+ & – Storm Surge
Hurricane Ian recently ravaged the Caribbean, Florida, and South Carolina.
At publication, over 100 people lost their lives to this massive storm. The number could rise significantly, as approximately 10,000 people are still missing. The tempest topped out as a Category 4 hurricane, with sustained winds of 155 miles per hour. The corridor of Florida across which the storm traveled received more than 10 inches of rain in under 24 hours; some spots got as much as 17 inches.
Wind and rain rightly receive a lot of attention with hurricanes. The unheralded killer, however, is often storm surge.
What is storm surge?
The phenomenon stems from cyclones (called tropical storms and hurricanes in our region of the world) and results in tsunami-like flooding. Storm surge is measured as the rise in water level over normal tides, excluding waves. If the storm surge is high enough, coastal flooding ensues.
Hurricane Ian produced some wicked surges. Check out the following webcam footage from Fort Meyers:
Unlike tsunamis, which are usually caused by earthquakes or landslides, storm surge is largely the result of wind.
Cyclones, by definition, create high-speed winds. These gales can become so strong that they can push huge amounts of water from the ocean. If the direction of the wind is toward land, the result is a storm surge. Imagine the power required to move walls of water, as seen in the videos above, from a body as large as the Gulf of Mexico.
How a storm surge susses out comes down to a few subfactors. Winds directly affect the water via an effect we call the Ekman spiral. This event causes surface currents at 45-degree angles to the wind, which has the consequence of pushing water downwind.
A hallmark of cyclones is extremely low atmospheric pressure. This anomaly can influence water levels. The happening is easy to visualize. Picture a large, solid block pushing down on a bathtub full of water. This high-pressure example would lower the water slightly. Normally, atmospheric pressure and ocean water are in balance, but, if the pressure above is low enough, the water level can actually rise. For every millibar the pressure drops, the sea rises approximately 10 millimeters. Hurricanes can see atmospheric pressure drop between 50 and 150 millibars.
The rotation of the Earth also plays a role. The Coriolis effect “bends” currents rightward in the Northern Hemisphere. If the angle of the bend lines up perpendicularly with a shoreline, storm surge can amplify.
Despite the heavy amounts of rain associated with cyclones, this water really does not impact storm surges. One notable exception occurs at the mouths of rivers and streams. Torrential rains can tax drainage systems, with the largest effects occurring where these waterways meet the oceans. When a wall of water attempts to empty into the ocean and it’s met by a larger wall being pushed the other way, the result can be a devastating storm surge in that location.
The topography of the region contributes to the surge outcome, as well. In open water, waves do not actually transport much water; rather, the energy moves through the water. However, near shore, if the surge is much higher than usual, waves can transport water significantly inland. Further, areas of shallow, constrained bays can suffer higher storm surges than areas where the water is deeper and the shelves are wider. In Florida, for example, the eastern shores often feature lower storm surges than the Gulf side because the Atlantic Ocean proper is deeper. On the western side of Florida’s peninsula, the average depth is much lower. Deeper, wider areas allow for surges to be displaced over a great area; shallower, narrower portions of shorelines can see water pushed up onto the mainland rather easily.
Though terrifying and deadly, storm surges do create one rather fascinating attribute. So far, we have discussed what meteorologists sometimes call “positive storm surge.” The wind pushes the water forward, onto land.
Most storm systems move in just a single direction, but hurricanes are furious, swirling balls. As you can see in the first animated image above, the storms rotate around an eye. Wind direction changes depending on the portion of the storm one experiences. The winds of a hurricane blow counterclockwise.
So, in the case of Hurricane Ian, as it made landfall in Florida, the southern half of the cyclone pushed water toward the mainland. These areas suffered the worst storm surges. But, in the northern half, a phenomenon sometimes called “negative storm surge” transpired. There, the winds blew away from the mainland.
In Tampa, the strong, outward gales pushed so much water away from the shore that the bay emptied!
The second video above features footage from 2017’s Hurricane Irma. In that clip, you can see people walking on the normally watery bay. That behavior is, of course, risky, as the water will eventually come back. And, as you can see from the videos from Hurricane Ian, when a positive storm surge hits, it can come in a hurry.
Further Reading and Exploration
Storm Surge Overview – National Hurricane Center
Storm Surge – National Geographic
Here’s why Hurricane Ian is sucking water out of Tampa Bay. – New York Times (printer-friendly; no paywall)