Internal gravity waves are like surface waves on the ocean, they both propagate under the restoring force of gravity. The restoring force is felt when there is a vertical variation in density such as the density difference between water and air. When the density difference is very large (at the interface between two fluids, like water and air) waves are trapped at the interface and you have surface waves. When the density varies gradually as in a stratified fluid internal gravity waves may exist. Such waves may move vertically upward as well as horizontally.
In the atmosphere, the effect of internal gravity waves can be seen on partly cloudy days. The rise and fall of the waves at just the right altitude can make wave condense and evaporate, respectively, and this leads to a banded cloud pattern.
In the Atlantic Ocean it flows westward between the equator and 20 degrees. Part crosses the equator and flows northwest along the coast of South America as the Guiana current . The rest turns to the left and flows south along the coast of Brazil as the Brazil current.
In the Pacific Ocean, the south equatorial current crosses the ocean from east to west between the latitudes of approximately 3 degrees and 10 degrees. Much of it turns south in mid-ocean forming a large anticyclonic whirl. The portion that continues across the ocean divides as it approaches Australia, part moving north toward New Guinea and part turning south along the east coast of Australia as the east Australia current .
In the Indian Ocean, the south equatorial current is displaced rather far to the south, and as it approaches the east coast of Africa it turns south, joining the Agulhas current .
References:
Department of Mathematical Sciences home page
Geophysical Fluid Dynamics home page