Hurricane activity in the North Atlantic has gone through extended, multi-decadal periods of generally above-normal and below-normal activity. "Active" periods (1926-1970, 1994-present) tend to have more named storms, hurricanes and major hurricanes compared to "quiet" periods (1900-1925, 1971-1994).
Why does North Atlantic hurricane activity exhibit this variability? What controls it? And can we predict active and quiet periods? These questions are essential to hurricane-risk assessment and to the re/insurance industries.
Academic researchers have made a substantial effort to find answers to these questions, but the attempt usually leads to more questions: Does the ocean or the atmosphere primarily drive sea-surface temperature changes thought to be responsible for hurricane variability? And if it's the ocean, is it the slow-moving global circulation of the ocean (including both deep and shallow waters)? Or is it only the shallow waters among the top ~100m of the North Atlantic? Do clouds play a role? What about volcanoes?
The answers are inexact, and it’s hard to draw sound conclusions because reliable observational records span only two active periods and two quiet periods.
To understand the various schools of thought on this issue, Swiss Re's Cat Perils teamed up with scientists at Columbia University in New York City to host a one-day workshop on the causes and predictability of North Atlantic hurricane variability. The Sept. 8 workshop featured talks by nine leading hurricane experts and vibrant panel discussions. In our opening address, we highlighted the relevance of the topic to the insurance industry and the importance of bridging the gap between academic theory and business/societal applications.
We invited representatives of important Swiss Re US clients to the workshop and attracted other industry stakeholders like brokers and cat modelers. With an active hurricane season underway, more than 75 academic and industry scientists attended! Primary insurers, reinsurers, and model vendors were all represented.
One speaker showed that multi-decadal variability in North Atlantic hurricane activity is driven by internal ocean dynamics, a pattern expected to continue in the future. Another speaker demonstrated how quiet hurricane activity in the 1970s/1980s was caused by European sulfate aerosols and African mineral dust that drifted over the Atlantic. (These pollutants reflect incoming sunlight, preventing the North Atlantic from warming up enough to form and sustain strong hurricanes.) The theory suggests that air-pollution regulations in the early 1990s allowed more sunlight to reach the Atlantic, warming it up again and ushering in another active period.
Despite the fundamentally different (and sometimes contrasting) views on what controls North Atlantic hurricane variability, it was remarkable to see the vibrant scientific dialogue. Pinpointing the right answers will take additional research and more hurricane records. But it was refreshing to see the academic and industry communities come together to discuss an issue that is highly relevant to both groups. We hope to continue this dialogue in the future.
Workshop funding was kindly provided by the Swiss Re Institute.
Authors: Marla Schwartz and Peter Zimmerli
Category: Climate/natural disasters: Climate change, Disaster risk, Floods/storms
Location: New York, NY, United States