Storm surge due to hurricane Fiona in eastern Canada

2022-10-04. Hurricane Fiona made landfall September 24, 2022, shortly after 5 a.m. ADT in the eastern end of mainland Nova Scotia between the towns of Canso and Guysborough. A weather station in nearby Hart Island then measured an atmospheric pressure of 932.6 mb, establishing a new historical record for Canada. This was a warning of terrible things to come for many living in Eastern Canada. By clicking on the image below, you can see a video showing the size and path of hurricane Fiona over eastern Canada.

I spent part of the weekend watching media coverage of the storm. I saw meteorologists present nice maps of observed peak wind gusts, atmospheric pressure, surface waves amplitude, etc. I also heard them talk about a large storm surge (Washington Post : what is a storm surge?) but did not see any map of observed storm surge amplitude. After noting this shortcoming, I challenged myself to create such a map. Hence this blog post which focuses on the storm surge in Newfoundland, Prince Edward Island and Nova Scotia. A complementary text written in French provides further details about the storm surge in Québec and New Brunswick. 

The real-time CHS tide gauge network is much denser west of Île-aux-Coudres than in the remainder of eastern Canada due to commercial navigation needs of under keel clearance in the St. Lawrence seaway. The single CHS station west of Île-aux-Coudres included here for analysis is at Québec City.

I selected four tide gauge stations to illustrate key features of local tides and of the Fiona storm surge.

For a storm surge to cause coastal erosion and damage to seaside buildings and roads, several conditions must be met at the same time. Damage is greatest when 1) the peak of the storm surge coincides with high tide; 2) the storm surge occurs during a period of spring tide; 3) strong surface waves break on the shore. Conditions 1) and 3) were met at Port-aux-Basques but not condition 2). The peak storm surge from Hurricane Fiona occurred about mid-way between neap and spring tides, during a high tide of 1.67 m. High tides during this spring-neap tidal cycle ranged from 1.47 m at neap tide to 1.87 m at spring tide. A worst-case scenario of peak storm surge coinciding with a high tide during spring tides would have caused total sea level to be 20 cm higher.

At North Sydney, located on the opposite side of Cabot Strait relative to Port-aux-Basques, peak storm surge happened during a low tide, so that total sea level observations were much lower than at Port aux Basques.

Charlottetown experienced flooding, but the total sea level could have been about 50 cm higher if the peak storm surge had coincided in time with the second high tide of September 24.

Strong waves accompanying the storm surge caused extensive coastal erosion in most of Prince Edward Island, as indicated by this set of before (August 21, 2022) and after (September 25, 2022) images taken from space by the Sentinel-2 satellite mission.

At Halifax, the most striking feature is that shortly after peak surge (0.62 m), fierce winds from the west-northwest pushed ocean waters away from shore, causing a 0.7 m (70 cm) sea level set-down on September 24.

A broad perspective of the maximum amplitude of the Fiona storm surge is shown below.

The Northumberland Strait region, located between Prince Edward Island and the provinces of New Brunswick and Nova Scotia, experienced the largest storm surges during the passage of hurricane Fiona.

Discussion

While the Fiona storm surge was quite large by Canadian standards, most of the coastal erosion and material damage along the coastline associated with Hurricane Fiona was primarily due to the extremely large waves breaking with brutal force on the shoreline. Individual waves heights exceeding 10 m were reported at Port-aux-Basques. The extra one meter in total sea level due to the storm surge certainly made things worse.

In the USA, a storm surge of 4.2 m was associated with Hurricane Sandy in New York, flooding streets, tunnels, and subway lines. And in 2005, a gigantic storm surge of 8.2 m accompanied Hurricane Katrina in Mississippi, causing flooding as far as10 km inland.

Methods

Real-time sea level data for the entire tide gauge network of the Government of Canada can be accessed here. Data from individual gauges can be obtained by zooming on the interactive map of Canada and clicking on the red triangle for the site of interest. 

Water level observations at each of the twenty-two tide gauge stations were obtained for the five-day time window from September 21, 2022 (18:00 UTC) through September 26, 2022 (17:45 UTC). The use of the UTC (Coordinated Universal Time) time zone has been preferred for all stations to avoid any confusion between the three time zones of Eastern Canada (EDT = UTC-4; ADT = UTC-3; NDT =UTC-2:30).

The astronomical tide, predicted solely from the position of the Earth relative to the Sun and Moon, is provided at 15-minute intervals. Observed total water levels are provided at 1-minute intervals. These one-minute water level observations were first averaged over 15 minutes. Residual sea level was then calculated as the difference between the 15-minute averaged sea level observations and predicted astronomical tides. This gives us a proxy of storm surge amplitude.

The storm surge, also called meteorological tide, is calculated by subtracting the predicted tide due to the Moon and Sun from the observed water level values.