(Reuters) - (The opinions expressed here are those of the author, a columnist for Reuters)

Many have doubted forecasts calling for the onset of the first La Niña in almost five years, believing that its failure to materialize in convincing fashion last summer - as originally predicted - means that it may be off the table for 2016-17.

But in recent weeks, the oceans and atmosphere have been pulling everything into place to facilitate a potentially stronger La Niña than previously thought, so those who follow commodities markets may want to take a second look.

Last Thursday, the U.S. Climate Prediction Center reissued the La Niña watch that was removed in early September. The watch indicates that conditions are favorable for the phenomenon’s development within the next six months.

El Niño-Southern Oscillation, with its cool phase La Niña and warm phase El Niño, is one of the most reliable long-term indicators for global climate. The ENSO phases can have drastically different impacts on commodities worldwide - from energy use to grain yields.

La Niña, characterized by cooler-than-normal surface waters in the equatorial Pacific Ocean, has not officially been in place since the first quarter of 2012. But recently, cooling sea surface temperatures in the key Niño 3.4 region have touched the levels of early 2012 (reut.rs/2e2lI7y).

CPC now says there is a 70 percent chance that La Niña will develop during the Northern Hemisphere autumn 2016 and there is a 55 percent chance it will persist during winter 2016-17. This is up from last month’s forecast of a 40 to 45 percent chance of development.

The environment has not fully committed to the La Niña cycle for Northern Hemispheric winter as the sea surface temperature anomalies have at times hesitated to maintain the downward plunge.

But there are larger-scale features in the oceans and atmosphere that appear consistent with soon welcoming a full-on La Niña, rather than a borderline event or a neutral ENSO.

PACIFIC COOPERATION

Over the past few months, there has been a considerable shift in the Pacific Decadal Oscillation or PDO, a large-scale, long-term climate variability in the North Pacific Ocean that is closely associated with ENSO cycles.

A positive PDO index is associated with anomalously cool waters and below-average sea level pressure in the interior North Pacific, while a negative PDO index is the inverse scenario. Positive PDO is often correlated with El Niño and negative PDO typically coincides with La Niña.

The PDO index has been unusually positive over the past two years, which is not surprising given the record-strong El Niño observed late last year. Since 1900, the first six months of 2016 and 2015 ranked third and fourth for highest average PDO index, behind 1941 and 1940 (reut.rs/2e2gB7l).

But the index has been on a steep decline over the past couple of months, and September’s reading of 0.45 was the lowest value since February 2014. PDO has not been negative since December 2013.

This substantial shift in overall oceanic-atmospheric conditions certainly makes a good case for La Niña, so a close watch on PDO trends over the next few months may hint at whether La Niña conditions could stick around, or whether neutral ENSO or a reversal back to El Niño is possible in 2017.

SHORTER-TERM TRENDS

Another climate indicator, the Madden-Julian Oscillation or MJO, has also been paving the path for La Niña via the trade winds, which are among the most important elements in determining ENSO phase.

MJO is a cluster of thunderstorms that circles the globe every 30 to 90 days and is instrumental in bringing rainfall to more drought-prone regions in the tropics. In recent weeks, the MJO cluster has been very prominent in the Western Pacific Ocean.

This is crucial for the development of La Niña because the persistent low pressure from the storm activity in the Western Pacific is highly conducive to strengthening the trade winds. The stronger the trade winds in this region, the more La Niña is favored.

The equatorial trade winds have become much more consistently favorable for La Niña than they had been just a couple months ago. While the cooler waters required for La Niña have existed within the ocean for some time now, the trade winds did not allow them to surface until late June (reut.rs/2eBNUxR).

Also highly influential in the ENSO cycle is the Southern Oscillation Index, or SOI, which is a measure of the pressure tendency between Tahiti and Darwin, Australia. A positive SOI index is supportive of La Niña and negative values favor El Niño.

While SOI and the trade winds often go hand-in-hand, they do not always follow the same tendency. This was particularly the case in late 2014 when conditions seemed to overwhelmingly favor development of a strong El Niño, but since SOI and the trade winds were out of sync, El Niño was not able to pick up notable strength until mid-2015 (reut.rs/2e2lxcv).

The trade winds and SOI are currently aligned in favor of La Niña to the greatest degree in three years. Together with MJO, SOI and the trades operate on a shorter time scale than features like PDO, but all three will play a key role in the degree to which La Niña arises in the Pacific this year.

IMPACTS

For the upcoming months, La Niña favors colder winters in the Northern United States and drier conditions across the southern tier. During the same time, the growing season for corn and soybeans in Argentina and southern Brazil can become unfavorably dry during a La Niña phase.

Parts of Southeast Asia that dried out during last year’s El Niño may be subject to heavier bouts of rain over the next few months as La Niña tends to introduce a wetter pattern to the Western Pacific Rim. Australia is included on this list, though the Aussies and several areas of Southeast Asia were saved last year from El Niño’s typical parching by an increase in MJO activity.

Heading into Northern Hemispheric spring and summer, Atlantic hurricane activity generally increases with La Niña and Eastern Pacific hurricanes are fewer. El Niño often suppresses Atlantic hurricane activity while amplifying it in the Eastern Pacific.

But there is a great debate within the meteorological community over whether La Niña and El Niño truly impact summer weather in the United States. Data generally suggests that La Niña will bring warmer summers to the main agricultural belt including the Midwest while El Niño is associated with wetter, possibly even cooler summers.

At this early stage, examining similar years may be the most feasible method when considering the possible impact of ENSO on the 2017 U.S. corn and soybean crops. Some preliminary U.S. summer analogs based on the recent and expected course of ENSO might include 1984, 1996, 2008 and 2011, all of which on average recorded moderately below-trend corn and soybean yields.

And in the longer term, history has shown that the stronger a La Niña episode becomes, the more likely it is to stick around in the following years. So there may be more at stake concerning the upcoming La Niña than just the impacts on the current season.

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Graphic- Weekly sea surface temperature anomalies (°C) in the Niño 3.4 region reut.rs/2e2lI7y

Graphic- Pacific Decadal Oscillation vs. El Nino-Southern Oscillation, monthly since 2013 reut.rs/2e2gB7l

Graphic- Equatorial trade wind anomalies: observed and forecasted reut.rs/2eBNUxR

Graphic- Southern Oscillation Index versus Western Pacific Trade Wind Index, 2013-2016 reut.rs/2e2lxcv

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