Courtesy of Mark Vogan
This summer, like recent ones have been somewhat of a dissapointment which extend all the way back to the soaker of 2007 where this ‘wet cycle’ appears to have began. You have you go back to 2006 to find the last truely warm and dry summer here for all of the UK. Summers before 2007 appeared to have trended warmer and drier. I recall the dry and very warm summers of 2003 which of course set new heat benchmarks here in the UK and for many countries across western and central Europe, 2004, 2005 and 2006 were all decent summers too.
During this meteorological summer of 2012, it has been the wettest in Great Britain for 100 years, making this the second wettest in recorded history. The wettest was back in 1912 but interestingly, the second wettest up until this year was the not so distant year of 2007.
Why have our summers turned so wet with 2007 appearing to be a turning point?
For the past couple of years I have noticed more and more that summers have become much more unsettled, wetter, even stormier but less and less days dominated by high pressure. Yes, there was summers since 2007 which weren’t too bad across the South of Britain but it’s when looking at ALL of Britain that you realise, something has changed and we seem to have entered a new cycle within our summer climatic pattern.
What could be the cause?
Note in this global SST map how the warmest waters in the N Atlantic compared toi normal are over the far north with tongue of cold stretching from central Atl to UK and the cold overall look of the Pacific (Courtesy of WeatherBELL Models)
As you know I am a great believer in ocean cycles and their 20-30 years of warm and cold phases. These giant heat reservoirs play a massive if not the biggest role in our climate. While the Atlantic has been warm over the past 15+ years and ended it’s cold phase at the start of the 1980s, the Pacific up until 2007 was likewise warm but the Pacific does tend to bob up and down a little depending upon the ENSO phase, which is of course the warming and cooling of the equatorial Pacific.
Generally when the North Atlantic is nearing the end of it’s warm phase, the warmest waters compared to average are usually up over the North Atlantic and surrounding Greenland while there is often a strip of cool with warmer than normal down towards the tropics. This ‘tripole’ of warm-cold-warm not only may be intensifying lows but this may be aiding the the enhancement of blocking which has been bias ‘negative’ in recent years, keeping a trough over us more times than not. These warm waters would certainly supply greater amounts of moisture for UK-bound systems to collect and dump on us. However why the sudden switch from dry in summer 2006 to super wet in 2007?
What’s interesting is the timing of the flip from warm to cold in the PDO.. When was that, 2007!
The jet stream in recent summers has been stronger and I believe a lot has to do with low pressures becoming more active and picking up more moisture thanks to the warmth of the N Atlantic and that tripole with each summer the waters warm more and more over the far N Atl. This could well be increasingly the rainfall amounts in the UK as well as dull days.
What’s interesting is that since the Pacific flipped to cold and the Atlantic is warm, I believe it is this that’s causing these very wet summers.
The reason for the super wet June, July may be down to the flip from a La Nina to El Nino which could have forced bigger rains across us, helped by the warm AMO and also the large transfer of moisture away from the US thanks to the cold PDO enhancing drier than normal conditions.
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