LONDON (Reuters) - El Niño will be present this winter, prompting many to seek comparisons to the most recent El Niño episode over the winter of 2009-2010. But links between El Niño and Europe are not commonly discussed, and there is no certainty that the relationship even exists.
Europeans probably remember the winter of 2010 as brutally cold and snowy, leading to a general notion that this is typical of El Niño. But this was not because of El Niño, nor could it have been confidently predicted several months ahead of time.
In late 2009, a very active climatic pattern had set up and ultimately handed control over Northern Hemispheric winter to the Arctic rather than the tropics. An uncommon and complex chain of atmospheric events through early 2010 caused the frigid temperatures to persist all winter in Europe.
Replicating the 2010 scenario in 2016 is statistically unlikely, though either way, El Niño does not ensure a grueling winter for Europe. Instead, there are a couple basic indices that can be followed as winter begins to forewarn of any potential cold blasts.
And the chance for tropical influence is isn’t out of the question yet. El Niño may indeed impact Europe’s winter, and there is evidence that the strength of this year’s event may produce favorable results.
The North Atlantic Oscillation (NAO) is a large-scale climate variability that is driven by differences in air pressure over the North Atlantic Ocean. To put it more simply, the NAO is partially responsible for how much cold Arctic air is allowed to pass down into the mid-latitudes.
Although the NAO operates year-round, its effects are felt most distinctly in the Northern Hemisphere during the winter. When the NAO index is in a negative phase, colder temperatures are likely to surface. The opposite is true for a positive phase.
Unlike El Niño, the NAO fluctuates on a much smaller scale, in the order of one to two weeks. That means there can be several rounds of positive and negative phases during one winter.
But 2010 was a standout. From December through February, the NAO was negative on all but four days. Further, the average value of the NAO was amongst the lowest of all time. This wintertime combination of longevity and strength has only been observed a handful of times, mostly in the 1960s, but none in recent memory aside from 2010.
As a result, Europe suffered a punishingly cold winter, and the position of the jet stream caused the snow to keep on coming. This led to record high soil moisture come the spring and combined with a hot summer, 2010-harvested winter crops performed poorly in many countries.
A similar index in the Northern Hemisphere, the Arctic Oscillation (AO), also hit record negative values during the 2010 winter. When the NAO and AO are negative at the same time, temperatures tend to plunge particularly low, explaining the extreme anomalies in 2010.
The winter of 2010 was also similarly cold for the same reasons in the United States, where El Niño would have otherwise been expected to produce warmer weather across most of the central and northern states.
The NAO and AO tendencies for winter of 2016 cannot be determined just yet. Both indices have reasonable predictability out ten days, and they should begin to be closely monitored in about one month’s time.
Even though connections between El Niño and winter in Europe are loose, the strength of this year’s event may work in Europe’s favor. With a near-record El Niño already in progress, there are only two other years that compare in strength to tack back to, 1982/83 and 1997/98.
In examining several of the past “standard-strength” El Niño winters, jet stream activity was rather weak directly over the European continent, allowing cold, dry Siberian air to sink down into Eastern Europe, particularly the Balkans, Ukraine and western Russia. These winters were a degree or two colder than normal in these areas but were generally normal elsewhere.
During the winters of 1983 and 1998, the active jet stream was positioned over Europe and took on a westerly to southwesterly direction, transporting warmer, wetter air towards the continent. As such, much of Europe and western Russia was warmer than normal. Further, European winter crops harvested in 1998 yielded much better than average.
With only two analogue years, it is almost impossible to tell whether the warmer 1983 and 1998 winters were purely coincidental, or whether the extraordinarily warm tropical Pacific waters influenced this atmospheric set-up over Europe. But if the NAO and AO trend positive this winter in addition to the massive El Niño, the odds for a mild winter across Europe multiply considerably.
Reporting by Karen Braun, editing by David Evans