(Repeats column first published on Dec. 20 with no changes)
* Chart 1: tmsnrt.rs/2hn3rOW
* Chart 2: tmsnrt.rs/2hn39HL
* Chart 3: tmsnrt.rs/2hn4uOV
By John Kemp
LONDON, Dec 20 (Reuters) - Weak La Nina conditions have persisted over the Pacific Ocean in the final quarter of 2016 but are likely to fade during the first three months of 2017.
La Nina conditions have been reported since July with sea surface temperatures in the central-eastern Pacific colder than normal.
But the sea surface temperature anomaly has barely met the threshold to qualify as a La Nina episode and U.S. government forecasters predict La Nina will give way to more neutral conditions during early in 2017. (tmsnrt.rs/2hn3rOW)
La Nina episodes are normally associated with a warm and dry winter across the southern United States while conditions are colder and wetter over the northern part of the country.
But correlations between the El Nino-Southern Oscillation (ENSO) cycle and winter weather conditions in the United States are not particularly strong.
In general, a strong El Nino or La Nina episode is more likely to produce a significant weather impact in North America.
Weak El Nino and La Nina episodes are less likely to have an observable effect as there are too many other influences on weather conditions.
La Nina has contributed to unusually warm weather over the southern United States since July, according to U.S. government forecasters.
But there have been fewer signs of consistent cold weather over the northern part of the country, where temperatures have mostly been above the long-term average.
The weakness of La Nina in 2016 is probably one reason why heating demand across the United States has been so low this heating season to date.
La Nina is the cold phase of the ENSO cycle of which El Nino is the better-known warm phase.
ENSO is an irregular cycle with an average periodicity of about 3 to 4 years but with repetitions varying from about 2 to 7 or even 10 years ("El Nino, La Nina and the Southern Oscillation", Philander, 1990).
Both El Nino and La Nina episodes vary considerably in terms of their intensity, duration and frequency, so although the general cycle is well understood prediction remains challenging.
La Nina episodes are characterised by a welling up of cold waters off the coast of Peru, carried into the central Pacific by the equatorial current.
During a La Nina episode, the trade winds are stronger than normal, accelerating the equatorial current and carrying cold water further west than usual.
The strong trade winds are themselves caused by a bigger than normal difference in atmospheric pressure between the eastern and western Pacific.
The pressure differential causes the trade winds to accelerate as well as increasing the return flow of air in the upper atmosphere.
The cooling of surface waters across the central and eastern Pacific in turn helps fuel the pressure differentials which drive the trade winds.
So the ocean and atmosphere circulations become locked in phase and reinforce one another in a classic example of positive feedback.
El Nino, the warm phase of ENSO, is characterised by the opposite phenomena to La Nina: warmer than normal waters off Peru and into the Pacific, weak trade winds and lower than average pressure differences.
The positive feedback mechanisms which sustain La Nina and El Nino are usually weakest between March and May, becoming progressively stronger between June and August and peaking towards the end of the year.
So the shift from La Nina to El Nino or vice-versa is most likely to occur in March and April each year ("Teleconnections linking worldwide climate anomalies", Glantz et al, 1991).
The unusually strong El Nino of 2015 broke down during the early months of 2016, as expected, and then transitioned towards La Nina during May and June.
But La Nina developed relatively late, rather than during April-May, and the cold cycle conditions never became particularly pronounced.
Sea surface temperatures have been about minus 0.5 degrees Celsius colder than normal since June, only just meeting the threshold for declaring La Nina to be present.
The sea surface temperature anomaly in the central-eastern Pacific never deepened as would be expected during a strong La Nina episode.
On the atmospheric side, pressure differentials have strengthened after the El Nino of 2015, but have never been much above their long-term average (tmsnrt.rs/2hn39HL).
Trade winds have also intensified following the end of El Nino, but again never much above the long-run average (tmsnrt.rs/2hn4uOV).
The result has been a marginal La Nina episode which already shows signs of having passed its peak ("La Nina forecast downgraded as trade winds remain moderate", Reuters, Sept. 21).
U.S. government forecasters predict La Nina conditions will prevail between December and February with a forecast probability of 51 percent.
But the probability of La Nina persisting will decline to 33 percent between February and April and then to just 24 percent between March and May.
Neutral ENSO conditions (not meeting the threshold for either La Nina or El Nino) are the most likely to prevail from the first quarter of 2017.
The probability of El Nino will increase steadily especially from March onwards ("Official probabilistic ENSO forecast", Climate Prediction Center, Dec 2016). (Editing by David Clarke)