(John Kemp is a Reuters market analyst. The views expressed are
* Chart 1: tmsnrt.rs/2cGjeuX
* Chart 2: tmsnrt.rs/2cOC4vQ
* Chart 3: tmsnrt.rs/2cOBVc5
* Chart 4: tmsnrt.rs/2d3J7o7
By John Kemp
LONDON, Sept 21 Sea surface temperatures in the
central Pacific have been significantly below the seasonal
average for the last 10 weeks, consistent with the formation of
mild La Nina conditions.
But U.S. government forecasters have cut the probability of
La Nina occurring this winter to 36 percent, down from an
estimated probability of 76 percent at the time of their May
The U.S. government now predicts conditions this winter are
likely to be neutral, with neither La Nina or El Nino evident,
and puts this probability at 56 percent, up from 21 percent in
Surface temperatures in the central Pacific have cooled but
not as fast or as far as expected earlier in the year, which has
caused the forecast probability of La Nina developing to drop (tmsnrt.rs/2cOC4vQ).
Many other phenomena associated with La Nina are either
absent or only weakly present, which has also caused forecasters
to downgrade their predictions.
Some models indicate a borderline La Nina this winter. But
the consensus among U.S. forecasters is for neutral conditions
based on the lack of significant support from other indicators.
The National Oceanic and Atmospheric Administration dropped
its "La Nina watch" in September, having been on the lookout
since April, according to the agency's latest forecast ("El
Nino/Southern Oscillation diagnostic discussion", NOAA, Sept.
The term El Nino, meaning the boy, and referring to the
infant Jesus, was originally employed by fishermen to describe
an unusual warm southward current appearing each year in the
cold waters off Peru just after Christmas.
Every few years, the current is unusually intense and
reaches further south, bringing heavy rains along the normally
Years in which the warm current was unusually strong were
known historically as anos de abundancia, or years of abundance
("El Nino, La Nina, and the Southern Oscillation", Philander,
The existence of the seasonal El Nino warm current was
formally reported by scientists in the 1890s but it was not
until the 1950s and 1960s that it was realised the warm surface
waters extended far out into the ocean.
In fact, the exceptionally warm waters associated with anos
de abundancia in Peru extend as far as the date line and are
associated with unusually weak trade winds as well as heavy
rainfall in the central Pacific.
El Nino has come to describe the presence of unusually warm
waters throughout a stretch of the central and eastern Pacific
and the associated weather phenomena over a much wider area.
The U.S. government now defines El Nino as sea surface
temperatures at least half a degree Celsius warmer than the
seasonal average, persisting for at least three months, in a
specific region of the central-eastern Pacific.
The critical area of the ocean stretches from 120 degrees to
170 degrees west of Greenwich and straddles the equator from 5
degrees south to 5 degrees north.
Sea surface temperature anomalies in this area, known as
Nino region 3.4, are the most closely correlated with all the
other phenomena associated with El Nino.
Oceanographers later came to realise El Nino was the warm
phase of an irregular cycle of sea surface temperatures. The
cold phase came to be called La Nina, the girl.
La Nina, the opposite of El Nino, is associated with
colder-than-normal waters off the coast of Peru stretching
towards the date line and trade winds that are even stronger
The Southern Oscillation was discovered separately by
Gilbert Walker, working as director-general of the Indian
Meteorological Department in the early 20th century.
Walker noted how pressure changes over the Pacific and
Indian oceans were inversely correlated with one another as well
as linked to the occasional failure of the Indian monsoon.
Walker observed that when atmospheric pressure at sea level
was high in the Pacific it tended to be low in the Indian Ocean
and vice versa, a cycle he named the Southern Oscillation.
The pressure swing was historically measured at
observatories located at Darwin in Australia and Tahiti in the
Pacific, both slightly south of the equator, but chosen for
convenience because they are on land.
More modern measurements of the Southern Oscillation are
usually taken by comparing sea level pressures over an area of
Indonesia straddling the equator with one in the eastern Pacific
Jacob Bjerknes at the University of California first
proposed the link between El Nino and the Southern Oscillation
in 1969. The combined cycle is now normally termed El
Nino/Southern Oscillation, or ENSO.
Bjerknes suggested, and it was subsequently confirmed, that
dry air sinks over the cold waters of the eastern Pacific
(causing local high pressure) and flows west along the equator
in the phenomenon known as the trade winds.
The air is warmed and picks up moisture as it flows across
the ocean until it rises and forms rainclouds over the western
Pacific (resulting in lower pressure there).
Drier air then returns across the Pacific in the upper
atmosphere to sink again off the coast of Peru, completing a
cycle Bjerknes called the Walker Circulation (here).
By warming the waters off Peru, El Nino weakens the trade
winds and the Walker Circulation (here).
In contrast, the cold coastal waters of La Nina accelerate
the trade winds and strengthen the Walker Circulation (here).
The Walker Circulation is therefore dampened and amplified
by the difference in ocean temperatures between the eastern and
western Pacific associated with El Nino and La Nina ("The Walker
Cycle: ENSO's atmospheric buddy", NOAA, August 2014).
TRADE WINDS, OCEAN CURRENTS
Causality also flows the other way. El Nino and La Nina are
themselves initiated and amplified by the weakening and
strengthening of the trade winds and the Walker Circulation.
Trade winds carry warmer surface water from the coast of
Peru far out into the Pacific in a fast-flowing equatorial
current that can carry water towards the date line at speeds
exceeding 1 metre per second.
As the warmer surface waters are driven off by the trade
winds they expose and are replaced by an upwelling of colder
water from the deep ocean.
When the trade winds accelerate, as in La Nina, the waters
off the coast of Peru cool even more than normal and the cold
zone extends further out into the ocean. When the winds slacken,
as in El Nino, the waters off the coast of Peru become warmer.
The ocean cycle of El Nino and La Nina is coupled with the
atmospheric Walker Circulation and Southern Oscillation, and the
two interact closely with one another.
La Nina describes the opposite set of ocean-atmosphere
interactions, with stronger trade winds cooling the eastern
Pacific more than normal, which in turn drives stronger trade
POSITIVE FEEDBACK LOOPS
El Nino and La Nina are examples of positive feedback
mechanisms in the coupled ocean-atmosphere circulations.
El Nino/La Nina and the Southern Oscillation are instances
of a complex system in which small changes in either trade winds
or sea surface temperatures can trigger large climate changes
The sensitivity of ENSO to small variances in atmospheric or
oceanic conditions is one reason why the timing, strength and
development of El Nino/La Nina episodes are hard to forecast.
It also explains why the accuracy of predictions
deteriorates rapidly when the forecast horizon extends forward
by more than a few months, which is why NOAA has had to scale
back its La Nina prediction since May.
Although El Nino and La Nina are defined by reference to sea
surface temperature anomalies, they are associated with a host
of supportive changes in sea-level pressure, wind speed and
Sea surface temperature, sea level pressure, wind speed and
convection anomalies are all closely though not perfectly
correlated with each other ("Why are there so many ENSO
indexes?" NOAA, January 2015).
But El Nino/La Nina and the Southern Oscillation are not the
only influences on ocean and atmosphere circulations in the
Pacific, so positive feedback mechanisms can fail to develop or
subsequently break down.
In the current episode, the moderate cooling of waters in
the eastern Pacific has not so far been accompanied by a strong
and supportive pickup in pressure anomalies and trade winds (tmsnrt.rs/2d3J7o7).
Without a pickup in pressure anomalies and wind speeds, the
positive feedback mechanisms that drive La Nina are absent or
only weakly present.
The lack of positive feedback as shown in other indicators
of sea level pressures and wind speeds is why forecasters have
become much less confident about the emergence of La Nina during
the winter of 2016/17.
(Editing by Dale Hudson)