The number of compound coastal flooding events driven by a combination of both storm surge and heavy precipitation has increased significantly over the past century for many major US coastal cities, finds a paper published online this week in Nature Climate Change. Such events are also shown to be more likely to occur on the Atlantic or Gulf coast than on the Pacific coast.

Nearly 40% of the US population resides in coastal counties and flooding in these usually low-lying, densely populated and highly developed regions can have wide-ranging social, economic and environmental consequences. When storm surge and heavy precipitation co-occur, the potential for flooding in low-lying coastal areas is often amplified compared with either phenomenon in isolation.

Thomas Wahl and colleagues determined the likelihood of the joint occurrence of these two phenomena for the contiguous United States. In the case of New York City, the authors determine that the observed increase in compound events is attributable to storm surge weather patterns that also favour high precipitation, as opposed to weather events primarily driven by high precipitation accompanied by storm surges. Under certain conditions, the combined event pushed the chances of flood risk to a once in 105 year event, compared with once in 245 years if precipitation and storm surges are thought of as independent. They find that long-term sea-level rise is the main driver for increased flooding along the US coastline, but changes in the joint distributions of storm surge and precipitation associated with climate change and variability further increase flood potential.