(John Kemp is a Reuters market analyst. The views expressed are his own)
By John Kemp
LONDON, Feb 12 (Reuters) - North America's bitter cold and Britain's intense storms and flooding are due to heavy rain, caused by higher coastal temperatures, in Indonesia and tropical western Pacific via shifts in the jet stream, the UK Meteorological Office says. ("Recent storms and floods in the UK" Feb. 11).
A "persistent pattern of enhanced rainfall" - 50 percent higher than normal in December and January - created westerly winds that pulled storms into the region from the northern Pacific, linking weather systems in the tropics with those closer to the polar regions.
"Extreme weather events on both sides of the Atlantic were linked to a persistent patterns of perturbations in the jet stream over the Pacific Ocean and North America," the Met Office and Centre for Ecology and Hydrology said in a comprehensive report updated on Tuesday.
"There is a strong association (between) the stormy weather experienced in the UK during December and January and the up-stream perturbations to the jet stream over North America and the North Pacific," the Met Office observed.
Deviations in the jet stream are similar to those associated with "La Nina," the cold phase of the El Nino Southern Oscillation (ENSO) cycle in which sea surface temperatures in the Pacific off Peru are colder than normal.
Sea surface temperatures across the equatorial Pacific are close to normal, however, and there is no sign of either La Nina or El Nino conditions.
But what matters for the jet stream is the heavy rainfall over Indonesia and the tropical western Pacific that comes with La Nina.
Rainfall across Indonesia pulls storms into the region from the North Pacific and helps deflect the jet stream.
This year the North Pacific jet stream was deflected much further north than usual, which brought bitterly cold polar weather sweeping down across Canada and the United States as far down as Texas when it turned south again.
As the cold polar air interacted with warmer than normal temperatures across the Atlantic, it strengthened the Atlantic jet stream and helped generate a series of unusually severe storms.
The North Atlantic jet stream was 30 percent stronger than usual in December and January, according to the Met Office. And a series of unusually intense depressions formed in the Atlantic, including storms which swept across Britain on Dec. 4-5, Dec. 24, Jan. 5-6, Feb. 5 and Feb. 7-8.
The depression which formed on Jan. 5 was so large it covered the entire North Atlantic, according to the Met Office.
The unusually strong and southerly track of the jet stream over the Atlantic took these storms straight across southern England.
As usual, the complete explanation is complex, and the Met Office report is worth reading in full (http://www.metoffice.gov.uk/media/pdf/1/2/Recent_Storms_Briefing_Final_SLR_20140211.pdf).
FOUR TYPES OF FLOODING
The result has been an inundation of biblical proportions. "Although no individual storm can be regarded as exceptional, the clustering and persistence of storms (in December, January and February) is highly unusual," according to the Met Office.
More than 37 cm (15 inches) of rain fell across central southern and southeast England during December and January, the highest two-month total since 1910. For the whole of England and Wales, the Met Office reckons this was one of the wettest winters in at least 248 years.
Britain's weather is notoriously variable, as its inhabitants and tourists will confirm. And it is quite common for the country to be hit by a succession of storms in a "cluster".
But the persistence of the recent storminess is unusual. The continued run of deep depressions through December, January and on into February is anything but normal according to the Met Office.
The result is that the country has been hit by no less than four types of flooding.
First came the coastal floods. The succession of massive storms have generated enormous waves for long periods and sent the bulk of the wave energy crashing onto the coasts of southwest Ireland and southern England, rather than further north. Prolonged, successive storms have gradually weakened coastal defences until many have failed.
By February, sea defences were already severely weakened, allowing the latest storms to seriously damage coastal infrastructure, including cutting the main rail line between London and the Cornish peninsula in the far southwest.
Next came flash floods. Intense storms and rainfall in early December caused extensive localised surface flooding, especially in southwest England, as drains, streams and rivers struggled to carry so much water away quickly.
Finally has come river and groundwater flooding. Groundwater levels were below average at the start of winter, but persistent heavy rain has saturated the ground over the last three months.
Aquifers are now full across the region. On Salisbury Plain, near the south coast, the water level in the chalk aquifer rose by 20 metres (22 yards) in just two weeks in late January.
With heavy rainfall continuing and nowhere for the water to soak away, the whole region is experiencing widespread flooding along river valleys and up from the ground.
Because the ground is totally saturated, even relatively moderate rainfall is now liable to cause severe flooding from now on. Southern England is likely to remain vulnerable to flash flooding, river flooding and groundwater flooding for months until the water table falls.
QUESTIONS ABOUT CLIMATE
"As yet, there is no definitive answer on the possible contribution of climate change to the recent storminess, rainfall amounts and the consequent flooding," the Met Office says. "This is in part due to the highly variable nature of UK weather and climate."
In other words it is hard to separate the signal (possible climate change) from the noise (the normal background variation in Britain's weather).
The number of storms forming over the mid-Atlantic and hitting Britain has not increased since 1871, but their strength and intensity does appear to have increased significantly.
"Current evidence does suggest an increase in storminess," the Met Office concluded, though it called for more research.
The storms may also be bringing more rain, though again it is hard to separate climate change signals from background noise. "UK rainfall is highly variable in space and time. Changes in monthly, seasonal or annual mean amounts are difficult to detect so far," the Met Office wrote.
But there is some evidence that what agency calls "daily heavy rain events" are becoming more common. Heavy rain which might have occurred on one day out of every 125 in the 1960s and 1970s is now occurring on one day out of every 85.
"This supports other evidence that UK rainfall is increasing in intensity," the Met Office said.
Whether this is attributable to climate change is a much harder question to answer. The answer so far must be "not proven," the verdict traditionally available to juries in Scotland that lies somewhere between guilty and not guilty.
But the Met Office observes: "Extreme rainfall events are becoming more intense, and the evidence is consistent with the what is expected from fundamental physics. There is no evidence to counter the basic premise that a warmer world will lead to more intense daily and hourly heavy rain events." (Editing by Louise Ireland)
Our Standards: The Thomson Reuters Trust Principles.