Ocean currents

Image of the Month - February 2003

Mean dynamic topography, i.e. oceanic relief corresponding to permanent ocean circulation, computed from hydrologic and drifting buoys, used jointly with altimetric measurements and Champ gravimetric satellite data (geoid model Eigen2). Arrows are proportional to current speed. This dynamic topography shows all the features of the general circulation. The ocean gyres and associated western boundary currents (e.g. Gulf Stream, Kuroshio, Brazil/Malvinas Confluence area) appear clearly on the map; so too does the Antarctic Circumpolar Current.

As at the eddy scale, global ocean circulation is made of "hills" and "valleys". Large scale ocean currents flow along the lines of equal dynamic topography (black arrows on the map). In the northern hemisphere, the flow is clockwise around the topography highs. In the southern hemisphere, the flow is counter-clockwise.

This mean circulation is not an immediate product of altimetry data. Those data give more the mean sea surface, which contains the marine geoid plus the sea elevation due to the mean oceanic circulation (see January 1999 Image of the Month "A glassy sea... of ridges and valleys"). So we have to combine altimetric data with others (in situ, gravimetric satellites...), to precisely determine the geoid, and by substracting it, compute the mean circulation.