An alarming feature of Hurricane Ida, which devastated Louisiana on Sunday, was how quickly it evolved from category 1 status in the Gulf of Mexico to category 4 at landfall. The storm’s sustained winds spun up from 85 miles per hour on Saturday to 150 mph the next day when they galloped onshore. The escalation in power was so quick and extreme that the National Oceanic and Atmospheric Administration’s National Hurricane Center (NHC) described Ida as a rapidly intensifying storm.
Forecasters at the NHC currently rely on one criterion for classifying such a storm: when the speed of its sustained winds increases by at least 30 knots (about 35 mph) within a 24-hour period. In the past, the term “rapid deepening” has been applied to a tropical storm if its central pressure drops by at least 42 millibars (about 0.61 pounds per square inch, or psi) in 24 hours. Remarkably, Ida’s pressure dropped by 56 millibars (about 0.81 psi) in 24 hours, making it something like a “super rapidly intensifying storm,” says atmospheric scientist Jennifer Francis of the Woodwell Climate Research Center in Falmouth, Mass. And Ida underwent that drop in pressure farther north than any other storm ever recorded in the Atlantic, tweeted Sam Lillo, a meteorology researcher at NOAA, on Sunday.
Francis has extensive experience studying rapid alterations in the Arctic climate system—research that has involved atmospheric influences on sea ice and transfers of heat and moisture from lower latitudes that are caused by climate change, among other subjects. That work led her to investigate their influence on weather patterns farther south, including extreme weather events such as winter storms and hurricanes.
Scientific American asked Francis to explain what caused Ida’s explosion in force.
[An edited transcript of the interview follows.]
What factors cause a storm to intensify rapidly?
First, it needs a substantial reservoir of energy in the ocean in the form of a deep layer of extra warm water. If that layer is shallow, it doesn’t contain sufficient energy to fuel rapid intensification. So the storm will deplete it quickly and not undergo intensification, especially a rapid one. The second need is for water vapor, which has been increasing over the past several decades because of the warming atmosphere and oceans. Warmer water evaporates more vapor into the air, and warmer air can hold more vapor. We’re already seeing about a 4 percent overall global average increase in the amount of water vapor in the atmosphere since the mid-1990s. That water vapor literally contains the fuel that the storm uses to intensify itself. When that water vapor, a gas that you can’t see, condenses into clouds as it does in a storm, it releases a lot of heat. That increases the upward motions in the atmosphere that lead to the big thunderstorms of a tropical storm. Wind shear tends to rip apart those updrafts of hot air that are occurring because of the condensation of water vapor. And when those become tilted or are ripped apart, you don’t get the formation of the big thunderstorms that feed into the development of a tropical storm.
Meteorologists have said that an eddy in the Gulf of Mexico played a role in the rapid intensification of Ida.
In the Gulf of Mexico, which is where both Hurricane Katrina and Ida underwent their rapid intensification, there was a blob of very warm, deep water that’s associated with a current called the Loop Current that comes from the Caribbean. You can think of it as a river in the ocean that flows into the Gulf from the warm Caribbean, then eastward, toward southern Florida, then up the East Coast, where it’s called the Gulf Stream. Sometimes the Loop Current forms a northward kink in the Gulf of Mexico, creating an eddy or pool of extra warm, deep water. It’s not an unusual occurrence, but when it does happen, and a tropical storm comes along and passes over it, it’s like giving the storm an energy drink. Energy plows into the storm from that pool of very warm water. That was the case in both Katrina and Ida.
Are rapidly intensifying storms becoming more common with climate change?
Yes, and that’s one of the clearest signals that we have of how climate change is affecting tropical storms. We’re warming the atmosphere with the burning of fossil fuels, and we’re warming the oceans. With those two factors, there’s more moisture in the atmosphere now because it can evaporate from the oceans more readily into a warmer atmosphere that can accept more water vapor. And this is all contributing to the fuel that tropical storms need to intensify.
Can you elaborate on the role of climate change in increasing the number of rapidly intensifying storms?
The ocean is absorbing about 90 percent of the heat that’s being trapped by the extra greenhouse gases that we’ve dumped into the atmosphere. So that is, all by itself, providing most of the ingredients needed for rapid intensification—just having that warm seawater, like a supercharged battery for storms, created by human-caused climate change. But in addition, when we do get a rapidly intensifying storm and a very strong storm, like we saw in Ida, that also creates stronger winds, so more wind damage, as we’ve certainly seen. A bigger storm surge, which, of course, is riding on top of higher sea levels, is another direct effect of climate change. The waves that are being generated by those stronger winds are also riding on top of a higher storm surge and sea-level rise. All of those factors are made even worse by climate change. And the increased amount of water vapor in the atmosphere is causing more frequent and heavier downpours—an increasing frequency of heavy precipitation events, whether it’s with a tropical storm or a thunderstorm or a nor’easter.
Can forecasters tell if a storm will intensify rapidly?
Satellites can only measure surface ocean temperatures. They can’t tell us how deep that layer of warm water is. That’s really one of the big impediments right now: we just don't have those data generally available in most areas of the ocean. We just don’t know how deep that layer is in most places. A focus of some research right now is figuring out how to get better information about how much energy is contained in, say, the upper 500 feet of the ocean, because that’s really where the energy is stored that feeds into the storms.
Approaches to obtaining these data include autonomous ocean gliders that swim beneath the ocean surface, measuring temperature, salinity and other characteristics. And satellites that can measure the height of the sea surface can be helpful because a warmer body of water takes up more room. So when you have a lot of warm water sitting in a location, generally, you’ll see a hump in the ocean surface there, which can be detected from space.
What can be done to mitigate and address rapidly intensifying tropical storms?
We humans have been using the atmosphere as a dumpster for more than 50 years now. We’ve been putting all of these waste gases, mostly from burning fossil fuels, into the atmosphere. And we have known for a very long time that these gases trap heat that largely goes into the ocean, thereby fueling these storms and also warming up the atmosphere. That’s really the underlying disease, so to speak, that we need to treat. And the way to go about that is to stop emitting these heat-trapping gases. There’s a role for all levels of society in this, all the way from national governments to companies and individuals. We know that solar and wind and other renewables can totally replace our electricity. We just need to get on with it faster.
What can individuals do?
We can make a lot of choices in our own lives, in our own communities and at the state level, such as incentives to support buying a car that's more efficient, to buy appliances that are more efficient or to insulate your home better. We have to pull out all the stops.
We also have to get ready. Our actions now will affect how [future] warming occurs. We are going to see more extremes; we are going to see more rapidly intensifying storms; we are going to see more heat waves and more fires. So we need to prepare for those.
When a home gets flooded out for the third time in a decade, does it make sense to spend tax dollars to help that person rebuild that home in the same spot? It does not. And you know, people don’t realize that a lot of our tax money is going toward things like this. We hear that “oh, emergency funds are going to help people rebuild on this low island off South Carolina.” People should be outraged. I think they’re not connecting the dots between the money they’re contributing and what it’s going toward in some locations where we really should not be rebuilding infrastructure.