"Planetary Waves": small amplitudes, large effects

Image of the Month - May 2002

Though the large currents are very important, the ocean and climate are also influenced by phenomena that are harder to see. "Planetary" waves cross the oceans along parallels and interact with general oceanic circulation. These are either Rossby waves, which go from East to West or Kelvin waves which move in the opposite direction. They intensify currents such as the Gulf Stream or the Kuroshio. In addition, they may be reflected off the continents and return in the opposite direction, or follow coastlines. These waves and their reflections play a key role, in particular in the El Niño phenomenon.

Map of sea level anomalies, on the left, and longitude-time diagram on the right (the time is indicated in Topex/Poseidon cycles). Diagrams like these bring out the variations of sea level over time along a particular parallel. The elevation at 90°E in cycle 20 (in yellow) can be found about 3 months (10 cycles) later at 85°E, 6 months later at 80°E, and so on. The westward motion of this elevation can be seen on the diagram as a sloped line. Another elevation follows the same path with a 3 month offset, creating a parallel trace. (Credits Southampton Oceanography Center).

Rossby waves had been predicted theoretically for over 50 years, but could not be observed until the advent of altimetric satellites. Their small amplitude (a few centimetres), their extent (an ocean basin) and their velocity (a few kilometres per day - they take several years to cross the Pacific Ocean at 30° latitude) made it nearly impossible to observe them with in situ measurements. Now that we know what we are looking for, we can also detect them in other kinds of satellite measurements. High precision altimetry, however, remains the best way of detecting these waves.

Comparison of longitude-time diagrams from three different sensors (altimeter, water colour, surface temperature). (Credits Southampton Oceanography Center).

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