(Reuters) - According to the latest forecasts, 2016/17 should be stamped in history as an official La Niña season. And although its relative weakness has somewhat downplayed its existence, La Niña is already halfway through its run.
La Niña is the cool phase of the El Niño-Southern Oscillation, which is very closely monitored by commodity markets as ENSO is one of the most reliable long-term indicators of weather patterns on the global scale.
In its monthly ENSO discussion last week, the U.S. Climate Prediction Center issued a “La Niña Advisory,” its first official recognition of the phenomenon’s presence. The agency left the probability for weak La Niña to persist through the Northern Hemispheric winter months unchanged at 55 percent.
Weekly sea surface temperatures in the key Niño 3.4 region have been hovering in La Niña territory since July, though it has taken the surrounding waters of the equatorial Pacific Ocean a little longer to cool down (reut.rs/2geLt40).
Other than just saying “time flies,” we can perhaps best explain how La Niña’s expected life cycle is already half over by examining its exact definition.
According to the National Oceanic and Atmospheric Administration, “La Niña is a phenomenon in the equatorial Pacific Ocean characterized by a five consecutive 3-month running mean of sea surface temperature anomalies in the Niño 3.4 region that is below the threshold of minus 0.5 C.”
This may sound like a mouthful, but the most important takeaway is that NOAA bases the requirements for La Niña and its warm counterpart El Niño on three-month averages rather than the monthly data itself in order to better isolate ENSO-related variability.
This three-month mean is known as the Oceanic Niño Index (ONI), which is the official data that classifies an ENSO event.
So far in the 2016/17 cycle, two of the five ONI readings that are required for La Niña are already in the book. At the conclusion of November, a third La Niña-qualifying ONI reading is nearly guaranteed (reut.rs/2fWWJzv).
This means that the ONI for the October through December and November through January periods must also come in equal to or less than minus 0.5 degree C, and this is also predicted to occur.
By the first week of February, we will know whether the ongoing event will officially join the ranks of La Niñas past.
The forecast for La Niña to wind down by the time the Northern Hemisphere enters spring has been pretty consistent and holds true in this month’s update.
The monthly models from CPC and the International Research Institute for Climate and Society predict that neutral conditions will finally prevail for the first full quarter of 2017, though there is a good deal of uncertainty moving forward (tmsnrt.rs/2fWYmxc).
Models now place greater odds on the return of El Niño conditions toward the middle of 2017 than on a comeback of La Niña. For the June through August period, the chances of El Niño stand at 30 percent and La Niña at 15 percent.
It is hard to place bets on either cycle with such low odds for both, though historically, the resurfacing of La Niña would be the much more likely outcome.
When considering officially classified ENSO cycles, there is only one instance since 1950 in which three consecutive years featured the pattern El Niño-La Niña-El Niño, which would be the case if El Niño comes back for the 2017/18 cycle. This occurred between 1963 and 1967, and the pattern was borderline between 2004 and 2007.
But in eight of the El Niño-turned-La Niña years, the third year has been followed by another La Niña or a negative ENSO phase, making a return to El Niño next year statistically unlikely.
Thus far, the 2016/17 ONI most closely resembles 1964/65 and 1983/84. The former returned to El Niño in 1965/66 and the latter turned even more negative in 1984/85 (reut.rs/2fWYTyZ).
(The opinions expressed here are those of the author, a market analyst for Reuters)
Reporting by Karen Braun in Chicago; Editing by Matthew Lewis