Climate scientist Tom Corringham talks to Down To Earth about bomb cyclones, atmospheric rivers and how global warming can effect these natural phenomena
Bomb cyclones and atmospheric rivers caused fierce rainstorms that brought flooding to California in January 2023. Photo: iStock
Intense snow and rain storms in December in the United States impacted 48 out of 50 states and killed more than 60 people and fierce rainstorms brought flooding to California in January. These events were caused by separate but interrelated weather phenomena called bomb cyclones and atmospheric rivers, respectively.
A bomb cyclone is an extratropical storm that forms on land and intensifies rapidly with a sudden pressure drop. It occurs due to the interaction of cold Arctic air with the warm air from the tropics.
On the other hand, an atmospheric river is a relatively long, narrow region in the atmosphere — like a river in the sky — that transports most of the water vapour outside of the tropics, according to the US government’s weather forecast agency National Oceanic and Atmospheric Administration.
Read more: ’Atmospheric river’ over California spurred by bomb cyclone
Down To Earth spoke to Tom Corringham, climate scientist at the Scripps Institution of Oceanography, University of California San Diego, to understand the interrelations between these two phenomena and the current and future impact of global warming, especially in the Arctic region, on their frequency and intensity. Edited exerpts:
Akshit Sangomla: Can bomb cyclones and atmospheric rivers occur independent of each other? If so, how would that happen?
Tom Corringham: Most (roughly 80 per cent of) atmospheric rivers are associated with extratropical cyclones, which often intensify the atmospheric rivers. Sometimes, extratropical cyclones are so strong and develop so rapidly that they are referred to as bomb cyclones.
In contrast, only half of all extratropical cyclones have paired atmospheric rivers. So a powerful extratropical cyclone can develop without an associated atmospheric river.
AS: Apart from North America, where else do such events occur?
TC: Atmospheric rivers generally and most characteristically make landfall in the mid-latitudes on the west coasts of the Earth’s major land masses, including western Europe, southern Africa, Australia, New Zealand, and South America.
Depending on one’s precise definition of an atmospheric river, it can be shown that similar phenomena occur in almost all regions of the globe outside of the tropics.
AS: Have atmospheric rivers and associated bomb cyclones increased in frequency and intensity in the past few decades?
TC: I cannot refer you to any studies showing increased frequency and intensity in recent decades, but higher-intensity storms are more likely today than in the pre-industrial era, according to global climate models.
The atmosphere can hold more moisture as the Earth’s climate continues to warm due to greenhouse gas emissions. We expect this will lead to more frequent extreme atmospheric rivers.
AS: How has global warming affected both these phenomena to date? What is the role of the rapidly warming Arctic region and the weakening of the Polar Jet Stream?
TC: The evidence indicates atmospheric rivers are becoming longer, wider and wetter with global warming. The Arctic is warming faster than anywhere else on Earth, reducing the temperature gradient between the equator and the poles.
Understanding the effects of these changes is an active area of research. The global atmosphere is a massive dynamical system. Because of industrial greenhouse gas emissions, we are altering some of the fundamental parameters of the system without understanding the potential impacts.
Read more: Atmospheric river storms can drive costly flooding — and climate change is making them stronger
AS: What could happen to atmospheric rivers and bomb cyclones and their destructive capabilities as we seem to be headed towards at least 1.5 degrees Celsius of warming?
TC: Our recent research projected tripling of flood damage associated with atmospheric rivers in the western United States due to climate change alone, assuming that patterns of development and real wealth remain constant at today’s levels.
The importance of adaptation and investment in increased resilience was highlighted by the research. If communities worldwide continue to build in flood-prone areas, we expect to see even greater increases in damage.
If we limit growth in flood plains and coastal regions and invest in improved flood control infrastructure instead, we can limit or even reduce future damage.
Even if we immediately reduce global greenhouse gas emissions to zero, we have already destabilised the global climate system. We will have to adapt to the new normal we have created. In our research, however, we do find that increases in damage can be limited by reducing global emissions.
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