(John Kemp is a Reuters market analyst. The views expressed are his own)
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LONDON, Sept 5 (Reuters) - Forecasts for an El Nino this winter have given way to the prospect of more La Nina-like conditions as sea surface temperatures in the central-eastern Pacific cool rapidly.
Surface temperatures in the critical area of the Pacific have fallen to 0.2 degrees Celsius below average, down from 0.7 degrees above average in the week centred on June 28. The rapid cooling has forced meteorologists to reassess the outlook for the northern hemisphere winter. (http://tmsnrt.rs/2wDJmMJ)
Until June, most forecasters were predicting a mild or moderate El Nino between December 2017 and February 2018.
But the rapid cooling of the sea's surface in July and August now points to a shift towards more neutral conditions, or even La Nina developing.
The U.S. National Oceanic and Atmospheric Administration (NOAA) has cut the probability of El Nino between December and February from 44 percent in its May forecast to just 16 percent in August.
At the same time, it has doubled the probability of La Nina between December and February from 14 percent to 28 percent.
The latest runs of NOAA's forecast models point to La Nina conditions developing by the end of 2017 and into early 2018.
Surface temperatures in the central-eastern Pacific area are monitored by meteorologists because they correlate with a broader set of oceanic and atmospheric changes known as the El Nino-Southern Oscillation (ENSO).
But the evolution of ENSO remains notoriously hard to predict. The warm phase forecast earlier this year proved to be very weak, failing to reach the threshold to qualify as a full El Nino, and then fizzled early.
Something similar could happen with the current predictions of La Nina. Even if La Nina conditions develop in the next few months, it remains hard to predict their intensity or duration.
Despite the rapid cooling of surface temperatures in the central-eastern Pacific, most of the other components of a La Nina episode are absent or only weakly present at the moment.
"Sea surface temperatures have cooled during the past several weeks, yet have remained within the neutral range," the Australian Bureau of Meteorology reported at the end of August. "Other indicators of ENSO ... are also at neutral levels."
The most likely outcome this winter is for ENSO to remain in a fairly neutral or slightly cool phase.
"The latest outlooks suggest that the central tropical Pacific Ocean will cool slightly in the next month, but remain within the neutral ENSO range for the rest of 2017," according to the Austrian meteorology bureau.
But if it does develop, a moderate to strong La Nina would bring a colder winter to the northern United States and Canada and milder conditions to the southern states.
It would also bring wetter weather to Southeast Asia and northern Australia, as well as northeast Brazil. ("The three phases of the El Nino-Southern Oscillation", Australian Bureau of Meteorology, 2011)
EL NINO-SOUTHERN OSCILLATION
ENSO alternates between warm and cool phases with a major and widespread impact on precipitation and temperatures around much of the world. ("Weather impacts of ENSO", U.S. National Weather Service)
ENSO swings predictably between warm and cool phases but their duration and intensity is highly irregular and much harder to predict, which is what makes seasonal weather forecasting so challenging.
Meteorologists label intense warm phases "El Nino" and strong cool phases "La Nina" but in reality they are just extreme positions in a continuous cycle.
The most common definitions require sea surface temperatures in an area of the central-eastern Pacific known as Nino region 3.4 to be at least 0.5 degrees Celsius above or below the seasonal average for several months to qualify as an El Nino or La Nina episode.
But the definition is arbitrary and it is possible to define strong, moderate and marginal instances of both El Nino and La Nina. From an analytical perspective, it is more useful to think about warm and cool phases of ENSO with varying intensities.
PACIFIC AND INDIAN OCEAN
ENSO has its most pronounced impact on countries around the Pacific and Indian Oceans because of their proximity but milder effects are felt much further away in North America and the Atlantic.
A strong El Nino or La Nina episode tends to have a significant impact on weather conditions while the impact of a milder episode is harder to identify.
ENSO is also only one of a number of large scale disturbances in the oceans and atmosphere so the effect of a mild phase may be lost among other variables.
The strongest impact is felt in Southeast Asia, the Maritime Continent and northern Australia, where ENSO shifts the location of rainfall and results in alternate droughts and flooding.
Similar shifts in rainfall occur in Latin America, the Indian subcontinent and eastern Africa, resulting in alternate heavy rains and dry seasons.
El Nino usually brings dry conditions to Southeast Asia, India and parts of South America and heavy rainfall in eastern Africa.
By contrast, La Nina brings wetter conditions to Southeast Asia and parts of South America, and drier conditions in eastern Africa.
UNITED STATES AND CANADA
The impact on the United States, Canada and the Caribbean comes from shifts in the strength and track of the Pacific jet stream. ("How El Nino and La Nina affect the winter jet stream and U.S. climate", NOAA, 2016)
El Nino is normally associated with a strong west-to-east air flow across the Pacific that brings stormier conditions to the southern United States and milder winters across much of the north of the country and Canada.
La Nina brings a more variable and weaker Pacific jet stream, allowing more cold polar air to enter from the north, bringing cold weather to the northern states and Canada but milder conditions in the south ("El Nino and La Nina-related winter features over North America", NOAA, 2005).
Because the United States and Canada are located further away from the heart of ENSO, the impact tends to be milder and more easily masked by other disturbances in the oceans and atmosphere.
ENSO therefore has a strong impact on weather in the United States and Canada only when there is a fairly strong El Nino or La Nina episode.
Mild episodes may not have a discernible effect on North American weather. ("Relationships between climate variability and winter temperature extremes in the United States", Higgins et al, 2002)
"El Nino signal is weakening in the Pacific", Reuters, June 14
"El Nino conditions are developing in the Pacific", Reuters, May 4
"La Nina has probably peaked and will fade in early 2017", Reuters, Dec 20 (Editing by David Clarke)