Ask a scientist: Hurricanes

Stanford experts explain how climate change affects hurricanes and typhoons, where hurricanes are likely to make landfall and bring damaging winds in the future, and more.

The Atlantic hurricane season, which peaks in late August, is shaping up to be more active than normal this year, with federal forecasters now predicting 14 to 21 named storms. The projection includes both tropical storms and hurricanes, which are types of tropical cyclones.

Hurricane Hilary — **Hurricanes and typhoons:** When a tropical cyclone reaches maximum sustained winds of at least 74 miles per hour, it is classified as a hurricane, typhoon, or tropical cyclone, depending on where in the world the storm originated. *(Image credit: NASA Earth Observatory)*

The above-normal activity, predicted to include up to five major hurricanes, is thanks in large part to record-high temperatures in the parts of the Atlantic Ocean where hurricanes are born.

So, how much is hurricane activity connected to climate change? In the wake of Tropical Storm Hilary’s landfall in southern California, which hasn’t seen a tropical storm in 84 years, what can scientists tell us about where hurricanes are likely to make landfall and bring damaging winds in the future? We’ve gathered four experts who study atmospheric dynamics, severe storms, and climate to answer these questions and more about hurricanes.

Jump to answers:

Morgan E O'Neill — **Morgan O’Neill, Assistant Professor of Earth System Science:** “I always wanted to be an astrophysicist, up until junior year of college, when I learned about climate change.” Read Morgan O’Neill’s story.

How do hurricanes form?
How does climate change affect hurricanes?
What makes a hurricane's path hard to predict?
Where are hurricanes likely to make landfall and bring damaging winds in the future?
Does climate change cause more tropical storms and hurricanes?

Our experts include Morgan O'Neill, an atmospheric scientist at the Stanford Doerr School of Sustainability who studies the fluid dynamics of storms to better understand how severe weather and climate interact with each other. “I primarily use computer models as a ‘lab’ of sorts, but try whenever possible to test my findings against real observations,” she said. Additional expertise comes from Aditi Sheshadri, a fluid dynamicist at the Stanford Doerr School of Sustainability whose research centers on large-scale climate features like the jet stream, storm tracks, and the polar vortex.

**Aditi Sheshadri, Assistant Professor of Earth System Science:** “Space propulsion was fascinating to me. I read a lot of science fiction – Arthur C. Clark and Isaac Asimov were two of my particular favorites. I guess that’s how it all started.” Read Aditi Sheshadri’s story.

Sheshadri and O’Neill are joined by Earth system science PhD students Adam Burnett and Ipshita Dey. Burnett followed an interest in physics, climate change, and the natural world to join Sheshadri’s research group in 2019. “What happens close to the surface of Earth is super complicated – hard to describe using clean physics,” Burnett said. “I thought that if I studied something more on the mathematical side in the atmospheric dynamics field, that might be a pretty good fit for me.”

Before coming to Stanford to join O’Neill’s Severe Weather and Climate group, Dey worked as a scientist in the Indian Space Research Organization. “We used satellite data to monitor tropical cyclones and other severe weather events and could observe their increasing impact. Tropical cyclones bring a lot of rainfall and floods outside of the monsoon season, which damages crops and can drive up their prices," Dey said. “The impacts are so direct in our lives, I thought it was important to study tropical cyclones, particularly in the context of a changing climate.”

How do hurricanes form?

Dey: Hurricanes typically form from an initial atmospheric disturbance (such as a tropical wave or a convection cluster) under a set of favorable environmental conditions, such as warm sea surface temperatures, abundant moisture in the atmosphere, and low wind shear amongst many others.

Fueled by unusually warm waters, Hurricane Idalia roared toward the Florida coast on the morning of August 29, 2023. *(Image credit: NASA Earth Observatory)*

How does climate change affect hurricanes?

O’Neill: We have little confidence about how climate change will impact overall hurricane and typhoon frequency, but basic physical principles suggest that storms are likely to get stronger (faster maximum winds) and wetter (more rain). Because these are among the most devastating impacts of hurricanes upon landfall, we don’t need to wait for a clear answer about the overall frequency to call for protective action in coastal communities.

What makes a hurricane’s path hard to predict?

O’Neill: The bulk of hurricane motion is due to steering by the planetary-scale high- and low-pressure systems the storms are embedded in. Sometimes hurricanes are not strongly influenced by these larger systems, which makes their tracks less predictable. That’s why, when you look at a historical map of all recorded tracks, some behavior happens over and over again, and some tracks look wacky and wiggle around in an ocean basin without precedent.

Dey: Advancements made in numerical models and observations have led to increased skill of tropical cyclone track forecasts. In fact, we are closing in on the intrinsic predictability limits of tropical cyclone tracks. It is the hurricane intensity that is difficult to predict, particularly for hurricanes that are undergoing rapid intensification. This may be due to an incomplete understanding of boundary layer and air-sea exchange physics, a lack of real-time measurements of ocean properties, and incorrect forecasts of environmental conditions.

Where are hurricanes likely to make landfall and bring damaging winds in the future?

O’Neill: Recent research shows that the tropics are expanding poleward slightly, and will continue to do so under climate change. Clusters of thunderstorms will have access to warmer waters in the subtropics than normal, which increases the ocean surface area that is capable of producing hurricanes. So more poleward locations are likely to be at increasing risk of hurricane landfall – though of course, the risk in any particular place in a given year is exceedingly small!

Dey: Scientists have observed a poleward shift of the latitude at which a hurricane reaches its maximum intensity in the western North Pacific which is a major concern for mid-latitude coastal cities. And we also know with some confidence that more intense hurricanes are becoming more frequent, and these can travel further inland causing more flooding and inundations.

It’s important to remember that hurricanes are not just coastal hazards and the risk of inland flooding may be more significant than expected. And while the Saffir-Simpson Scales or Categories that we use to assess hurricane impact provide information about wind speed, these categories do not fully capture the potential for other hazards like storm surges and heavy rainfall.

Sheshadri: Beyond the two-week horizon, if we want to predict how many hurricanes there will be in a given year or in a given climate scenario, we come back to basic questions about what sets the number of tropical cyclones.

Does climate change cause more tropical storms and hurricanes?

Burnett: Tropical storms and hurricanes are both types of tropical cyclones. Climate change is expected to increase the strength of the strongest tropical cyclones. That might mean that more tropical storms cross over the threshold where they become hurricanes. But scientists don’t know yet if warming the climate will increase or even decrease the total number of tropical cyclones.

Tropical cyclones are discrete features that originate out of this chaotic, continuously variable atmosphere. There’s some amount of randomness in that.

We only have a few decades of very complete satellite observations, where we know we’ve seen every tropical cyclone that has occurred. Those few decades are also the time scale over which climate change has been having a stronger impact on our climate. So, we don’t have a long enough observational record to see whether there’s a correlation between warming temperatures and changing numbers of tropical cyclones. There are other things that have gone on during those decades, including a reduction in aerosols, that may have influenced tropical cyclone numbers independently of climate change.

Sheshadri: We know that warmer air can hold more water, and warmer sea surface temperatures tend to favor tropical cyclone formation. I have encountered only one physics-based, large-scale constraint on the number of tropical cyclones occurring each year that I find plausible. That’s the latitude of the Intercontinental Convergence Zone, or ITCZ. As you move the ITCZ poleward, we seem to see an increase in the number of tropical cyclones.

Why Earth spawns a surprisingly consistent number of cyclones each year

Between 80 and 100 tropical cyclones occur on our planet every year. But what on Earth makes that number so consistent has long been a mystery.

Using a simplified computer model of our planet, Stanford atmospheric scientist Aditi Sheshadri and PhD student Adam Burnett found the number of tropical cyclones is set largely by where the warmest band of ocean water lies and where trade winds from the north and south converge. The discovery, which relied on a computer model of an Earth covered entirely in ocean, could eventually help improve predictions for how often hurricanes, typhoons, and big tropical storms will brew in a warming world.

“The most complex models can tell you very precisely what happens, but if you want to answer some questions about why, it can be helpful to reduce your model to the simplest possible system that still simulates the thing you’re trying to understand,” Burnett explained.

From space, the zone where trade winds converge looks like a band of clouds around Earth’s midsection, often peppered with flashes of thunderstorms. Known to sailors as the doldrums, the zone is famous for windless waters that can becalm sailing ships for weeks. Scientists call it the Inter-Tropical Convergence Zone, or ITCZ.

The ITCZ usually extends about five degrees north and south of the equator, but it gradually shifts from season to season. As the convergence zone moves north of the tropics, Earth’s rotation exerts stronger influence on air movement because the planet spins faster at the equator than it does at the poles.

The effect of Earth’s rotation on air movement is expressed in a mathematical term known as the Coriolis parameter. As this value increases, it can lead to more disturbances or wobbles in otherwise smoothly circulating air, which then spawn tropical cyclones under the right conditions.

“Wobbles form at the edge of the ITCZ and then are intensified by the Coriolis parameter. If the ITCZ is at the equator and the disturbances form there, they don’t want to ‘spin up’ and form cyclones. But that changes as you move the ITCZ even slightly towards the pole,” said Sheshadri.

The modeling results, published in 2021, indicate the ITCZ accounts for only a fraction of the total number of tropical cyclones. “In some ocean basins, tropical cyclones that have the ITCZ as a precursor are a significant story. In the Atlantic, they’re only about half the story,” Sheshadri said. An important question now centers on how the position of this tropical rainbelt over different ocean basins and times of year may shift under climate change.

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