CHICAGO (Reuters) - The potential for the massive El Niño to transition into La Niña later in the year is one of the hottest topics in commodities markets right now.
These fluctuations of sea surface temperatures in the tropical Pacific Ocean can have drastically different impacts on global weather depending on which phase is present - El Niño, the warm phase, or La Niña, the cool phase. They are one of the few clues to seasonal weather patterns several months or even years in advance.
The short question-and-answer session would look like this: Are we headed for La Niña toward the end of 2016? Looks that way. Will it be a big one? Not sure.
The answers may help dictate whether drought is likely in South America, winter will be cold in the United States or abundant rains will return to Southeast Asia, among other things.
Although strength is yet to be determined, the progression of certain atmospheric and oceanic variables will provide clues on the possible entry to La Niña. Particular insight will be offered by the timing of these events, as it might be the difference between strong La Niña and nothing at all.
It seems unlikely that we will stay in El Niño, though. Of the 14 El Niño events since 1950, excluding this year, only one of them remained El Niño into the following year, while three others lingered in neutral to weak El Niño territory. So the odds favor La Niña, but in meteorology, odds alone are not enough.
A La Niña environment has already begun to develop. Cooler waters are building beneath the surface in the Pacific Ocean and El Niño-supporting trade winds have lessened. But sea surface temperatures, or SSTs, in the defining region of the Pacific remain very warm, so we are still amid a strong El Niño event.
It is helpful to look for historical instances in which El Niño turned into La Niña through the course of a year. This has occurred only a handful of times since 1982, but there are enough similarities among these analogs that we can use them to inform this year’s likely outcome.
Selected analog years suggest that huge dropoffs of SST anomalies into negative, La Niña-defining territory are likely to take place between April and July. These analogs also suggest that when the SST anomalies cross into negative territory later than June, a weaker La Niña event is likely to follow (tmsnrt.rs/1UkgzEC).
Although the journey to La Niña has already begun, there are still some variables that need to fall into place in order to lock in this forecast solidly for the end of 2016.
El Niño decay is unlikely to proceed without the shutdown of its two key mechanisms: strongly reversed trade winds in the western Pacific Ocean and net warmth below the ocean’s surface. Check, and check.
One of the first anti-El Niño signals was the abrupt strengthening of western Pacific trade winds back to near-normal levels last November. It is probably no mistake then that El Niño reached maximum strength that month (tmsnrt.rs/1UkhtB0).
But this has not weakened El Niño as much as it would seem. Localized but strong westerly bursts of wind over the central and eastern Pacific – the Niño region – have propped up SSTs in recent weeks, though the wind bursts have been less frequent since early March.
Enter the ocean temperature anomalies. Water temperatures just below the surface across the entire Pacific Ocean have turned net cool, and this massive, cold blob is now lurking below the surface waiting for its chance to turn up. The colder the anomaly becomes, the bigger the potential for La Niña becomes (tmsnrt.rs/1Lt5VZC) (tmsnrt.rs/1UkhRzs).
The cold blob can surface in the Niño region with the help of the trade winds. The winds must continue to strengthen, and the pace at which they do so will determine just how soon we might enter La Niña.
If the cold blob can keep losing heat at the same pace as recently, other similar years seem to suggest that Niño 3.4 SST anomalies are likely to cross into negative territory one to two months after western trade winds begin their big upward push. This was demonstrated well in 1998 and 2010, both of which resulted in very strong La Niña events.
The trade winds have been somewhat mixed in strength this month, though, so we may have to wait until April at the earliest to see a large strengthening in the winds. That means that we could see SST anomalies turn negative as early as June, but there are other factors that could hold this back.
Although the cold blob is growing and the trade winds are strengthening, pressure tendencies – the Southern Oscillation portion of ENSO – still highly favor El Niño.
The very warm ocean waters over the past year have led to the persistence of lower air pressure near Tahiti relative to Darwin, Australia. This ultimately helps reverse the trade winds and sustain El Niño, and is reflected by a highly negative Southern Oscillation Index, or SOI.
But the juice could be running out. Over the past few weeks, the cold blob has surfaced in areas of the Pacific Ocean outside the immediate influence of Niño-region trade winds. The latest SST data shows an expanding cold pool that, if it should close in on Tahiti, may soon and quickly reverse the trend in SOI to favor La Niña (tmsnrt.rs/1Lt66E9).
SOI tends to fluctuate a lot, even in the midst of a strong El Niño or La Niña event, but there is a clear separation between the stronger La Niña years and the weaker ones. In the stronger years (1988, 1998, 2010), considerably higher SOI values are already in place by July, while the weaker years tend to bounce around for a bit longer (1983, 1995, 2005, 2007) (tmsnrt.rs/1Lt627j).
Another ENSO-supporting variable is the outgoing longwave radiation, or OLR, which is a proxy for thunderstorm activity in the central equatorial Pacific Ocean. Because warmer waters favor atmospheric convection, El Niño is often associated with negative OLR anomalies. This implies an increased presence of towering storm clouds that block solar radiation from escaping back into space.
Last month’s OLR anomalies were record negative for the month of February, but like SOI, we would expect these anomalies to start turning positive at some point in the next couple of months in order to assist with the wind-down of El Niño (tmsnrt.rs/1UkhtAY).
If we do not see positively persisting SOI and OLR anomalies by July or August, then the potential impending La Niña may be weaker and/or delayed, all other factors aside.
But history would suggest that a late-forming La Niña – after about August – does not usually go on to become a strong La Niña, even a few months down the road. So if we do not see convincingly negative SSTs by September then we may be headed for either weak La Niña, or possibly “La Nada.”
Editing by Matthew Lewis
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