(John Kemp is a Reuters market analyst. The views expressed are
* Chart 1: tmsnrt.rs/2hn3rOW
* Chart 2: tmsnrt.rs/2hn39HL
* Chart 3: tmsnrt.rs/2hn4uOV
By John Kemp
LONDON, Dec 20 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
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
La Nina has contributed to unusually warm weather over the
southern United States since July, according to U.S. government
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
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
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
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
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)