Separating planetary waves from eddies

Image of the Month - August 2008

Sea level anomalies Longitude-time diagram (at 27°S in the Pacific), top original, middle filtered so as to remove the eddies (showing planetary waves), bottom filtered so as to remove the planetary waves (showing mostly eddies). Such possibility enable to study the ones without the others -- and vice-versa (Credits Esa).


Eddies and planetary waves as observed in altimetric maps can be similar in their effects on sea surface height: propagation speed and direction, height and spatial scale can fall within a comparable range. To study the ones without the others, or to estimate their relative contributions to sea surface height, you have to separate them.  Research has been undertaken to better understand their respective role, by simulating planetary waves and eddies (planetary waves from theory, eddies from observation) and use those simulations to make a filter that enable to separate in the observations eddies from planetary waves.
Planetary waves were predicted by theory long before they were observed. With altimeters'  precision down to about 2 cm, since Topex/Poseidon, they have been observed continuously and globally since nearly 16 years -- as well as eddies. They both have impacts on the overall climate picture, since planetary waves are producing the El Niño phenomenon, and since eddies can transport heat.

Sea level anomaly maps, top modelled planetary waves, middle modelled waves plus eddies, bottom output of the filter (eddies removed). The latter enable to check the coherence of the filter (Credits Esa)

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