El Niño has arrived when temperatures in the eastern Pacific Ocean along the equator west of South America warm by at least half a degree Celsius (0.9 Fahrenheit) above normal

La Niña has arrived when temperatures in the eastern Pacific Ocean along the equator west of South America cool by at least half a degree Celsius (0.9 Fahrenheit) below normal

The last reversal was about 780,000 years ago. Reversals are not instantaneous; they happen over a period of hundreds to thousands of years, though recent research indicates that at least one reversal could have taken place over a period of one year.

1,013.25 hectoPascals (hPa), 29.9212 inches Mercury (inHg), or 14.696 pounds per square inch (psi)