Sunday, February 9, 2014

UK Storms: "All the evidence suggests there is a link to climate change." Dame Julia Slingo

The UK MET Office has issued a report on the recent extreme UK and global weather. Prior to the issuance of the report, Dame Julia Slingo, chief scientist of the MET, gave an interview in which a few excerpts are included in a February 9 BBC article.
 She stated that, "all the evidence suggests there is a link to climate change," and "there is no evidence to counter the basic premise that a warmer world will lead to more intense daily and hourly rain events."

According to the BBC, she also commented that "the UK had seen the "most exceptional period of rainfall in 248 years".

Unsettled weather at this time of year was not unexpected - but the prolonged spell of rain, as well as the intensity and height of coastal waves, was "very unusual".
"We have records going back to 1766 and we have nothing like this," she said. "We have seen some exceptional weather. We can't say it is unprecedented but it is exceptional."

See: Met Office: Evidence 'suggests climate change link to storms' ,
What is of real interest, in relation to the previous blog post, is the report's support for the mangled jet stream and Rossby waves that are deemed the main contributors to the North American and UK weather extremes, linked to the warmer West Pacific sea surface temps.
The MET blog states,
"The severe weather in the UK coincided with exceptionally cold weather in Canada and the USA. These extreme weather events on both sides of the Atlantic were linked to a persistent pattern of perturbations to the jet stream, over the Pacific Ocean and North America.
The major changes in the Pacific jet stream were driven by a persistent pattern of enhanced rainfall over Indonesia and the tropical West Pacific associated with higher than normal ocean temperatures in that region. The North Atlantic jet stream has also been unusually strong; this can be linked to exceptional wind patterns in the stratosphere with a very intense polar vortex."
The UK MET report, pp. 12-13, provides the details:
"During December and January 2013/14 the pattern of winds over the North East Pacific and North America was very disturbed (Figure 13, right panels). The North Pacific jet was deflected a long way north, with a secondary branch extending southwards into the tropical Pacific accentuating the separation of the Pacific and Atlantic jet streams. The effects of this over North America and into the North Atlantic were profound. The deflection of the jet to the north led to colder air being carried south over Canada and the northern US ... to enter the North Atlantic jet and establish a stronger than normal temperature gradient at the entrance of the North Atlantic Jet. This acted to strengthen the jet and provide the conditions for active cyclogenesis, which in turn led to a sequence of strong storms across the UK throughout December and January ... the North Atlantic jet was, on average, as much as 30% stronger than normal. 
In regard to Rossby Waves interaction with the jet streams it comments in page 17, "As is evident in Figure 13, the ‘buckling’ of the jet stream over the Pacific and North America became much more pronounced during January 2014, as the precipitation anomaly over Indonesia and the West Pacific strengthened (Figure 17). A notable feature of this anomalous area of tropical precipitation is its northwards extent into the winter hemisphere where it is able to interact with the North Pacific jet and generate Rossby waves6 that propagate along the jet and act to reinforce the huge meander of the jet stream off the west coast of North America. At the same time, Rossby waves propagate along the southern branch of the jet stream and enter the tropical East Pacific through the westerly duct, creating weather disturbances that can then get caught up in the entrance region of the Atlantic jet stream.
These Rossby wave interactions are very complex but appear to be fundamental to understanding this winter’s weather."
In regard to the Polar vortex, it notes on page 19 the elongation and also the wind speeds,
"Above the troposphere, westerly winter winds in the polar night jet stream9 were very strong during December and January. The polar night jet exceeded twice its normal strength at times during the winter, reaching speeds in excess of 100ms-1 in the upper stratosphere. A strengthening of the polar night jet often precedes periods of a strong Atlantic jet stream below and a positive North Atlantic Oscillation pattern, as was seen during the whole December to January period and consistent with the increased winter storminess this year."
Continuing on page 20, "In terms of the recent extreme cold event over Canada and the US, the evidence suggests that the polar vortex has been stronger than normal, consistent with the increased winds in the polar night jet; the structure of the vortex has also been stretched with the core of the vortex extending southwards over Canada (Figure 19)."

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