During which months do ocean surface elevations begin to rise along the equator, indicating the beginning of an el niño?

At the beginning of El Niňo, the sea surface rises typically around December which can last up to 9 months or even 2 years. Luckily, this phenomenon only happens every two to seven years. El Niňo is considered the warm phase of a long cycle, wherein sea surface temperatures rise from the central and east-central equatorial regions of the Pacific. 

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