A flooded community in West Sussex (Photo courtesy of Eddie Mitchell/BBC)
It’s been a tough time for folks across the South of England this year with record drought which extended back a good 2 years to now exceptional rainfall and flooding. I guess one could link this to the onset of the El Nino rather than global warming given the flip from drought to deluge across hard hit drought areas including Texas and the Southeast US as well as down across Australia.
Why such rains here in the UK?
I blame the unusual cold to the north and summer warmth to the south over the North Atlantic, as you know, warm and cold don’t mix and so the jet stream which is the ribbon of air which seperates the two conflicting air masses at 30-40,000 feet. This blows faster when the temperature contrast between warm and cold is greater. Recently it has been blowing a wintertime speeds of 150 mph across the Atlantic. The UK, like the Pacific Northwest of the USA has been the hot bed for persistent lows due to a trough or dip in the jet. These dips allow westward moving air to hit up against a wall and so air starts to pile up and spin, forcing a low pressure centre to form. As these systems wind up, they tap air from both the north, drawing in cold air over top of warm, moist surface air flowing up from the subtropical regions. This pulling in of warm and cold air both fuel storm intensity as well as get really bring down some tremendous rainfall.
With the trough oriented over the UK with a cold front extending down into the warm, humid Azores region, this air gets drawn north along the front. As this fuel runs underneath the colder upper level environment, it gets ‘lifted’ up into the into the abnormally cold air within the trough. This is the perfect setup for heavy rains.
The prolonged nature of this heavy rainfall, especially over Southern England over the past 36 hours can be attributed to a high pressure system further east which slows the forward speed of the system and so with a constant support of subtropical moisture surging up from the Azores and the near stalling of the system, this is what brings a months worth of rain within a 24-hour period.
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