Mobile version

ASAR vs altimetry South of Africa

Image of the Month - September 2010

Mean radial velocity of the Agulhas current by altimetry (left, Mean Dynamic Topography) and by ASAR on Envisat (right, 2007-2009 mean). With the new Mean Dynamic topography (MDT_CNES_CLS09), both are very close, with a strong current jet following the 1000 m isobath (200, 500, 1000, 2000 and 4000 m isobaths plotted). (Credits Cape Town University/Nersc/CLS/Ifremer)

The Agulhas current is one of the most powerful currents of Earth's oceans, often studied with several means of observations (see Images of the Month April 2008: Diving in an eddy, or July 2000: Where currents meet). This current is strongly steared by topography, following the 1000 m isobath. It can be seen on altimetry maps, but, since it is the main (and in part steady) flow of the current, this relies on precise knowledge of the mean dynamic topography. From these altimetry topography, the current velocity can be estimated.

Synthetic Aperture Radar is another technique entirely. The Envisat ASAR instrument enables to estimate mean radial velocity from the measured Doppler shift anomaly, and thus provides a mean to assess the strength and variability of the surface current and upper layer dynamics. In particular where the current lies close to the coast, altimetry measurements are somehow limited and the geoid is not properly resolved. However, SAR acquisitions are programmed and thus mostly not repetitive like altimetry (thus providing repeat global coverage). But at selected ocean sites, such as for the Agulhas Current regime, repeat acquisitions are ensured.

Comparisons between the two techniques in open ocean show very good agreement. It must be noted that the new Mean Dynamic Topography distributed by Aviso (MDT_CNES-CLS09), based on integrated use of altimetry, surface drifters, Argo floats and GRACE-based geoid, significantly improves previous geostrophic current estimates in the Agulhas Current, and thus the agreement.

These two distinct satellite-based observation techniques have their strengths and limitations. In combination, on the other hand, they can be largely overcome, leading to more accurate retrieval methods and products. In this context the Sentinel-1 C-band SAR secures the continuity of the Doppler shift anomaly method, while Saral, Sentinel 3 and Jason-3 will ensure altimetry continuity.

See also:

Other websites on this subject:

References

  • M. J. Rouault, A. Mouche, F. Collard, J.A. Johannessen and B. Chapron, Mapping the Agulhas Current from space: an assessment of ASAR surface current velocities, Journal of Geophysical Research (in press), 2010
  • This work is supported by ESA through the study contract no. 18709/05/I-LG
EnglishFrançais
Rss feedsContactPartnersFAQLexiconLegal noticePersonal Data Protection PolicyMobile
You must enable cookies in your browser preferences to save the modifications
You are using an older version of Internet Explorer, please update to take full advantage of Aviso+ features

Customize the home page

View all widgets
Reduce display
Public profile
User profile

Select/Deselect widgets

Discover AVISO+

Customizable home, reading tools, products search guide, etc.. Discover step by step the features of the new site AVISO+.

Customizable homepage

This home page is composed of blocks called "widgets".

Rearrange the blocks with a "Click & Drop".

Hide a widget with the cross at the top right. Reset or add a widget with the "wrench" icon. The "User" and "Public" profiles offer a predefined organization.

All the modifications are automatically saved as cookies on your machine so as to enable you to have your own specific AVISO+.

Reading tools

Ease your reading by choosing the page background colour (black/white) and/or by modifying the font size.

Browsing menu and breadcrum trail

To ease your browsing through AVISO+, you can use at any time the main banner items with the interactive submenus or use the browsing menu on the left side of the screen (it is open by default). For more readiness of the page contents, you can fold and unfold the browsing menu by clicking on the tab. At any time you can know where you are thanks to the browsing menu or the breadcrum at the top of the page.

Products search guide

The first submenu of the "Data" banner item leads you to a new search form enabling you to find quickier the product that suits your needs. The list of results is updated according to your selection criteria.

AVISO+ Multimedia

The first submenu of the "Multimedia" banner item leads you to AVISO+ collections of images and videos linked to altimetry and its applications. Have a nice visit!