Tide “canceled” by Hurricane Dorian
In addition to causing significant property damage in the Magdalen Islands, Hurricane Dorian had another astonishing consequence: it seems to have “canceled” a tide, so to speak. This effect was largely ignored, but it was measured throughout the St. Lawrence to Quebec.
“I’ve been doing the ferry between Trois-Pistoles and Île aux Basques for 29 years, and I’ve never seen that. The water was just not rising, “says the guardian of the island of Basques, Jean-Pierre Rioux. Like other captains in the area, he needs high tide to leave the marina of Trois-Pistoles. Normally, tides are easy to predict since they are governed by the gravity of the Moon and Sun. But on Sunday, September 8, the planned tide simply did not arrive. And when Mr. Rioux arrived with two tourists to embark at the end of the morning, his boat was still lying on the seabed instead of floating.
“So I thought we’d give ourselves another 45 minutes, because the tide was still rising. But in the meantime, I see the ferry Trois-Pistoles-Les Escoumins [… which finally] never got closer than 30 meters from the wharf and he had to return to Les Escoumins, “says Rioux.
Research scientist in Ocean Forecasting at Fisheries and Oceans Canada, Denis Lefaivre confirms that Dorian has prevented the tide from climbing as high as expected that day. There was indeed a high tide that was recorded and measured, notice, but its amplitude was much smaller than expected – and as the tide forecast that day was already the weakest of the month, the total effect was spectacular. In Cap-aux-Meules, the “high” tide was 40 cm lower than expected. In Rimouski, the difference was 52 cm. And since the tidal effect is in a way “concentrated” and amplified by the conical shape of the St. Lawrence, a gap of 1.15 meters was measured in Quebec City, says Mr. Lefaivre.
“It is mainly the very low atmospheric pressure of Dorian and its rapid passage in the Gulf that are involved. We were at 960 millibars […] which is the strongest depression we’ve seen in the last fifty years, I would say, “he says. The normal pressure is 1013 millibars.
When the pressure is very low in one place it creates a kind of suction on the surface of the water that raises the level, explains Mr. Lefaivre. But this water must come from somewhere, so that around the low pressure, its level drops.
So when Dorian went to the Islands, he raised the waters, but when the depression came out of the Gulf to go to the Atlantic, it was in that area that the “aspiration” moved. This then prevented the tidal water from entering the Gulf – hence the tide partly “canceled”.
In other sectors, the opposite has occurred: it was low tide that was “canceled”. For example, the University of Quebec Rimouski oceanographer Daniel Bourgault notes, the measurements of the St. Lawrence Global Observatory showed a high tide much higher than expected in Rivière-au-Renard (at the end of Gaspésie) during the passage of Dorian , by about 40 cm. But the low tide that theoretically was supposed to follow only finally dropped to a level comparable to that of the next high tide.
It may be thought that Rivière-au-Renard was included in the area where Dorian was raising the water – the Gaspesian point is not so far from the Islands, after all. It is also possible, adds Mr. Bourgault, that the extremely strong winds that were blowing then could push water towards Rivière-au-Renard, thus amplifying the effect of the depression until “jumping” at low tide.
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