El Nino is an oceanographic phenomenon first observed in the eastern Pacific near Peru. It occurs periodically with an irregular period of about 4--7 years, and consists of anomalous warming of the waters near Peru and cooling near the Philippines. It involves major changes in both the atmospheric and oceanic circulation over the Equatorial Pacific, and is a clear example of large-scale air--sea coupling and short term climatic variation. The first observations noted an unusual warming of the waters off Peru (which came usually around Christmas, hence the name, which means ``Christ child''). The warming diminishes the usual upwelling of cold and nutrient-rich waters to the surface, so it leads to a disastrous decrease in anchovies and other fish production with large economic consequences.

A radar altimeter measures the distance from the satellite to the surface of the ocean, and if the position of the satellite in space is known, this measurement allows the ocean surface deviations from the level corresponding to no motion to be inferred on time scales of a few days to years. The time scale of significant large scale variation of wind forcing is a few days and is called synoptic time scale.

Gravity waves are waves that propagate at the air--sea interface under the action of gravity. Note that the displacement of the heavy fluid represents available potential energy and the resulting equations are somewhat similar to those governing waves on a string.

GIN Sea: Greenland--Iceland--Norwegian Sea.