Coastal communities in the US are highly vulnerable to floods, which can be caused by storm surges, extreme rainfall, and upstream river discharges. Traditionally, it was assumed that rainfall and storm surges occurred independently, and thus their potential compounding was not taken into account in flood risk assessment. In reality, storm surges and rainfall may be highly dependent, since a single storm system can often generate both flood mechanisms. For example, hurricanes are one of the largest drivers of coastal flood losses, and a single hurricane can generate both extreme storm surges and extreme rainfall. Moreover, as the climate continues to warm, causing rising sea levels and potential changes in storm climatology, the risk of joint rainfall-surge events may increase significantly.
The study in the journal Nature Climate Change, led by CEE Associate Professor Ning Lin and her graduate student Avantika Gori, is among the first to assess how climate change could impact the frequency of extreme rainfall-surge events. Dazhi Xi (another graduate student in Lin’s lab) and Kerry Emanuel (MIT) are coauthors of the paper. Integrating climate projection, hurricane modeling, and rainfall and storm surge simulations, the researchers investigate the frequency of the joint extreme event, defined as one that exceeds the 100-year level (magnitude that has <1% chance of occurring in any year) of both 24-hour rainfall and peak storm tide based on historical analysis. They found that a drastic increase under the high emission scenario SSP5 8.5 by 2100 in the frequency of joint extreme rainfall-surge events. In particular, along the Gulf of Mexico the joint extreme events occur on average once every 200-500 years in the historical climate, but they may occur as often as every 10-30 years in the future. For regions in the Northeast US, the increase in the joint event frequency is even larger such that in the future the Mid-Atlantic and New England regions may experience the joint extreme events on average once per 5-10 years.
Most interestingly, they found that, rather than sea level rise, projected changes in storm climatology (especially increases in storm intensity and decreases in storm translation speed) are a dominant driver of the increases in the join extreme events for most coastal areas. Consequently, the findings motivate greater consideration of changing hurricane activity in addition to sea level rise and adaptation to joint hazards in coastal communities.