Meteorologists measure El Niño strength by comparing sea-surface temperature changes in an imaginary box encompassing the eastern equatorial Pacific.
The "Niño-3.4 region" is 2.7 degrees (1.5 Celsius) above average, meeting the Weather Service's unofficial criterion for a strong El Niño.
However, the atmosphere differs from previous intense El Niños. The reason
An uncommon warm spot in the western Pacific. Professor of atmospheric science Paul Roundy at the University of Albany suggests that this is reducing conventional El Niño signs.
Rising air is scarce over the eastern Pacific. Typically, El Niño causes rising air due to warmer eastern Pacific waters heating the air above. That causes low air pressure, rains, and thunderstorms.
Air subsides over the Atlantic. However, the eastern Pacific ascent area is small and diffuse. The out-of-place western Pacific warm blob may be heating and rising there, and some of the air is sinking in the eastern Pacific.
El Niño's irregular pattern may be a result of three consecutive La Niña winters. Crawford speculates that a delayed La Niña response may remain.
Rising ocean temperatures from human-caused climate change may also warm the western Pacific.