4. Eddies: the Oceanic Storms

From surface slope to current speed

Alongside the major currents, the sea surface is scattered with rotating vortices, called eddies. These circular-shaped or elongated features, 50 to 300 km wide, are the oceanic analogy to storms in the atmosphere, and are clearly seen in maps of sea surface anomaly from altimeters. Eddies are subject to the Coriolis force and to a good approximation can be considered geostrophic

• so that a warm anticyclonic eddy (i.e a positive bump in the sea surface) will rotate clockwise in the Northern Hemisphere, anticlockwise in the Southern Hemisphere,
• while a cold cyclonic eddy (a depression in the sea surface) will rotate in the opposite direction.

The same happens for anticyclones (high pressure) and cyclones (depressions) in the atmosphere.

Some large, elongated eddies off the East coast of Australia as seen in a Sea Surface Height Anomaly from a combination of satellite altimeters. The black lines indicate the direction of geostrophic currents around the two main eddies.

Eddies show clearly also in Sea Surface Temperature (SST)- here is a map of Sea Surface Temperature from the MODIS instrument on board NASA's Aqua Satellite, showing a number of meanders and eddies in the region of the Gulf Stream

Importance of eddies

Eddies are important both for ocean circulation and climate, as they transport heat and momentum, and for biology and fisheries, and they mix different oceanic waters influencing the conditions that allow the marine food web to develop. Moreover they may be important for shipping, as explained in the diagram below, negotiating the wrong side of an eddy may well mean losing a couple of knots in speed.

To go from A to B, the vessel has a faster route on that side of the eddy where the eddy rotation adds to the vessel’s speed.