MA in French philology and translation science, PhD in German literature and intercultural studies, interested in tropical weather since 2005.
By: taistelutipu , 12:20 AM GMT on July 11, 2013
The next couple of blogs will deal with Prof Vaughan's public lecture held at Bangor University (N-Wales, UK) on 24.6.2013.
News page of the University and abstract of the lecture can be found here: http://www.bangor.ac.uk/oceansciences/full.php.en ?nid=14571&tnid=14571
On Monday night Prof. Geraint Vaughan (Manchester Unversity but a native North Walian) gave a lecture on the astonishing turn in weather patterns over the UK in the first half of 2012. After an exceptionally dry initial three months period, April 2012 turned out to be the wettest April on record. From predicting a drought for the spring and summer months, the UK met office had to adjust their mid-range forecast to an increasingly wetter one for the rest of the year. Vaughan’s aim was to explain to the audience why there was such a discrepancy between the initial mid-range forecasts for 2012 and what actually happened. In the following, I’m presenting a write-up of the notes I took during the lecture with my own comments in parentheses and italics.
He first explained what exceptionally wet meant in UK terms. Average annual rainfall for England and Wales is 915mm (1845-1979 long term average) but there is a high regional variation between over 4400mm in Snowdonia and around 600mm in East Anglia. Of the 10 wettest years, 6 were after 1995 [my addition, since I know that the Atlantic entered an active phase with more active hurricane seasons. That this could also have an effect on the precipitation in Western Europe was one of my personal conclusions I took from the lecture].
2012 was a very unusual year with the extraordinary comeback from the driest January-March period to being the second wettest year on record. Jan, Feb, Mar and May were all drier than usual, while April, June, July and December all recorded regional or even national precipitation maxima.
Fig. 1: 10 wettest years on record:
[disclaimer: Ok, the blog does not deal well with tables created in word, so a simple 2 column solution has to do. Also, I did not catch all figures, concentrated on getting the highest and the lowest first to illustrate the range. The departure for the wettest year 2000 is thus 146% and 1923, the 10th wettest, is 133% wetter than average]
Winter 2011/12 was very dry, especially in NE-England and Scotland, while central regions experienced a slight drought, e.g. N-Wales, the SE being drier again. Vaughan explained this with the dipole pattern which also affected France and Spain, so more or less the entire Western part of Europe had less than average precipitation.
[He did not go into detail but this paper summary explains the dipole pattern which determines precipitation patterns in Europe in normal years: http://nora.nerc.ac.uk/13543/2/N013543PO.pdf In an anomalous year such as 2012, the NAO was first very positive and then flipped to very negative in April which basically switched the climate from very dry to very wet within mere weeks]
March was the driest month, in some regions only 20% of the average precipitation fell, some places in Scotland even below that. In SE-England, the driest region of the UK, a hose pipe ban was issued. March was very anti-cyclonic with high pressure dominating proceedings. To illustrate that, Vaughan showed the MLSP chart for March 2012 and the jet stream chart and the 500 mbar heights. They showed mainly high pressure, the jet stream taking an anomalous dip over the central ATL which meant that Britain was under the upwards loop to the East of it, receiving warm air from Southern Europe and N-Africa. The 500mbar heights were abnormally high which also indicated warm air. The whole pattern looked somewhat like this :
[I'm playing around with the special characters in Word since I don't have a copy of the graphs, only my pencil drawings in my notebook which I try to replicate, apologies for the less than professional look ;-)]
The big loop over the C-Atlantic was dominated by cold polar air masses while Britain sat in warm air, a condition which lasted for a couple of weeks. Vaughan called it a blocking pattern, when an anomalous dip in the jet stream persists for several weeks and causes the forecasters headaches because they don't know how long it will last.
In March 2013, Britain and most of Western Europe experienced another blocking pattern. This time, however, the dip had moved further east, so Britain now sat in the cold air pocket and registered its coldest March on record.
.........╚═════╝E-Europe now enjoying the warm air.
This was part 1 of the series. Part 2 will illustrate and explain the sudden change in the weather patterns from March to April and show the effects this had on various regions of the UK. Later parts of the series will also include a possible super cell storm in Scotland and a visit of ex-Hurricane Nadine to the UK, or mainly her moisture, which inundated large parts of the UK in the last week of September 2012, making things a lot worse than they already were.
Thank you very much for reading and I hope to see you back for the next parts of the series. Enjoy the summer (or the winter if you are on the Southern hemisphere), watching the hurricane season unfold and stay safe wherever you are.
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Updated: 4:36 PM BST on October 06, 2015