The west coast of Wales, Ireland, and Scotland have been buffeted with unusually strong winds in the last couple of days.
Decoded Science has named Extreme Weather Event Courgette, which promises to be more extensive than just this one storm — a multi-week spell of unusually stormy weather — even for this stormy part of the world.
The popular press has dubbed this a ‘weather bomb’ and ‘explosive cyclogenesis.’ What do these phrases mean?
Weather Bombs And Cyclogenesis
Meteorologists commonly refer to any extreme weather event, particularly a deep, extratropical low pressure system, as a ‘bomb.’ The term has no precise meaning, and shouldn’t be taken seriously.
Cyclogenesis is the process of cyclone formation in the mid-latitudes. Adjacent air masses of contrasting temperatures release their potential energy, converting it to energy of motion (wind). As a storm strengthens and the central pressure falls, the pressure gradient increases; wind speed is directly proportional to pressure gradient, so winds increase as cyclogenesis occurs.
In the end the air masses are realigned with warm air on top of cold — a lower potential energy state.
The Initial Phase Of Extreme Weather Event Courgette
In the case of Courgette, the popular media have misrepresented the cause and effect: Cyclogenesis occurred days earlier over Greenland. The storm moved east with the jet stream and reached Ireland and Scotland on Tuesday and Wednesday.
A wind speed of 144 miles per hour was reported on Hirta in the remote St. Kilda Islands off the Scottish coast, but this looks suspiciously high, even for a place where 30 miles per hour would be considered a gentle zephyr and 60 miles per hour a refreshing breeze. Reports of wind speeds in the 85 mile per hour range, received from many exposed locations, are probably better representations of the power of this storm.
Inland areas of the British Isles received copious amounts of rain, and where temperatures fell below freezing, icy roads caused hazardous driving conditions.
The Role Of The Jet Stream In Strong Surface Winds
The jet stream is a reflection of the temperature gradient below it; when the gradient is strong, so is the jet stream. A very powerful jet stream has camped over the eastern North Atlantic Ocean, and Courgette is the result. Cyclogenesis produced a deep low pressure center and the pressure surrounding this low affected the UK as it traveled eastward.
Winds at the surface are proportional to the strength of the pressure gradient, but they are exacerbated by vertical mixing. Since the jet stream winds are very strong, and since a cyclone contains rising and falling air currents, some of the fast-moving air from the jet stream can be transported to the surface.
The Role Of The Polar Vortex In Courgette
The Polar Vortex is a circumpolar whirlpool, which occasionally — more frequently in recent years — becomes of two minds and splits apart into a bi-polar Polar Vortex. Last winter a strong spin-off of the polar vortex took up residence in south-central Canada and laid the groundwork for a nasty winter in the eastern US. Now we have vortices over Greenland and the Central Pacific Ocean. Each is associated with a very powerful branch of the jet stream.
The jet stream over the Pacific is responsible for Extreme Weather Event Cucumber, which is a series of powerful storms, the first of which struck California on Wednesday and Thursday.
Similarly the vortex over Greenland and Iceland has spawned Courgette, which began affecting Scotland on Wednesday in the form of the ‘weather bomb,’ and will bring intermittent periods of strong wind and rain to the UK for at least ten days.
The Jet Stream Wave Pattern
Dips in the jet stream have a typical wavelength of several thousand miles. In the middle latitudes, where one circumference is about 16,000 miles around at 45 degrees, there are normally four or five troughs (dips). Waves can have any length, but just as ocean waves have a preferential wavelength of a few tens of feet as they approach the beach, so the atmosphere prefers the wavelengths of a few thousand miles.
From the east-central Pacific to the northeast Atlantic is a distance of around seven thousand miles — about the perfect distance for three troughs, three or four thousand miles apart, in the jet stream. The middle trough is near the east coast of the United States, where a storm is just departing, and the troughs in the Pacific and over Greenland and Iceland — more pronounced — have spawned Cucumber and Courgette.
The Connection Between The Position Of The Polar Vortex And Climate Change
One of the most noticeable effects of global warming has been the extreme change near the North Pole. The dramatic rise in temperature at the Pole has had the result of pushing the zone of maximum temperature contrast, and thus the jet stream, southward. The unusually strong jet streams causing Cucumber and Courgette seem to be the culmination of this trend.
Models of the atmosphere that incorporate increased greenhouse gases predict more extreme weather events. Are Cucumber and Courgette a result of global warming? No single storm, or even series of storms, can conclusively be proven to have a connection with global warming. But these events are consistent with it.
The mountain of evidence consistent with anthropogenic climate change is now very high.
How Does Decoded Science Name Storms?
The naming of the weather events Cucumber and Courgette may seem frivolous, and though the names are meant to be mildly whimsical, they serve a purpose: We now have some systematic way to refer to unusual weather events.
Decoded Science has chosen to use the names of vegetables for extreme extra-tropical weather occurrences this year. Cucumber was the third, following Artichoke and Broccoli, all of which affected the US.
When it came to naming the current situation, we considered it appropriate to use a Continental Vegetable. Courgette is the European name for what Americans call Zucchini. We used a C-vegetable because of the meteorological similarities of Cucumber and Courgette.
And besides, it seemed unfair to make poor Zucchini wait so long to have anything named after it.