Say goodbye to El Niño, and hello to its less popular sibling, La Niña.

Tropical Pacific Ocean waters are cooling rapidly after record warmth during much of 2015 and 2016 so far, signaling an impending shift.

A new climate outlook released on Thursday puts the odds of a La Niña event developing in the tropical Pacific Ocean at 75 percent by the September through November period of this year. Forecasters' confidence in a developing La Niña event is high enough that the Climate Prediction Center (CPC) has issued a "La Niña watch."



SEE ALSO: How the ongoing El Nino compares to the mother of all El Nino events

La Niña conditions are said to exist when a specific region of the tropical Pacific Ocean has a three-month average temperature departure from average of at least 0.5 degrees Celsius, or 0.9 degrees Fahrenheit, below average. In other words, a La Niña is characterized by cooler-than-average ocean temperatures, whereas El Niño events are characterized by unusually warm waters.

Sea surface temperature anomalies in tropical Pacific Ocean. Image: Climate Prediction center

"It’s possible the transition from El Niño to La Niña will be quick, with forecasters slightly favoring La Niña developing this summer," wrote Emily Becker, a scientist at the Climate Prediction Center (CPC) in Maryland, in a blog post.

Like El Niño, La Niña can exert a significant influence on global weather patterns. If it kicks in early enough, it will likely allow for a more active hurricane season in the Atlantic Ocean compared to last year. La Niña events tend to suppress hurricane activity in the eastern tropical Pacific.

The new climate outlook prepared by the U.S. Climate Prediction Center and Columbia University's International Research Institute for Climate and Society (IRI), puts the odds of a La Niña conditions during the height of the hurricane season, from July through September, at 65 percent.

Upper ocean heat anomalies in tropical Pacific Ocean. Image: Climate prediction center

La Niña events are also tied to more severe tornado seasons in the southern U.S. A moderate La Niña was present during the deadly 2011 tornado season, when tornadoes killed 553 people, mostly in the south central states.

Such events also tend to damper global average surface temperatures somewhat, and may put an end to the record-long string of warmest months on record. Through March, that stood at 11 months, based on data from the National Oceanic and Atmospheric Administration (NOAA).

"Most computer models are predicting El Niño conditions will come to an end in the early summer, and that sea surface temperatures will continue to drop, potentially passing the La Niña threshold (0.5°C below average) sometime in the summer. Some areas of near- or below-average sea surface temperatures have already appeared in the eastern Pacific," Becker wrote.



In addition to computer models and trends in sea surface temperatures, another strong sign of an impending La Niña is the increasingly abundant supply of cooler-than-average waters under the surface of the Pacific.

"This large pool of cool water stretches across the entire Pacific, along the Equator, and extends down from just below the surface to around 500 feet," Becker wrote.

Not every El Ni ño event is followed by a swing to La Ni ña conditions. However, some strong El Ni ño's have been succeeded by major La Ni ña events, as occurred after the 1997-98 El Ni ño.

The 2015-16 El Niño was on par with, if not more intense than, the 1997-98 event, suggesting that the upcoming La Niña may be significant. However, the CPC states that "there is clear uncertainty over the timing and intensity of a potential La Niña."

According to the CPC, there have been 14 La Niña events since 1950, while El Niño events have occurred 23 times during the same period.