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Mean Dynamic Topography

Current relief

The Coriolis force, generated by the Earth's rotation, diverts currents to the right of prevailing winds in the Northern Hemisphere, and to the left in the Southern Hemisphere. The higher the latitude, the larger the diverting force. (Credits JPL - CLS)

The major ocean currents generated by prevailing winds are deflected from their course by the shoreline, and by the Earth's rotation. Ocean circulation thus causes water to accumulate at the western edge of ocean basins.

Large scale ocean currents flow along the lines of equal dynamic topography. In the northern hemisphere, the flow is clockwise around the topography highs. In the southern hemisphere, the flow is counter-clockwise.

By measuring sea level variations, altimetry satellites enable us to observe ocean currents.

 

Dynamic topography and ocean currents viewed from space

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. 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. This dynamic topography shows all the features of the general circulation with gyres and associated western boundary currents (e.g. Gulf Stream, Kuroshio, Brazil/Malvinas Confluence area).

Mean dynamic topography, i.e. oceanic relief corresponding to permanent ocean circulation. Arrows are proportional to current speed. (Credits CNES/CLS, 2012).

Variability of ocean circulation

The world's oceans are subjected to many movements and variations. These movements are a lot "livelier" in some regions than others, chiefly in the strongest currents near the western seaboard of the oceans, such as the Gulf Stream in the North Atlantic, the Malvinas Current in the South Atlantic, the Kuroshio Current near Japan, and the Agulhas Current in the Indian Ocean, south of Madagascar. The strong variations in these zones are caused by intense eddying generated by instabilities in these powerful ocean currents. Sea level variations in the vicinity of these currents, caused by eddies tens or hundreds of kilometers across, can be as much as 30 centimeters. The Antarctic Circumpolar Current is another highly energetic current unbounded by any continent.

On the other hand, there are zones on either side of the Equator that hardly vary at all. Other intermediate zones exhibit phenomena that are more intermittent than ocean currents. The El Niño phenomenon in the Equatorial Pacific Ocean is a good example.

Satellites provide a global picture spanning several years to help us monitor these ocean variations more closely.

RMS of the Sea level anomalies over the whole Jan 1993-March 2010 period. Red areas are the one where the sea surface heights change the most (Credits Cnes/CLS)

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