I am sure you can learn everything about El Niño Southern Oscillation (ENSO) on its Wikipedia page: El Niño - Wikipedia But I would like to make you aware that it is not a “special” thing. read more
I don't know of any evidence either way, but it's worth noting that the Pacific existed in something comparable to its present form - covering mucht of one half of the globe. read more
El Niño and La Niña, two opposing modes of the El Niño Southern Oscillation (ENSO), are notorious for altering both regional and global climate on short timescales (i.e. on the order of 2 to 7 years). read more