Linking global precipitation intensification to coastal river and compound flooding

Destructive coastal floods are increasing in frequency and may be caused by global warming intensifying precipitation, through an increase in atmospheric moisture content. Such connections through climate, watershed, and river processes is poorly understood because of complex interactions in transitional fluvial-marine environments where flooding is caused by rivers, marine storm surge, or both in compound events. Long-term observations of precipitation, discharge, and water levels from watersheds in the northern Gulf of Mexico (Pascagoula, Tombigbee-Alabama, Apalachicola) were analyzed with nonstationary methods. Results show intensifying precipitation shortened precipitation-discharge lag times, doubling river-flood frequency and increasing the likelihood of compound events in fluvial-marine transitions. This occurred when the Atlantic Multidecadal Oscillation and El Niño Southern Oscillation began strongly affecting river discharge through the advection of moist air, intensifying precipitation. Flooding frequency also tripled in a downstream direction, reaching a maximum in the fluvial-marine transition. Along the transition, compound events were identified as overlapping surge and river flood waves with dynamics that routinely formed compound events exclusively in inland reaches. However, they did not exceed the size of non-compound river floods, suggesting the largest flood hazard along most of the fluvial-marine transition is solely from river discharge. Our results demonstrate precipitation and river discharge play critical roles in coastal flooding and will likely escalate flooding as the climate continues to warm and intensify precipitation.