I understand that an ENSO event is precipitated by the weakening of the equatorial trade winds, allowing warm equatorial sea water to flow eastward across the Pacific Ocean. What I don't understand, and can find little information on, is the reason why the trade winds weaken in the first place. What do meteorologists and climatologists think causes the trade wind weakening? Is this at all related to the piling up of warm water in the western equatorial Pacific, or is that not related? Thanks!