On the Changing Pattern of Seasonal Flooding Along the U.S. East Coast

Acceleration in the frequency and duration of flooding along the U.S. east coast is linked to high rates of local sea level rise. However, large variations are found in the severity and timing of floods at different locations along the coast, so that planning strategies for adaptation and mitigation efforts will require regional approaches. The seasonal pattern of flooding shows, for example, a distinct regional pattern where over 75% of the minor floods occur during the fall in Florida, but during the winter and spring in Boston. Two mechanisms that found to contribute to the seasonal sea level and flooding pattern are the annual and semiannual tidal cycles and variations in the Gulf Stream (GS) flow. Moreover, nor’easter storms affect flooding in the north during winter and spring, while hurricanes and tropical storms affect flooding in the south during the fall. In recent decades the seasonal pattern seemed to have shifted so that increased flooding is seen on the northeastern coasts during spring and summer, while in the Mid-Atlantic and southeastern coasts a dramatic increase in flooding is seen almost exclusively during the fall. A long-term change in the mean zonal wind pattern, especially during summer is a potential factor in the recent shift in the seasonal flooding pattern. The increased risk of flooding in the southeastern coasts especially during the fall is a combination of a peak in annual and semiannual tides, a weakening GS and the hurricane season, all occurring at the same time of the year. Recent studies also found that hurricanes can have a long-lasting indirect impact on increased flooding during the fall through their disruption of the Gulf Stream flow.