Changing patterns of compound coastal flooding along the Pacific coast of North America: precipitation-surge interactions under climate change

Abstract

As climate changes, coastal communities face increasing threats not only from individual hazards, but from their combined impacts. This study investigates compound coastal flooding (CCF) potential arising from concurrent precipitation and storm surge along the Pacific Coast of North America, focusing on how these interactions may intensify under future scenarios. Using high-resolution multi-model RCP8.5 projections from the Canadian Coastal Climate Risk Information System, we analyzed CCF pattern changes across three future periods through 2100 to capture the evolving nature of joint hazard occurrence. We applied two approaches to identify compound events: (1) conditional block maxima (BM), with separate conditioning on annual maxima of surge and precipitation, and (2) peaks-over-threshold (POT), which isolates co-occurring events where both drivers exceed their 95th percentile thresholds within a short matching window. The choice of method significantly affects the resulting dependence structure. The conditional BM approaches reveal a distinct latitudinal gradient, peaking with positive dependence in the mid-latitudes, while the POT approach captures uniformly lower, but structurally important, correlations domain-wide. Projected joint return periods (JRPs) for extreme compound events exceeding the 98th percentile return level reveal severe, methodology-dependent hazard intensification. BM approaches, particularly when conditioned on annual maximum surge, project substantial northern JRP reductions by 2100; however, weak ensemble signals render these shifts largely indistinguishable from internal climate variability. Backed by strong multi-model consensus for the far future, POT projects a drastic escalation in northern hazard frequency, with compound events in Alaska and British Columbia becoming up to 5 to 10 times more frequent. These findings emphasize that resilient coastal planning requires diverse analytical frameworks.