El Niño 2014: What it is, when it's coming, and what to expect

El Niño, a tropical-climate phenomenon with a global reach and following, is stirring in the Pacific, although forecasters aren't ready to pronouncing it awake just yet.

It's warming effect on Earth's climate can lower winter heating bills in some regions and reduce the formation and growth of Atlantic hurricanes. But it also alters rainfall patterns in ways that increase the risk of floods in some areas and drought in others.

The '97-'98 El Niño, the strongest on record, is estimated to have caused $35 billion to $45 billion in damage globally and 23,000 fatalities.

Here’s a look at what to expect this time:

1. What is El Niño and how does it work?

Marcio Jose Sanchez/AP
Visitors get a view of the Pacific Ocean from an overlook at Pigeon Point Lighthouse on June 6, 2014, in Pescadero, Calif.

El Niño occurs when easterly trade winds in the tropical Pacific relax – even reverse – to allow a vast pool of warm water piled up in the western tropical Pacific to move east until it reaches the west coast of Central and South America, leading to higher-than-normal sea-surface temperatures across the equatorial Pacific.

As the ocean releases its heat and moisture to the atmosphere, intense thunderstorms once cooped up over the western Pacific spread along the equator as well. The cumulative effect of this activity changes large-scale circulation patterns at higher latitudes, altering storm tracks that change the typical distribution of rain and snowfall, as well seasonal temperatures.

La Niña, El Niño's sibling, throws the process into reverse, bringing cooler-than-normal sea-surface temperatures to much of the tropical Pacific. They form a cycle known as the El Niño-Southern Oscillation, or ENSO. El Niños occur on average every four to seven years and can last up to two years.

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