Sack Lunch Seminar (SLS)

The large-scale geostrophic flow-field and eddy variability as seen from the TOPEX/Poseidon and Jason-1 Tandem Mission


Geostrophic surface velocity anomalies are used to analyze the variations of the large-scale geostrophic currents and of the Eddy Kinetic Energy (EKE) field of the ocean circulation. Using the Jason1-TOPEX/Poseidon (JTP) tandem mission it has been possible for the first time to calculate the zonal and meridional geostrophic velocity components separately without the assumption of isotropy, and the resulting EKE subsequently along the satellite track for each along-track position. The underlying geostrophic currents were estimated from the JTP tandem altimetric sea surface height (SSH) measurements using the ``parallel-track-approach'' with a 6.2 km along-track resolution. The analysis covers the entire 3-year period of the tandem mission (109 repeat cycles) from September, 2002, to September, 2005.


The talk will concentrate on the importance of an anisotropic consideration of the ocean flow-field as discovered by the JTP tandem mission. It will be shown, that the ocean circulation has a slightly higher meridional variability by 10 to 20 % in mid latitudes, while in the tropics the EKE field is dominated by the variability of the mostly zonal current field which clarifies that in some regions it is important to assume anisotropy.


Furthermore, the study underlines the importance of both JTP velocity component-estimates for the understanding of the spatial structures of flow changes on the annual period. Results reproduce conclusions drawn from several studies which are available on a regional basis, as for example the clear dominance of the seasonal flow changes in low latitudes. However, they are extended to the basin-scale and into a global context. At the same time, complex structures of the seasonally changing flow-field reveal. In the western Pacific, for instance, they appear as zonally coherent jet-like structures. Furthermore, complex phase structures are found in mid- and high latitudes.


To describe all temporal and spatial scales seen by the JTP tandem mission, the frequency and wavenumber spectral behaviors of the large-scale geostrophic currents and of the eddy kinetic energy (EKE) field will be presented. This has been done in previous studies, however, for the first time it has been possible to calculate frequency and wavenumber spectra for both, the zonal and meridional, geostrophic velocity components separately as well as for the resulting EKE along a satellite track.