Three national agencies have confirmed that the natural phenomenon has arrived, but not in time to bring much-needed rains in the West.

A warm band of water has appeared in the equatorial Pacific Ocean, heralding the return of El Niño. The condition, which can bring unusual weather patterns ranging from drought to floods, will probably last at least through the end of the year, the National Weather Service’s Climate Prediction Center announced.

The El Niño Southern Oscillation is a natural cycle of temperature oscillations in the tropical eastern Pacific. Water temperatures go from warmer to cooler than average, and an El Niño year marks the warmest phase. Ocean temperatures and global weather patterns are linked, so each peak in this cycle is accompanied by extreme weather in certain parts of the world.

Past El Niños have been blamed for devastating floods and droughts, landslides, fishery collapses and crop losses. The most severe El Niño in recent history, which occurred in 1997 and 1998, caused between $10 billion and $25 billion in damages in the United States.

Last month, the Climate Prediction Center noted the appearance of the features of a weak El Niño, such as some surface water warming and changes in wind patterns. Trade winds had become more westerly and upper-level winds had weakened. Those conditions have strengthened and now reflect a “weak to moderate” El Niño, according to the center’s most recent alert. Government meteorologists in Japan and Australia have concurred that 2015 is an El Niño year.

“The onset of El Niño in Australia in 2015 is a little earlier than usual. Typically El Niño events commence between June and November,” Neil Plummer at the Australian Bureau of Meteorology noted in a statement.

Scientists expect this year’s El Niño to strengthen, but they do not yet know how strong it will get or how its effects will play out around the globe. There are trends in El Niño years: In local summer, much of Southeast Asia and India tend to dry out, while mountain ranges in the U.S. West usually see more wintertime rain and snow. But these effects do not necessarily happen during every El Niño event. Australia has experienced widespread drought in 17 of the last 26 events, but this year, warmer-than-average Indian Ocean temperatures may dominate the region, bringing rain instead of dry weather.

In the United States, El Niño’s impacts occur mostly in winter. The northern part of the country usually gets warmer, and the Ohio Valley gets drier. The southern parts of the United States, from California to Florida, tend to receive more rain. This last bit of the pattern has led some people to hope that El Niño could relieve the California drought.

But with El Niño’s May arrival, it is too late to help California. The drought is expected to persist or worsen over the next few months, and even a strong El Niño would have no effect unless it lasted through the upcoming winter. Should conditions in the Pacific continue or even strengthen, El Niño could bring the rains next winter that would offer some relief to parched Californians.

Source - http://www.smithsonianmag.com/