At around 6pm on Jan 9 1993, the North Atlantic was just about to get a surprise. All the ingredients were in place for a small, innocuous-looking disturbance on the chart to turn into what would be, 24 hours later, the deepest North-Atlantic low pressure in history; the Braer Storm.
The Braer Storm was named after its victim, the MV Braer, an oil tanker that it smashed to pieces off Shetland. The remarkable thing about the storm, apart from its record central pressure of 914 mb, was the fact that the pressure plummeted over 75 mb in 24 hours. Which makes it one of the most notable examples of a phenomenon called explosive cyclogenesis.
Explosive cyclogenesis, as the term suggests, is the explosive genesis of a cyclone; in other words, the extremely rapid development of a storm. More specifically, a storm can be categorised as having explosive cyclogenesis if the central pressure drops more than 24 mb in 24 hours.
I was taught the ‘24 in 24’ rule in my meteorology classes more than 20-yearsago, when nobody had ever heard of explosive cyclogenesis. In those days, it was just another geeky scientific term along with baroclinic instability, geostrophic balance and potential vorticity, which, even if you didn’t understand them, could still be used to chat up girls at parties.
Nowadays, every time there is a deep low on the chart, the words ‘explosive cyclogenesis’ can be seen on any TV channel or newspaper, and are almost as common as ‘Brexit’ or ‘Trump’.
One of the first times explosive cyclogenesis was explained, was in a ground-breaking paper published in 1980 by Fred Sanders and John Gyakum from MIT. Sanders and Gyakum suggested that the minimum pressure drop to qualify for explosive cyclogenesis (24 mb in 24 hrs) would give the storm a rating of one Bergeron, after the legendary meteorologist Tor Bergeron. The Bergeron rating would increase if the pressure drop was faster. This gave an easy way to measure the explosiveness of storms: the higher the Bergeron rating, the more bomb-like.
The Braer Storm, with its 75 mb drop over 24 hours, came in at an unbeatable 3.1 Bergerons. More recently, the storm we all remember – Hercules – dropped 48 mb in the 24 hours between 3rd and 4th January 2014, giving it a rating of 2.0 Bergerons.
Now, what are the main causes behind explosive cyclogenesis? Well, there isn’t really a ‘secret ingredient’ to storms like the Braer to make them deepen so quickly. It’s just the same factors that make a normal storm to deepen, but more intense and more synchronous. These are typically:
*A strong upper airstream, pumping energy into the atmosphere at the surface (see my jet-stream article HERE);
*The merging of two systems. For example, if a peripheral low is gobbled up by its own mother system, which is kind of what happened with the Braer Storm;
*A strong gradient in the sea surface temperature along the path taken by the storm. For example, the Braer Storm developed along the northern edge of the Gulf Stream, where warm water meets cold.
And finally, note that the UK Met Office predicted the Braer Storm to have a central pressure of 909 mb. In the end it only got down to 914 mb, but still remains to this day the deepest low every recorded in the North Atlantic. Keep an eye on the pressure charts by heading over here.
Cover shot, Nazare by Helio Antonio