Local and Remote Storm Surge Contributions to Total Water Levels in the Gulf of St. Lawrence During Hurricane Fiona

Post‐tropical Hurricane Fiona generated a large storm surge that resulted in pronounced flooding and coastal erosion in Atlantic Canada in September 2022. In this study we apply a regional barotropic storm surge model in the Gulf of St. Lawrence, a semi‐enclosed sea, to demonstrate a method of evaluating different contributions to the total water levels. These include the surge generated over the ocean, the surge generated by the cyclonic winds over the gulf, and the tides. The results indicate that the highest storm surge occurred in the southeastern region, a combination of locally and remotely generated components. The surge that entered from the ocean was greater than the surge generated over the gulf; however, these were not in phase. To investigate the case where the local and remote surges are coincident, we shift the wind field relative to the timing of the boundary conditions and find the near “perfect storm” with significantly higher storm surge elevations. These findings highlight the importance of basin morphology and storm conditions in controlling the interactions of surge components, and this approach can be applied to simulate a range of storm‐driven hazard outcomes for future extreme events. Plain Language Summary Strong winds and low air pressure over water can cause the water level to rise. This storm surge can flood land, change the shape of beaches, and cause damage to roads and buildings. In this study, we use a computer model to calculate the storm surge during Hurricane Fiona, one of the most intense storms ever to hit eastern Canada. We use the model for different conditions, and determine that part of the storm surge was generated over the Atlantic Ocean, while another part was generated over the Gulf of St. Lawrence. These two parts of the storm surge did not occur at exactly the same time, and were not coincident with high tide. If all three of these components of the total water level peak simultaneously in a future storm, coastal areas could experience even more damage. Key Points A numerical model is applied to compute the storm surge driven by Hurricane Fiona in the Gulf of St. Lawrence Water level changes caused by the storm surge entering from the ocean, the storm surge generated by local winds, and tides are evaluated Results reveal spatial and temporal variability of different surge components that drive maximum water levels and regional coastal change