The temperature of the seas changes abruptly at around 1000 meters' depth, so roughly speaking there are two layers of stratified fluid, and as a result, waves also exist there. Although no wall or boundary exists at the equator, symmetrical Kelvin waves that sandwich the equator can lean against each other as they progress because Coriolis' force reverses at the equator. This kind of Kelvin wave is called the equatorial Kelvin wave. In December when the trade wind weakens, warm sea water that is blown by the trade wind to the west side of the Pacific Ocean returns to the shores off Peru to the east as equatorial Kelvin waves. This is when the fishing season for anchovy in cold seas ends. This was originally called El Niño.