What exactly is the El Nino phenomenon? And how does it cause such severe weather conditions?

The effect is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific. During an El Nino event, the warm water lingers in the region, rather than flowing westward, resulting in higher than average amounts of rainfall.

Meanwhile, over in the west of the tropical Pacific, the lack of warm sea currents causes severe drought. El Nino is also accompanied with regional air pressure changes, which can affect regions beyond the tropical Pacific.