(John Kemp is a Reuters market analyst. The views expressed are
* Chart 1: tmsnrt.rs/2pBM2Zq
* Chart 2: tmsnrt.rs/2p0HPiV
* Chart 3: tmsnrt.rs/2pJdQJt
* Chart 4: tmsnrt.rs/2pc7y4i
By John Kemp
LONDON, May 4 El Nino conditions are developing
across the Pacific with an increasing probability that a
full-fledged El Nino episode will occur during the second half
Pacific equatorial winds have slackened since the start of
the year and a characteristic tongue of warm water has begun to
form stretching from Peru towards the international dateline.
Both are consistent with the development of El Nino and are
likely to strengthen during the second and third quarters.
The U.S. government’s Climate Prediction Center (CPC) last
month forecast El Nino conditions would prevail by the end of
the northern hemisphere summer, but put the probability at only
Most El Nino indicators have strengthened since then so the
probability is likely to be revised higher when the CPC issues
its next forecast later in May.
But the strength of any future El Nino remains highly
uncertain as does its impact on temperatures and precipitation
across North America this winter.
El Nino and the Southern Oscillation (ENSO) describe closely
linked oceanic and atmospheric processes that stretch across the
Pacific and the neighbouring maritime continent of South East
The oceanic component sees cold water well up from the deep
ocean off the coast of Peru and move west across the Pacific
carried on the surface equatorial current.
There is a return flow of warm water eastwards across the
Pacific on countercurrents to the north and south of the equator
and also on an equatorial undercurrent deeper in the ocean.
The atmospheric component sees warm moist air rise over the
maritime continent and western Pacific, then flow east through
the upper atmosphere towards South America.
The air cools and sinks over the colder waters off South
America, before returning towards Asia in a steady westward flow
known to mariners as the trade winds.
The oceanic and atmospheric circulations are coupled, with
the equatorial winds helping drive the surface equatorial
current, and sea surface temperature differentials reinforcing
the rising and falling air columns.
But the strength of the circulations and the degree of
coupling varies at seasonal, annual and inter-annual scales.
When the circulations are particularly strong and tightly
coupled, the equatorial winds accelerate, the equatorial current
picks up and colder than average water is carried further from
the Peru coast towards the dateline.
When the circulations are weak and uncoupled, the winds
slacken, the current slows and warmer than normal water extends
from Peru to the dateline.
The stronger, cooler phase of this cycle is termed La Nina
while the weaker, warmer phase of the cycle is El Nino (“El
Nino, La Nina and the Southern Oscillation”, Philander, 1990).
ENSO cycles back and forth between strong and weak phases
with an average periodicity of 3 to 4 years but with repetitions
varying from as little as 2 years to 7 or even 10 years.
ENSO effects tend to be weakest during March and April when
the cycle is most likely to start transitioning from El Nino to
La Nina or vice versa.
Effects tend to strengthen as the year progresses and peak
between October and January, when El Nino or La Nina becomes
The atmospheric and oceanic circulations that lie at the
heart of ENSO involve very large thermal masses of air and water
which means they exhibit a lot of short-term persistence.
The short-term stability (especially the thermal stability
of the ocean) makes the ENSO cycle somewhat forecastable.
The trickiest time of year to make forecasts is during the
first quarter of the year when ENSO effects are weakening and
the possibility of a phase transition (from El Nino to La Nina
or vice versa) is greatest.
By the second and third quarter, the oceanic and atmospheric
circulations are likely to become locked in either a strong/cool
or a weak/warm phase which makes forecasting to the end of the
year more accurate.
While forecasters have had a fair amount of success in
predicting phase shifts in ENSO, forecasting the strength of an
El Nino or La Nina episode has proved much harder.
El Nino and La Nina episodes vary enormously in their
intensity. The winter of 2015/16 saw an unusually intense El
Nino but it was followed by a very weak La Nina episode in
Forecasts for the development of the ENSO cycle and its
intensity are based on observations of the strength of the
oceanic and atmospheric circulations.
Since the start of 2017, and especially since the start of
March, all these indicators have pointed to a weakening of
existing La Nina conditions and the shift towards a potential El
Sea surface temperatures in the central-eastern Pacific, an
area known as Nino region 3.4, have warmed much more quickly
than normal since January and are now above the long-term
seasonal average (tmsnrt.rs/2p0HPiV).
More recently, pressure differentials between Indonesia and
the eastern Pacific, measured by the Equatorial Southern
Oscillation Index, have started to weaken, and the equatorial
winds have slackened (tmsnrt.rs/2pJdQJt and
The components for El Nino are all now in place and ENSO is
clearly transitioning from a strong/cool to a weak/warm phase.
But as the signals are still weak it remains uncertain if
they will strengthen sufficiently to qualify as a full-fledged
El Nino episode later in the year.
ENSO has a major impact on temperatures and rainfall around
the Pacific and Indian basins and a smaller impact on the
But the main effects are felt within the tropics, with a
much more limited effect on weather outside the tropics in the
Researchers have found links between ENSO and temperatures
and precipitation in some parts of North America owing to its
impact on the position of the Pacific and polar jet streams.
ENSO’s impact on the United States tends to be regional
rather than national. Higher temperatures in some areas are
offset by lower ones in others (“North American precipitation
and temperature patterns associated with the El Nino/Southern
Oscillation”, Ropelewski et al, 1986).
El Nino tends to be associated with a warmer than normal
winter in the Pacific North West of the United States and
Canada, and a colder and wetter winter in the U.S. Southwest and
La Nina tends to bring colder and wetter weather to the
Pacific Northwest but dry and warm conditions to the U.S.
Southeast and U.S. Southwest.
ENSO is not the only atmospheric circulation that drives
temperature and precipitation across North America so the impact
tends to be felt clearly when El Nino or La Nina are especially
Temperature and precipitation in North America and Europe
are more driven by the Arctic Oscillation/North Atlantic
Oscillation (AO/NAO) and to some extent its interaction with
ENSO impacts tend to be visible only when ENSO is
particularly strong (for example the strong El Nino of 2015/16)
or when ENSO and other circulations reinforce rather than
counter one another.
If El Nino develops later this year it should bring warmer
temperatures to Washington state and neighbouring areas but
colder, wetter weather to a belt of southern states stretching
from California through Texas to Florida.
The exact impact depends critically on the strength of the
El Nino phase and its interaction with nearer atmospheric and
oceanic circulations, which remain impossible to predict this
(Editing by Mark Potter)