RECENT EXTREME WEATHER CAUSED BY EL NIÑO PLUS CLIMATE CHANGE

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1 january 2016

Record flooding in the U.K. is just the latest symptom of both El Nino and climate change

By Chelsea Harvey, Washington Post January 1, 2016

December witnessed a spate of extreme global weather events, from deadly tornadoes in the southern U.S. to bushfires in Australia — and the latest development is a series of record-breaking floods in the United Kingdom, brought on by torrential rain starting in the first week of the month. Thousands of homes in the north of England are believed to have been affected already, and the region is reeling again from a new wave of flooding just brought in this week with the onset of Storm Frank.

There’s been much discussion about the causes behind the surprising rash of winter storms in the region (Frank is the third major storm to hit within a month), and equal suspicion has fallen on the effects of climate change and the influences of this year’s particularly potent El Niño event. It can be difficult to parse exactly what’s going on, though.

Nicola Maxey, a press officer from the Met Office (the U.K.’s national weather service), noted in an email to The Washington Post that it was too early to say for sure whether climate change was a major contributor to this winter’s extreme rainfall — but added that evidence from both physics and the study of weather systems suggests that it may have played a part.

The Met Office, in fact, recently published a report in the Bulletin of the American Meteorological Society examining the causes behind dozens of extreme weather events in 2014, including similarly severe rainfall in the U.K. in the winter of 2013/2014. Using models, the report concluded that anthropogenic climate change likely had a hand in the extreme conditions that winter — the highest rainfall since 1931 — and that climate change increases the chances of extreme rainfall during a time period of 10 consecutive winter days by a factor of seven.

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What’s been happening to our weather?


Met Office News Blog, 31 12 2015

December 2015 will go down in meteorological history as one of the wettest – and warmest – on record. It will also be remembered for the devastating floods in Cumbria, Lancashire, Yorkshire and Scotland. The extensive flooding of homes and businesses, loss of electrical power, major damage to roads and bridges, and disruption to the rail network have caused great misery and incurred huge losses
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In this blog our Chief Scientist, Professor Dame Julia Slingo, discusses what factors may have influenced the record breaking weather we have seen in recent weeks.

As with all high-impact weather, the meteorological set-up was critical in defining the severity of these events. Throughout the month, the winds have come from the south or southwest, bringing both extreme warmth but also very high levels of moisture.

There has been a lot of debate whether this has been associated with El Nino – an intermittent warming of the equatorial Pacific Ocean which has been very strong this year – or whether this is a sign of a changing climate. The links to El Nino are certainly very clear in the set up of large waves (troughs and ridges) in the atmospheric circulation, which we expect to see in these events.
Latest monthly anomalies in sea surface temperatures showing the strong El Nino lying along the equator, the warmth of the north-east Pacific and western Atlantic and the colder than normal ocean temperatures of the northern North Atlantic

Latest monthly anomalies in sea surface temperatures showing the strong El Nino lying along the equator, the warmth of the north-east Pacific and western Atlantic and the colder than normal ocean temperatures of the northern North Atlantic

However, it does seem that this year the unusual warmth of the North East Pacific Ocean may have altered the position of these waves across North America and into the Atlantic sector, setting up the conditions for the devastating tornadoes in the US and for the southerly feed of moisture-laden air into the UK.
Circulation anomalies in the middle troposphere for 1-30 and 24-30 December

Circulation anomalies in the middle troposphere for 1-30 and 24-30 December, showing a persistent pattern of troughs (blue/purple) and ridges (green/orange) across the US, North Atlantic and into Europe. The trough over the western US set up the conditions for tornadoes along the confluence of cold air from the north with very warm air from the Gulf (see elevated sea temperatures above). The southerly airstream from Spain to the North Pole is established by the gradient between the trough over the North Atlantic and the ridge over Europe.

Storm Desmond in early December was associated with a strong west-south-westerly flow around the ridge over the eastern seaboard of the US, reaching far back across the Atlantic, as far as the Caribbean. With ocean temperatures well above normal in the southern part of the North Atlantic (see above) – possibly due to the much weaker than normal hurricane season this year associated with the current El Nino – the air was primed with more moisture than normal. This river of atmospheric moisture fed the storms that formed on a stronger than normal jet stream, and as the air impinged on the mountains of Cumbria, large quantities of rainfall were released.

Later in the month the southerly flow intensified, with a high pressure system to the east of the UK over continental Europe providing a block to the normal passage of the westerly jet. With colder than usual ocean temperatures over the northern part of the North Atlantic (see above), a strong temperature gradient formed which acted to strengthen the jet and set up the conditions for the formation of rapidly deepening cyclones, such as Storm Frank. These cyclones drew in warm, moist air from far south leading again to heavy rainfall and further flooding on already saturated ground. And the southerly winds on the eastern flank of Storm Frank, and strengthened by the high pressure to the east, enabled extremely warm air to penetrate, temporarily, the deep Arctic leading to very high temperatures.

The potential for December to be stormy and wet was picked up in the three-month outlook and is consistent with what we expect in early winter when there is a strong El Nino in place. However, early analysis suggests that the specific nature of this December’s extreme weather might be linked to the detailed structure of this El Nino, to the warmth of the north-east Pacific Ocean and to their combined effects on the atmospheric circulation.

As for whether climate change has played a role, we know that the overall warming of the oceans increases the moisture content of the atmosphere by around 6% for every 1°C warming. This extra moisture provides additional energy to the developing weather system, enabling even more moisture to be drawn in to the system, so that the overall enhancement of rainfall when the moisture-laden air impinges on the mountains of Wales, northern England and Scotland may be even more significant. So from basic physical understanding of weather systems it is entirely plausible that climate change has exacerbated what has been a period of very wet and stormy weather arising from natural variability.


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