The Process of Bombogenesis
Just days after California received a severe thrashing from the combination of a bomb cyclone and the Pineapple Express, New England was whipped by rain and 90 mph winds from a brutal nor-easter. Over 600,000 people in the US Northeast lost power as another bomb cyclone exploded over the North Atlantic Ocean. Meanwhile, in the Mediterranean off the coast of Italy, an intense cyclone called a “Medicane” drenched Sicily with a year’s worth of rain in 48 hours.
The connection between these events is a warming planet. Scientists often note the link between climate change and the increasing frequency of extreme weather. Many factors contribute to the weather we experience, but warmer oceans are one important factor behind all these recent events. Earth’s oceans have absorbed about 90 percent of the heat from global warming over the past 50 years.
Some areas warm faster than others, and the Mediterranean is warming more quickly than other oceans due to its small size and geographic position. Warmer water leads to two important storms components — more moisture is pumped into the atmosphere from evaporation, and extra warm patches of ocean water create convection hot spots where storms nucleate. Two key drivers of hurricanes and cyclones are warm water and a stable upper atmosphere.
Forming the Vortex
Think of a situation where an ocean hot spot transmits moisture and heat to the atmosphere. The air over the hot spot is warmer than the surrounding atmosphere, and it rises, carrying with it the moisture. The air is evacuating at the ocean’s surface, and this loss of air creates a low-pressure zone. Nature abhors a vacuum, so air from the surrounding areas rushes in to fill the developing void.
Our planet is constantly in motion, spinning on its axis. Because it is round, points on the equator are traveling faster than points near the poles. This velocity differential imparts a momentum effect to flowing water and air currents called the Coriolis force. This force causes currents to naturally curve or deflect to the right in the northern hemisphere.
Because of the Coriolis effect, air can’t flow in a straight line. As the wind blows towards the developing low-pressure center of a storm, it keeps veering to the right causing the air to circle the low-pressure center before it enters and ascends. In the northern hemisphere, this creates a cyclone rotating counterclockwise around its low-pressure center.
A storm grows by feeding on the ocean’s heat, sending increasing amounts of warm, moist air upward. The warmer the air, the more moisture it can hold. As the air ascends, it cools, and the moisture condenses into clouds, and finally, it precipitates as rain or snow. But this process requires the second component, a vertically stable atmosphere. Strong upper-atmosphere wind shear can chop off the top of a developing storm and effectively kill it. But when wind shear is reduced, the frequency of dangerous storms increases. This effect played a part in the very active 2020 and 2021 storm seasons for Hurricane Alley, affecting the Gulf of Mexico and the southeastern USA.
The term “bombogenesis” frequently appeared in the news during this recent flurry of bomb cyclones. Borrowing from NOAA: “Bombogenesis, a popular term used by meteorologists, occurs when a midlatitude cyclone rapidly intensifies, dropping at least 24 millibars over 24 hours….The formation of this rapidly strengthening weather system is a process called bombogenesis, which creates what is known as a bomb cyclone.”
A rapidly developing storm poses two threats. The first threat is the raw destructive power unleashed. Secondly, a rapid escalation in storm intensity may prevent adequate warning and preparation in populated areas. This problem is particularly true for coastal areas where rain and high winds accompany the storm as it makes landfall. When pressures in the center of the vortex rapidly lower, wind speed increases as the atmosphere strains to fill the rapidly developing void in the eye of the storm.
This type of rapid intensification also occurs in hurricanes, and in 2020 Hurricane Laura slammed into the Louisiana coast during the early morning hours, about 40 miles from the Texas border — a Category 4 storm with sustained winds of 150 miles per hour (mph). Twenty-four hours before landfall, Laura was a Category 2 storm with winds reaching only 110 mph. So how did she transform into a monster storm in just 24 hours?
Typically, a Category 4 storm develops slowly over days or even weeks, but very warm seas and a calm upper atmosphere provided the right conditions for Laura to explode into a dangerous storm, and she blasted from Category 2 to Category 4 in just 24 hours.
Climate change is now pushing the term “bomb cyclone” into our everyday vocabulary.
Hurricane Laura Goes Rogue (by WM House; ArcheanWeb)
Warmer Oceans Drive Tropical Storms (by WM House; Medium)
Ocean warming dwarfs atmospheric warming (by WM House; ArcheanWeb)
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Deadly Sicilian cyclone drops a year’s rain in 48 hours (by Tom Kington; The Times)
More than 600,000 without power as bomb cyclone brings hurricane-force winds, heavy rains to Northeast (by John Bacon; USA Today)