The tropical western Pacific ocean regions experience lower surface air pressure during strong ENSO events.
ENSO (El Nino Southern Oscillation) is a natural phenomenon that reverses the temperature pattern in the Pacific. The trade winds are slowing down or even reversing their direction, so the warm water is no longer pushed west, so the western Pacific cools down, while the eastern Pacific is warming up. The pressure on the western Pacific is low, while on the eastern Pacific it's high.
