{Back in November, El Ni\u00f1o reached a fever pitch, vaulting into the ranks of the strongest events on record and wreaking havoc on weather patterns around the world. Now it is beginning to wane as the ocean cools, so what comes next?}<\/p>\n
It’s possible that by next fall, the tropical Pacific Ocean could seesaw into a state that is roughly El Ni\u00f1o’s opposite, forecasters say. Called La Ni\u00f1a, this climate state comes with its own set of global impacts, including higher chances of a dry winter in drought-plagued California and warm, wet weather in Southeast Asia.<\/p>\n
But El Ni\u00f1os and La Ni\u00f1as are particularly difficult to predict at this time of year, so exactly what happens remains to be seen.<\/p>\n
Warm-Cool Cycle
\nEl Ni\u00f1o and La Ni\u00f1a are part of a cycle that runs over the course of three to seven years. While El Ni\u00f1o features warmer-than-normal ocean waters in the central and eastern tropical Pacific \u2014 much warmer in the case of this exceptional El Ni\u00f1o \u2014 La Ni\u00f1a features colder-than-normal waters in the same region.<\/p>\n
Those changes in ocean temperatures are accompanied by changes in the atmosphere: During El Ni\u00f1o, convection and rains shift eastward and the normal east-to-west trade winds weaken or even reverse, while during La Ni\u00f1a, the normal dry state of the eastern Pacific intensifies along with the trade winds. Those atmospheric effects set off a domino effect around the world that can shift normal weather patterns.<\/p>\n
This El Ni\u00f1o reached a peak in ocean temperatures in November and those waters have been cooling off ever since, following the normal progression. That decline means” it’s almost a certainty that [the tropical Pacific Ocean is] going to go back to neutral in about two months,” Anthony Barnston, chief forecaster at Columbia University’s International Research Institute for Climate and Society, said.<\/p>\n
What’s still up in the air is whether it stays neutral or continues to cool until it reaches a La Ni\u00f1a state.<\/p>\n
La Ni\u00f1a’s don’t always follow after El Ni\u00f1os, but seem more likely to do so after a strong El Ni\u00f1o, based on the historical record. That record is quite short, though, which makes it hard to draw firm conclusions from it.<\/p>\n
But the underlying physics of the El Ni\u00f1o cycle offers some reason to think that strong El Ni\u00f1os do tend to lead to La Ni\u00f1as.<\/p>\n
El Ni\u00f1os generate large-scale waves in the ocean (these aren’t like the waves that break on the water’s surface). One set, called Kelvin waves, travel from west to east and cause warming, enhancing the El Ni\u00f1o.<\/p>\n
The other, called Rossby waves, travel in the opposite direction until they reach Indonesia, where they bounce off the landmass and head back east. Eventually, the Rossby waves catch up to the El Ni\u00f1o and cause cooling, in something of an act of self-sabotage.<\/p>\n
“The El Ni\u00f1o sort of kills itself,” Barnston said.<\/p>\n
The stronger the El Ni\u00f1o, the stronger the Rossby waves it generates. If those waves are strong enough, they can not only kill off the El Ni\u00f1o, but “overshoot” in the other direction, driving the system towards a La Ni\u00f1a state, Barnston said.<\/p>\n
Current Cooling
\nThe Rossby waves usually disrupt the El Ni\u00f1o pattern about six months after it peaks, or, right about now. Indeed, forecasters have noted a cool down below the surface of the eastern tropical Pacific in recent weeks, though surface water temperatures are still firmly in El Ni\u00f1o territory. They will gradually follow the subsurface cool-off, though, likely reaching neutral territory by late spring.<\/p>\n
If a La Ni\u00f1a is in the offing, those waters should be cooling further by mid-summer, though, like El Ni\u00f1o, it wouldn’t peak until late fall or early winter.<\/p>\n
Right now Barnston puts the odds at slightly better than 50 percent that a La Ni\u00f1a does develop.<\/p>\n
What is very unlikely to happen is a return to El Ni\u00f1o conditions, which almost never occur in back-to-back years because of that self-sabotage mechanism. (It only tends to happen when there is an unusually late-developing El Ni\u00f1o that can then persist and peak again the following year.)<\/p>\n
La Ni\u00f1a, on the other hand, can last for two to three years because the large-scale waves it generates aren’t as well-defined. “It’s not equal and opposite to what you get during El Ni\u00f1o,” Barnston said, so La Ni\u00f1a doesn’t tend to undercut itself the way El Ni\u00f1o does.<\/p>\n
It’s far too early to tell how strong any La Ni\u00f1a that does develop might be, forecasters say.<\/p>\n
“It’s difficult to forecast strength of events. An added difficulty is that things change pretty quickly when an event is decaying \u2014 this is the time of year when the accuracy of forecasts is lower,” Catherine Ganter, a senior climatologist with Australia’s Bureau of Meteorology, said in an email.<\/p>\n
Barnston said they should have a better idea of the potential strength by August, possibly a bit sooner if there is a very sharp cool down in Pacific Ocean temperatures.<\/p>\n","protected":false},"excerpt":{"rendered":"
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