Ocean currents are sometimes compared to ‘sea highways’, because ships follow their flow to gain speed. But this picture is not wholly accurate. In fact, ocean currents are more like secondary roads branching off into narrow country roads and lanes, winding their way around hills and natural obstacles. The main flow of these currents is often disturbed by eddies that form at their edge. So the ocean is really more like a vast, slowly swirling whirlpool bath than a swimming pool divided into perfectly straight lanes.

sdt_dev
Standard deviation computed from 12 years of altimetry data (all available satellites during the 1992-2004 period)
(Credits Aviso)

The ocean is a turbulent environment. The most energetic kind of ocean circulation variability is associated with so-called mesoscale variability (eddies, meandering currents or fronts, squirts and filaments), i.e. features on a 50-500 km scale, lasting 10-100 days, with currents of a few kilometres per hour. The energy of these mesoscale processes generally exceeds that of the mean flow by an order of magnitude or more. They also transport heat, salt, carbon and nutrients as they propagate through the ocean. Ocean eddies play an important role in ocean circulation and heat transport, as well as in the ocean’s biogeochemical cycles. The ability to monitor them from space has applications in navigation, offshore operations, fisheries, hurricane and climate forecasting, among others.
Before the advent of satellite observations, this turbulence was mostly underestimated, or even ignored. For improved monitoring of such features, altimetry needs merged data from at least two satellites (see How altimetry works: Multiple satellite for high-precision altimetry).

Further information:

Le Traon, P.Y. and R. Morrow, Ocean currents and eddies, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001
Morrow, R. and P.Y. Le Traon, Mesoscale Eddy Dynamics observed with 15 years of altimetric data,15 years of progress in radar altimetry Symposium, Venice, Italy, 2006