El Niño is the frustrating guest that RSVPs "maybe" to your big climate party.
After much hype during the spring about the impending onset of a moderate to strong El Niño event in the tropical Pacific Ocean, which would have had significant ripple effects on weather patterns worldwide, climate forecasters now favor a weak El Niño event beginning in the late fall.
In their latest El Niño forecast update, issued Thursday morning, the Climate Prediction Center, which is part of the National Oceanic and Atmospheric Administration (NOAA), put the odds of an El Niño occurring during the October through January time period at 60 to 65%, which is down slightly from the 65% figure given in early August. (The odds for the November through January period stayed the same.) Previously, the odds of an El Niño had been as high as 80%.
In addition, forecasters said the odds are highly skewed in favor of a weak El Niño, which is unwelcome news for California, where parched residents have been hoping that a major El Niño would supercharge their rainy season and help relieve the multibillion dollar drought that is affecting the state. In the past, some strong El Niño events, though not all, have led to increased rainfall along the West Coast.
In California, 39.3 million people are living in areas that are in either "extreme" or "exceptional" drought, the two most severe categories, according to the U.S. Drought Monitor.
The shift in expectations largely has to do with the interactions between the waters of the tropical Pacific Ocean and the atmosphere above. For an El Niño to occur, the air and sea need to both play ball. For example, trade winds usually blow from east to west across the tropical Pacific, which has the effect of upwelling cold, deep water to the surface in the eastern Pacific, and piling up warmer water in the west, near Indonesia and Papua New Guinea.
However, westerly wind bursts that move from west to east can help push pulses of warm water eastward, from near Indonesia to the eastern tropical Pacific, through a mechanism known as an underwater Kelvin wave. Once they reach the eastern Pacific, these waves, like huge bubbles of warm water, rise to the surface, causing sea surface temperatures to increase.
El Niño events are characterized by warmer than average sea surface temperatures in the central and eastern equatorial Pacific, and they add heat to the atmosphere, thereby warming global average temperatures. Already, in June, global ocean temperatures were the warmest they had ever been since recordkeeping began in 1880, thanks in large part to above average sea surface temperatures in the Pacific.
El Niño events typically occur once every three to seven years and can also alter weather patterns around the world, causing droughts and floods from the West Coast of the U.S. to Australia and eastern Africa.
In the spring, there was an unusually strong series of Kelvin waves, triggered by a temporary reversal of the trade winds. But then the air and sea went their separate ways for a while, like a couple that couldn't reconcile their differences after dating for a short time.
“It was pretty apparent when we were seeing fairly warm sea surface temperatures across the tropical pacific, and across the board there was a fairly lackluster atmospheric response,” according to Michelle L'Heureux, a scientist with the Climate Prediction Center in Maryland.
Now, though, the trade winds and other indicators are "a little more of a mixed bag," she said, which may indicate the emergence of an El Niño in the next few months.
There have also been indications of another eastward-moving Kelvin wave, she said, which could soon increase ocean temperatures in the eastern Pacific again.
L'Heureux told Mashable that most of the computer models that forecasters use to help predict El Niño events had been predicting an earlier onset of an event, which has not been the case. Now, those models are showing a later onset, and most still favor the occurrence of an El Niño.
“We’re not completely clear on what season it is going to emerge in,” she said. “The onset time is being progressively pushed back.”
Interestingly, she said that in the past 10 years, many El Niño and La Niña events have kicked off later in the year, in either late fall or early winter, compared to previous decades. However, it is not yet clear why that is, and whether it is linked to natural climate variability, manmade climate change, or both.
The intensity of an El Niño, if indeed one occurs at all, will help determine what the impacts will be. According to L'Heureux, for weak El Niños, the impacts are harder to predict and more subtle than they are with strong El Niños.
L'Heureux said that based on observations and computer model forecasts, it's "very unlikely" that a 2014-2015 El Niño would be a strong event. “Our consensus forecast is that folks should at least expect a weak event," she said.
The El Niño forecast from the Climate Prediction Center says: "At this time, the consensus of forecasters expects El Niño to emerge during September-October and to peak at weak strength during the late fall and early winter."