'Bomb cyclones' are becoming frequent extreme weather events

A severe storm described as a “once-in-a-decade” event recently devastated the Pacific Northwest, causing widespread power outages, significant damage, and resulting in at least two deaths. 

Data indicates that such storms – referred to as bomb cyclones – are becoming more frequent, raising questions about the role of climate change as a potential driver.

A powerful low-pressure system 

Frightening satellite images captured a powerful low-pressure system swirling approximately 300 miles off the coast of Washington. 

Known as a “bomb cyclone” due to the extremely rapid drop in atmospheric pressure that intensifies its strength, the system lashed Northern California, the Pacific Northwest, and British Columbia with hurricane-force winds and heavy rain. 

Tens of thousands of homes lost power, trees were uprooted, and at least two people lost their lives.

What is a bomb cyclone?

The term “bomb cyclone” originated as slang to describe an extratropical cyclone that strengthens quickly, similar to a rapidly intensifying hurricane. 

“This term has been in use for about 80 years but was popularized and formalized 40 years ago when it entered the published literature,” said Brian McNoldy, a senior research associate at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science.

Authors of the paper “Synoptic-Dynamic Climatology of the ‘Bomb’,” published in the October 1980 issue of the American Meteorological Society’s Monthly Weather Review, labeled the weather phenomenon a bomb because it develops “with a ferocity we rarely, if ever, see over land.”

“Another term used to describe this is explosive cyclogenesis – rapid formation and development of a cyclone,” McNoldy explained. “To officially qualify as a bomb, the central pressure of the low must fall at least 24 millibars in 24 hours at 60 degrees latitude, or 14 millibars in 24 hours at 30 degrees latitude.”

Differences in bomb cyclones and hurricanes

Although satellite imagery of the recent bomb cyclone, captured by the National Oceanic and Atmospheric Administration‘s GOES 18 satellite, resembles that of a hurricane, the two systems differ structurally.

Hurricanes are vertically upright, with their structure extending uniformly from the ocean surface to the upper atmosphere. This vertical alignment allows for efficient wind flow and intensification.

“But a midlatitude cyclone is actually tilted in the vertical,” explained Ben Kirtman, a professor of atmospheric sciences and the William R. Middelthon III Endowed Chair of Earth Sciences at the Rosenstiel School.

“What’s happening in the upper atmosphere sits to the west with what’s happening at the surface, and it’s that vertical tilt that is so critical for the development of the cyclone.” 

“And so, the wind shear is a critical source of energy in the development of the cyclone as opposed to hurricanes, where it’s vertically stacked, and it’s the warm water below that is the source of energy for hurricanes.”

A common feature of winter weather

Bomb cyclones are winter storms that typically occur from October through March. They also form off the U.S. East Coast, where New England’s notorious nor’easters often qualify as bomb cyclones.

The recent powerful bomb cyclone that struck the Pacific Northwest eventually combined with an atmospheric river – a long, narrow band of water vapor in the atmosphere that transports moisture from tropical regions to cooler areas. Meteorologists are calling it a “once-in-a-decade” storm.

Data from the National Weather Service shows that such weather systems are becoming more common, especially along the East Coast. Between 1980 and 2020, the number of bomb cyclones in the Atlantic basin increased by approximately 40 percent.

Climate change and bomb cyclones

The question arises: is climate change driving the increase in bomb cyclones? The answer is complex. Kirtman noted that it’s probably connected to warmer ocean temperatures.

He explained that as the climate system warms, the higher latitudes warm faster than the lower latitudes, and the energy for the system of midlatitude cyclones is that contrast between north and south. 

“And so, your first knee-jerk reaction would be, ‘If it’s warming in the higher latitudes more than the southern latitudes, there’s less energy for these cyclones, less contrast in temperature.’ But that’s not the whole story,” said Kirtman.

“The challenge is what happens when you weaken that polar-equator temperature gradient – you get a wavier pattern in the midlatitude wind jet stream. And what that means is it’s able to suck down cold air because it’s wavier; we’re getting these bigger dips.”

“So, you’re getting cold air that’s penetrating further south, and that sets off these midlatitude cyclones. It’s that cold air injection that sets them off. And so, we can get an increase in these systems.”

Image Credit: Pixabay/CC0 Public Domain

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