–2 cm accuracy. However, in the upcoming context of high resolution wide-swath altimetry, and increasing need for high resolution coastal products, the merging of GOCE data with higher resolution sources of data will be mandatory.